array assignment copy

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Copy assignment operator.

A copy assignment operator of class T is a non-template non-static member function with the name operator = that takes exactly one parameter (that isn't an explicit object parameter ) of type T , T & , const T & , volatile T & , or const volatile T & . For a type to be CopyAssignable , it must have a public copy assignment operator.

[ edit ] Syntax

[ edit ] explanation.

The copy assignment operator is called whenever selected by overload resolution , e.g. when an object appears on the left side of an assignment expression.

[ edit ] Implicitly-declared copy assignment operator

If no user-defined copy assignment operators are provided for a class type ( struct , class , or union ), the compiler will always declare one as an inline public member of the class. This implicitly-declared copy assignment operator has the form T & T :: operator = ( const T & ) if all of the following is true:

• each direct base B of T has a copy assignment operator whose parameters are B or const B & or const volatile B & ;
• each non-static data member M of T of class type or array of class type has a copy assignment operator whose parameters are M or const M & or const volatile M & .

Otherwise the implicitly-declared copy assignment operator is declared as T & T :: operator = ( T & ) . (Note that due to these rules, the implicitly-declared copy assignment operator cannot bind to a volatile lvalue argument.)

A class can have multiple copy assignment operators, e.g. both T & T :: operator = ( T & ) and T & T :: operator = ( T ) . If some user-defined copy assignment operators are present, the user may still force the generation of the implicitly declared copy assignment operator with the keyword default . (since C++11)

The implicitly-declared (or defaulted on its first declaration) copy assignment operator has an exception specification as described in dynamic exception specification (until C++17) noexcept specification (since C++17)

Because the copy assignment operator is always declared for any class, the base class assignment operator is always hidden. If a using-declaration is used to bring in the assignment operator from the base class, and its argument type could be the same as the argument type of the implicit assignment operator of the derived class, the using-declaration is also hidden by the implicit declaration.

[ edit ] Deleted implicitly-declared copy assignment operator

An implicitly-declared copy assignment operator for class T is defined as deleted if any of the following is true:

• T has a user-declared move constructor;
• T has a user-declared move assignment operator.

Otherwise, it is defined as defaulted.

A defaulted copy assignment operator for class T is defined as deleted if any of the following is true:

• T has a non-static data member of a const-qualified non-class type (or array thereof);
• T has a non-static data member of a reference type;
• T has a non-static data member or a direct base class that cannot be copy-assigned (overload resolution for the copy assignment fails, or selects a deleted or inaccessible function);
• T is a union-like class , and has a variant member whose corresponding assignment operator is non-trivial.

[ edit ] Trivial copy assignment operator

The copy assignment operator for class T is trivial if all of the following is true:

• it is not user-provided (meaning, it is implicitly-defined or defaulted);
• T has no virtual member functions;
• T has no virtual base classes;
• the copy assignment operator selected for every direct base of T is trivial;
• the copy assignment operator selected for every non-static class type (or array of class type) member of T is trivial.

A trivial copy assignment operator makes a copy of the object representation as if by std::memmove . All data types compatible with the C language (POD types) are trivially copy-assignable.

[ edit ] Eligible copy assignment operator

Triviality of eligible copy assignment operators determines whether the class is a trivially copyable type .

[ edit ] Implicitly-defined copy assignment operator

If the implicitly-declared copy assignment operator is neither deleted nor trivial, it is defined (that is, a function body is generated and compiled) by the compiler if odr-used or needed for constant evaluation (since C++14) . For union types, the implicitly-defined copy assignment copies the object representation (as by std::memmove ). For non-union class types ( class and struct ), the operator performs member-wise copy assignment of the object's bases and non-static members, in their initialization order, using built-in assignment for the scalars and copy assignment operator for class types.

[ edit ] Notes

If both copy and move assignment operators are provided, overload resolution selects the move assignment if the argument is an rvalue (either a prvalue such as a nameless temporary or an xvalue such as the result of std::move ), and selects the copy assignment if the argument is an lvalue (named object or a function/operator returning lvalue reference). If only the copy assignment is provided, all argument categories select it (as long as it takes its argument by value or as reference to const, since rvalues can bind to const references), which makes copy assignment the fallback for move assignment, when move is unavailable.

It is unspecified whether virtual base class subobjects that are accessible through more than one path in the inheritance lattice, are assigned more than once by the implicitly-defined copy assignment operator (same applies to move assignment ).

See assignment operator overloading for additional detail on the expected behavior of a user-defined copy-assignment operator.

[ edit ] Example

[ edit ] defect reports.

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[ edit ] See also

• converting constructor
• copy constructor
• copy elision
• default constructor
• aggregate initialization
• constant initialization
• copy initialization
• default initialization
• direct initialization
• initializer list
• list initialization
• reference initialization
• value initialization
• zero initialization
• move assignment
• move constructor
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4 Ways to Copy an Array in Java

When you’re working with arrays in Java, you may decide that you want to create a copy of an array. For instance, if you’re running a coffee shop and want to create a seasonal menu, you may want to create a copy of your original menu on which the new menu can be based.

In Java, there are a number of ways in which you can copy an array. This tutorial will explore four common methods to copy arrays and discuss how they work line-by-line. After reading this tutorial, you’ll be a master at copying arrays in Java.

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Java Arrays

In Java, an array is a container that holds values that hold one single type. For example, an array could be used to store a list of books, or a list of scores players have earned in a game of darts.

Arrays are useful when you want to work with many similar values because you can store them all in one collection. This allows you to condense your code and also run the same methods on similar values at one time.

Suppose we want to create an array that stores the coffees sold at our coffeeshop. We could do so using this code:

String[] coffees = {“Espresso”, “Mocha”, “Latte”, “Cappuccino”, “Pour Over”, “Flat White”};

In this example, we declare an array called coffees which stores String values. Our array contains six values.

Each item in an array is assigned an index number, starting from 0, which can be used to reference items individually in an array.

Now that we have explored the basics of Java arrays, we can discuss methods you can use to copy the contents of your array.

Copy Array Using Assignment Operator

One of the most common clone methods used to copy an array is to use the assignment operator. 

The assignment operator is used to assign a value to an array. By using the assignment operator, we can assign the contents of an existing array to a new variable, which will create a copy of our existing array.

Let’s return to the coffeeshop. Suppose we want to create a copy of the coffees array upon which we will base our summer coffee menu. We could use this code to create a copy of the array:

Our code returns:

Espresso, Mocha, Latte, Cappuccino, Pour Over, Flat White,

Let’s break down our code. On the first line of code inside our CopyAssignment class, we declare an array called coffees which stores our standard coffee menu.

On the next line, we use the assignment operator to assign the value of coffees to a new array called summer_coffees . Then we create a “for-each” loop that iterates through every item in the summer_coffees array and prints it out to the console.

There is one drawback to using this method: if you change elements of one array, the other array will also be changed. So if we changed the value of Latte to Summer Latte in our summer_coffees list, our coffees list would also be changed.

Loop to Copy Arrays

The first approach we discussed to copy an array — using the assignment operator — creates what is called a shallow copy. This is the case because we have assigned an existing array object to a new one, which means that when we change any object, they will both be changed — the two objects are linked.

However, we often need to create a deep copy. Deep copies copy the values from an existing object and create a new array object. When you create a deep copy, you can change your new array without affecting the original one.

One approach that can be used to create a deep copy is to create a for loop which iterates through the contents of an array and creates a new array.

Suppose we want to create a deep copy of our coffees array called summer_coffees. This needs to be a deep copy because we plan on changing the contents of the summer_coffees array to reflect the new coffees we will offer in the summer months. 

Here’s the code we would use to create a deep copy using a for loop:

When we run our code, the output is as follows:

[Espresso, Mocha, Latte, Cappuccino, Pour Over, Flat White]

As you can see, our code has created a copy of our original array. Let’s explain how it works step-by-step:

• We import java.util.Arrays which includes the toString() method, we’ll use to print our array to the console at the end of the example.
• We declare an array called coffees which stores the list of coffees on our standard menu.
• We initialize an array called summer_coffees which will store six values.
• We use a for loop to iterate through every item in the coffees list.
• Each time the for loop runs, the item at index value i in summer_coffees will be assigned the item at the index value i in coffees.
• We use Arrays.toString() to convert summer_coffees to a string, then print out the new array with our copied elements to the console.

Using copyOfRange() Java Methods

The Java copyOfRange() method is used to copy Arrays.copyOfRange() is part of the java.util.Arrays class. Here’s the syntax for the copyOfRange() method:

Let’s break down the syntax for the copyOfRange() method:

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• DataType is the type of data the new array will store.
• newArray is the name of the new array.
• oldArray is the array whose values you want to copy to the newArray .
• indexPos is the position at which the copy operation should begin in the oldArray .
• length is the number of values that should be copied from oldArray to newArray .

Let’s walk through an example to demonstrate the copyOfRange() method in action. Say we want to create a copy of our coffees array from earlier. We could do so using this code:

Summer coffees: [Espresso, Mocha, Latte, Cappuccino, Pour Over, Flat White]

Let’s break down our code:

• We import the java.util.Arrays library which stores the copyOfRange() and toString() methods that we’ll use in our example.
• We declare an array called coffees which stores the coffees on our standard menu.
• coffees is the name of the array we want to copy.
• 0 specifies we want to copy values starting at index position 0 from the coffees array.
• coffees.length specifies we want to copy every value in the list.
• We print out “Summer coffees: “ followed by the resulting array called summer_coffees to the console.

Now we’ve created a copy of our “ coffees list called summer_coffees .

Using arraycopy() Java Methods

The arraycopy() method is used to copy data from one array to another array. The arraycopy() method is part of the System class and includes a number of options which allow you to customize the copy you create of an existing array.

Here’s the syntax of the arraycopy() method:

System.arraycopy(sourceArray, startingPos, newArray, newArrayStartingPos, length);

Let’s break down this method. copyarray() accepts five parameters:

• sourceArray is the name of the array you want to copy.
• startingPos is the index position from which you want to start copying values in the source_array .
• newArray is the name of the new array where values will be copies.
• newArrayStartingPos is the index position at which the copied values should be added.
• length is the number of elements you want to copy to the new_array .

Let’s return to the coffeeshop. Suppose we wanted to copy every value in our coffees array to a new array called summer_coffees . We could do so using this code:

Let’s break down our code step-by-step:

• We import the java.util.Arrays package at the start of our program. This includes the toString() method we’ll use to print the array copy we create at the end of our program.
• We initialize an array called summer_coffees which will hold 6 values.
• coffees is the array we want to copy.
• 0 is the position at which we want our copy to start in the coffees array.
• summer_coffees is the array in which we want our copied values to be added.
• 0 is the position at which we want the copied values to start being added in the summer_coffees array.
• coffees.length is the number of array elements we want to copy. In this case, using coffees.length allows us to copy every element from the coffees list.
• We print out a message stating “Summer coffees: “, followed by the list of summer coffees we have created.

Copying an array is a common operation when you’re working with lists. This tutorial explored four ways in which you can copy an array in Java.

First, we discussed how to create a shallow copy using the assignment operator, then we proceeded to explain how to create a deep copy using a for loop. Then we explored how to use the copyOfRange() method to create a copy of an array, and how the arraycopy() system method is used to copy an array.

Now you’re ready to start copying arrays in Java like a professional!

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Blog about the Xojo programming language and IDE

Working with Arrays: Assignment vs Copy

If you’re starting to program with Xojo, then you’ll probably soon find yourself in a situation where you need to use one of the Container types: Array. In this case, it’s also probable that you might be confused at first as to why you don’t get what you expected when assigning the contents of a source Array to a target Array via the assignment operator (“=”).

After all, when using the following snippet of code we could (wrongly) expect that the contents stored in the ATarget Array would be the same that the ones stored in the ASource Array. Of course, and at a first glance, this is the case. Try to write the following fragment in the Opening Event of a new Xojo Project or Method created by yourself (don’t forget to call such method, if you decide to follow that path):

If you set a breakpoint in the last line of code and execute your test app, then you’ll see that, as exepected, the contents of the ATarget() Array are the same that the ones stored by the ASource() Array. (Navigate through the Debugger Inspector Panel to inspect these variables)

However, what happens if we add the following line of code at the end of our fragment of code?:

In this case we will be redimensioning the storage capacity of our ASource() Array with a total of 0 items. That is, in practice we will be emptying the Array.

As result of this operation you could expect that, while the contents of the ASource() variable are emptied, your ATarget() Array will continue storing their items. 

The truth is that it doesn’t.

This is how Array Assignment Works

When we use the assignement operator (=) to set the values (items) of an Array to another one, what in fact happens behind the scenes is that the target Array will store only a reference to the source Array (the right-side value of the assignement operator), and not a copy of the values/objects stored.

Thus, and because they are references, all the operations we do on the source Array will be reflected in the target Array.

If you want to see that for yourself, try to delete the previous breakpoint and put a new one in the last line of code added.

When you run the now modified code, using the IDE Debugger feature to run the line of code where the Debugger has stopped, you’ll see how both Array variables ( ASource and ATarget ) reveal that their dimensions are -1. That is: when you clean the source Array using the ResizeTo method that change is also reflected in the target Array.

Why does this happens?

Better Understanding: Reference vs Copy

All the apps you execute in your computer or any other device are stored in memory, both the code to be executed and the Objects created and used during all the application lifetime.

For example, when we declare a variable in our example code (in this case an Array of Integer type), we can think about that variable as a mere label pointing to the starting cell of memory where will be stored the values later. We could visually represent that as this:

So, this could be the visual representation of the memory when we add new items on the Array, although the real Xojo implementation of the stored values for an Array can differ from this. It doesn’t needs to be a contiguous region of memory or even the same structure to represent an Array in memory:

However, when we use the assignment operator (“=”) to set the contents of our ASource() Array to the ATarget Array what happens is that the value set for the ATarget() variable is the same memory position used by the ASource() variable; so all the operations made on the ASource() object (memory region) will be also reflected on ATarget() :

Conversely, when we iterate the items stored in the ASource() Array and use the Add method on the ATarget() Array, what happens is that the target Array will get and use its own memory region to store its own copy of the added values and, thus, is independent of the memory region used by the source Array.

This way, all the opeations made on the items pointed by the source Array will not be reflected on the target Array:

Copying Array Items

So, if what we really want to do is have our target Array stores its own copies of a source Array items, then we need to change from a simple assignment to an iteration and addition of the iterated values.

For that we need to use the Add Method on the target Array; while to access every item stored in the source Array we need to employ its index position in the Array (for example, ASource(0) will get the first item stored in the Array, while ASource.LastIndex will gives us the index position of the last added value).

Replace the previous code example with this one:

Now, if you set the a Debugger breakpoint in the last line of code and execute the application, then you’ll see, after executing the last line of code, our ATarget() Array will keep its own set of values, even after redimensioning the ASource() Array with the equivalent to “reserve storage for no items”, thus cleaning the region of memory used by such variable.

You can see all this in action in the following video (in Spanish):

Javier Rodri­guez has been the Xojo Spanish Evangelist since 2008, he’s also a Developer, Consultant and Trainer who has be using Xojo since 1998. He manages  AprendeXojo.com and is the developer behind the GuancheMOS plug-in for Xojo Developers, GuancheID, AXImageCanvas, Markdown Parser for Xojo, HTMLColorizer for Xojo and the Snippery app, among others.

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Object.assign()

The Object.assign() static method copies all enumerable own properties from one or more source objects to a target object . It returns the modified target object.

The target object — what to apply the sources' properties to, which is returned after it is modified.

The source object(s) — objects containing the properties you want to apply.

Return value

The target object.

Description

Properties in the target object are overwritten by properties in the sources if they have the same key . Later sources' properties overwrite earlier ones.

The Object.assign() method only copies enumerable and own properties from a source object to a target object. It uses [[Get]] on the source and [[Set]] on the target, so it will invoke getters and setters . Therefore it assigns properties, versus copying or defining new properties. This may make it unsuitable for merging new properties into a prototype if the merge sources contain getters.

For copying property definitions (including their enumerability) into prototypes, use Object.getOwnPropertyDescriptor() and Object.defineProperty() instead.

Both String and Symbol properties are copied.

In case of an error, for example if a property is non-writable, a TypeError is raised, and the target object is changed if any properties are added before the error is raised.

Note: Object.assign() does not throw on null or undefined sources.

Cloning an object

Warning for deep clone.

For deep cloning , we need to use alternatives like structuredClone() , because Object.assign() copies property values.

If the source value is a reference to an object, it only copies the reference value.

Merging objects

Merging objects with same properties.

The properties are overwritten by other objects that have the same properties later in the parameters order.

Copying symbol-typed properties

Properties on the prototype chain and non-enumerable properties cannot be copied, primitives will be wrapped to objects, exceptions will interrupt the ongoing copying task, copying accessors, specifications, browser compatibility.

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Given an array, we need to copy its elements in a different array, to a naive user below way comes into mind which is however incorrect as depicted below as follows:

Output: 

Output Explanation: When we do “b = a”, we are actually assigning a reference to the array. Hence, if we make any change to one array, it would be reflected in other arrays as well because both a and b refer to the same location. We can also verify it with code as shown below as follows:

We have seen internal working while copying elements and edge cases to be taken into consideration after getting through errors as generated above, so now we can propose out correct ways to copy array as listed below as follows:

• Iterating each element of the given original array and copy one element at a time
• Using clone() method
• Using arraycopy() method
• Using copyOf() method of Arrays class
• Using copyOfRange() method of Arrays class

Method 1: Iterating each element of the given original array and copy one element at a time. With the usage of this method, it guarantees that any modifications to b, will not alter the original array a, as shown in below example as follows:

Method 2: Using Clone() method  

In the previous method we had to iterate over the entire array to make a copy, can we do better? Yes, we can use the clone method in Java . 

Method 3: Using arraycopy() method

We can also use System.arraycopy() Method. The system is present in java.lang package. Its signature is as : 

Parameters:

• src denotes the source array.
• srcPos is the index from which copying starts.
• dest denotes the destination array
• destPos is the index from which the copied elements are placed in the destination array.
• length is the length of the subarray to be copied.

Method 4: Using copyOf() method of Arrays class  

If we want to copy the first few elements of an array or a full copy of the array, you can use this method.

Syntax:  

• Original array
• Length of the array to get copied.

Method 5: Using copyOfRange() method of Arrays class

This method copies the specified range of the specified array into a new array.

• Original array from which a range is to be copied
• Initial index of the range to be copied
• Final index of the range to be copied, exclusive

Lastly, let us do discuss the overview of the above methods:  

• Simply assigning references is wrong
• The array can be copied by iterating over an array, and one by one assigning elements.
• We can avoid iteration over elements using clone() or System.arraycopy()
• clone() creates a new array of the same size, but System.arraycopy() can be used to copy from a source range to a destination range.
• System.arraycopy() is faster than clone() as it uses Java Native Interface
• If you want to copy the first few elements of an array or a full copy of an array, you can use Arrays.copyOf() method.
• Arrays.copyOfRange() is used to copy a specified range of an array. If the starting index is not 0, you can use this method to copy a partial array.

This article is contributed by Ashutosh Kumar . Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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Array. Copy Method

Some information relates to prerelease product that may be substantially modified before it’s released. Microsoft makes no warranties, express or implied, with respect to the information provided here.

Copies a range of elements in one Array to another Array and performs type casting and boxing as required.

The following code example shows how to copy from one Array of type Object to another Array of type integer.

Copy(Array, Array, Int32)

Copies a range of elements from an Array starting at the first element and pastes them into another Array starting at the first element. The length is specified as a 32-bit integer.

The Array that contains the data to copy.

The Array that receives the data.

A 32-bit integer that represents the number of elements to copy.

sourceArray is null .

destinationArray is null .

sourceArray and destinationArray have different ranks.

sourceArray and destinationArray are of incompatible types.

At least one element in sourceArray cannot be cast to the type of destinationArray .

length is less than zero.

length is greater than the number of elements in sourceArray .

length is greater than the number of elements in destinationArray .

The sourceArray and destinationArray parameters must have the same number of dimensions. In addition, destinationArray must already have been dimensioned and must have a sufficient number of elements to accommodate the copied data.

When copying between multidimensional arrays, the array behaves like a long one-dimensional array, where the rows (or columns) are conceptually laid end to end. For example, if an array has three rows (or columns) with four elements each, copying six elements from the beginning of the array would copy all four elements of the first row (or column) and the first two elements of the second row (or column).

If sourceArray and destinationArray overlap, this method behaves as if the original values of sourceArray were preserved in a temporary location before destinationArray is overwritten.

This method is equivalent to the standard C/C++ function memmove , not memcpy .

The arrays can be reference-type arrays or value-type arrays. Type downcasting is performed, as required.

When copying from a reference-type array to a value-type array, each element is unboxed and then copied. When copying from a value-type array to a reference-type array, each element is boxed and then copied.

When copying from a reference-type or value-type array to an Object array, an Object is created to hold each value or reference and then copied. When copying from an Object array to a reference-type or value-type array and the assignment is not possible, an InvalidCastException is thrown.

If sourceArray and destinationArray are both reference-type arrays or are both arrays of type Object , a shallow copy is performed. A shallow copy of an Array is a new Array containing references to the same elements as the original Array . The elements themselves or anything referenced by the elements are not copied. In contrast, a deep copy of an Array copies the elements and everything directly or indirectly referenced by the elements.

An ArrayTypeMismatchException is thrown if the arrays are of incompatible types. Type compatibility is defined as follows:

A type is compatible with itself.

A value type is compatible with Object and with an interface type implemented by that value type. A value type is considered connected to an interface only if it implements that interface directly. Disconnected types are not compatible.

Two intrinsic (predefined) value types are compatible if copying from the source type to the destination type is a widening conversion. A widening conversion never loses information, whereas a narrowing conversion can lose information. For example, converting a 32-bit signed integer to a 64-bit signed integer is a widening conversion, and converting a 64-bit signed integer to a 32-bit signed integer is a narrowing conversion. For more information about conversions, see Convert .

A nonintrinsic (user-defined) value type is compatible only with itself.

Enumerations have an implicit conversion to Enum and to their underlying type.

If every element in sourceArray requires a downcast (for example, from a base class to a derived class or from an interface to an object) and one or more elements cannot be cast to the corresponding type in destinationArray , an InvalidCastException is thrown.

If this method throws an exception while copying, the state of destinationArray is undefined.

This method is an O( n ) operation, where n is length .

• ConstrainedCopy(Array, Int32, Array, Int32, Int32)

Copy(Array, Array, Int64)

Copies a range of elements from an Array starting at the first element and pastes them into another Array starting at the first element. The length is specified as a 64-bit integer.

A 64-bit integer that represents the number of elements to copy. The integer must be between zero and Int32.MaxValue , inclusive.

length is less than 0 or greater than Int32.MaxValue .

Copy(Array, Int32, Array, Int32, Int32)

Copies a range of elements from an Array starting at the specified source index and pastes them to another Array starting at the specified destination index. The length and the indexes are specified as 32-bit integers.

A 32-bit integer that represents the index in the sourceArray at which copying begins.

A 32-bit integer that represents the index in the destinationArray at which storing begins.

sourceIndex is less than the lower bound of the first dimension of sourceArray .

destinationIndex is less than the lower bound of the first dimension of destinationArray .

length is greater than the number of elements from sourceIndex to the end of sourceArray .

length is greater than the number of elements from destinationIndex to the end of destinationArray .

The sourceArray and destinationArray parameters must have the same number of dimensions. In addition, destinationArray must already have been dimensioned and must have a sufficient number of elements starting from the destinationIndex position to accommodate the copied data.

When copying between multidimensional arrays, the array behaves like a long one-dimensional array, where the rows (or columns) are conceptually laid end-to-end. For example, if an array has three rows (or columns) with four elements each, copying six elements from the beginning of the array would copy all four elements of the first row (or column) and the first two elements of the second row (or column). To start copying from the second element of the third row (or column), sourceIndex must be the upper bound of the first row (or column) plus the length of the second row (or column) plus two.

Copy(Array, Int64, Array, Int64, Int64)

Copies a range of elements from an Array starting at the specified source index and pastes them to another Array starting at the specified destination index. The length and the indexes are specified as 64-bit integers.

A 64-bit integer that represents the index in the sourceArray at which copying begins.

A 64-bit integer that represents the index in the destinationArray at which storing begins.

sourceIndex is outside the range of valid indexes for the sourceArray .

destinationIndex is outside the range of valid indexes for the destinationArray .

Additional resources

• Python »
• 3.11.5 Documentation »
• The Python Standard Library »
• Data Types »
• copy — Shallow and deep copy operations
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copy — Shallow and deep copy operations ¶

Source code: Lib/copy.py

Assignment statements in Python do not copy objects, they create bindings between a target and an object. For collections that are mutable or contain mutable items, a copy is sometimes needed so one can change one copy without changing the other. This module provides generic shallow and deep copy operations (explained below).

Interface summary:

Return a shallow copy of x .

Return a deep copy of x .

Raised for module specific errors.

The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):

A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.

A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

Two problems often exist with deep copy operations that don’t exist with shallow copy operations:

Recursive objects (compound objects that, directly or indirectly, contain a reference to themselves) may cause a recursive loop.

Because deep copy copies everything it may copy too much, such as data which is intended to be shared between copies.

The deepcopy() function avoids these problems by:

keeping a memo dictionary of objects already copied during the current copying pass; and

letting user-defined classes override the copying operation or the set of components copied.

This module does not copy types like module, method, stack trace, stack frame, file, socket, window, or any similar types. It does “copy” functions and classes (shallow and deeply), by returning the original object unchanged; this is compatible with the way these are treated by the pickle module.

Shallow copies of dictionaries can be made using dict.copy() , and of lists by assigning a slice of the entire list, for example, copied_list = original_list[:] .

Classes can use the same interfaces to control copying that they use to control pickling. See the description of module pickle for information on these methods. In fact, the copy module uses the registered pickle functions from the copyreg module.

In order for a class to define its own copy implementation, it can define special methods __copy__() and __deepcopy__() . The former is called to implement the shallow copy operation; no additional arguments are passed. The latter is called to implement the deep copy operation; it is passed one argument, the memo dictionary. If the __deepcopy__() implementation needs to make a deep copy of a component, it should call the deepcopy() function with the component as first argument and the memo dictionary as second argument. The memo dictionary should be treated as an opaque object.

Discussion of the special methods used to support object state retrieval and restoration.

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Java Copy Arrays

In Java, we can copy one array into another. There are several techniques you can use to copy arrays in Java.

1. Copying Arrays Using Assignment Operator

Let's take an example,

In the above example, we have used the assignment operator ( = ) to copy an array named numbers to another array named positiveNumbers .

This technique is the easiest one and it works as well. However, there is a problem with this technique. If we change elements of one array, corresponding elements of the other arrays also change. For example,

Here, we can see that we have changed one value of the numbers array. When we print the positiveNumbers array, we can see that the same value is also changed.

It's because both arrays refer to the same array object. This is because of the shallow copy. To learn more about shallow copy, visit shallow copy .

Now, to make new array objects while copying the arrays, we need deep copy rather than a shallow copy.

2. Using Looping Construct to Copy Arrays

Let's take an example:

In the above example, we have used the for loop to iterate through each element of the source array. In each iteration, we are copying elements from the source array to the destination array.

Here, the source and destination array refer to different objects (deep copy). Hence, if elements of one array are changed, corresponding elements of another array is unchanged.

Notice the statement,

Here, the toString() method is used to convert an array into a string. To learn more, visit the toString() method (official Java documentation) .

3. Copying Arrays Using arraycopy() method

In Java, the System class contains a method named arraycopy() to copy arrays. This method is a better approach to copy arrays than the above two.

The arraycopy() method allows you to copy a specified portion of the source array to the destination array. For example,

• src - source array you want to copy
• srcPos - starting position (index) in the source array
• dest - destination array where elements will be copied from the source
• destPos - starting position (index) in the destination array
• length - number of elements to copy

In the above example, we have used the arraycopy() method,

• System.arraycopy(n1, 0, n2, 0, n1.length) - entire elements from the n1 array are copied to n2 array
• System.arraycopy(n1, 2, n3, 1, 2) - 2 elements of the n1 array starting from index 2 are copied to the index starting from 1 of the n3 array

As you can see, the default initial value of elements of an int type array is 0.

4. Copying Arrays Using copyOfRange() method

We can also use the copyOfRange() method defined in Java Arrays class to copy arrays. For example,

In the above example, notice the line,

Here, we can see that we are creating the destination1 array and copying the source array to it at the same time. We are not creating the destination1 array before calling the copyOfRange() method. To learn more about the method, visit Java copyOfRange .

5. Copying 2d Arrays Using Loop

Similar to the single-dimensional array, we can also copy the 2-dimensional array using the for loop. For example,

In the above program, notice the line,

Here, the deepToString() method is used to provide a better representation of the 2-dimensional array. To learn more, visit Java deepToString() .

Copying 2d Arrays using arraycopy()

To make the above code more simpler, we can replace the inner loop with System.arraycopy() as in the case of a single-dimensional array. For example,

Here, we can see that we get the same output by replacing the inner for loop with the arraycopy() method.

Table of Contents

• Copying Arrays Using Assignment Operator
• Using Looping Construct
• Using the arraycopy() Method
• Using the copyOfRange() Method
• Copying 2d Arrays Using Loop
• Using arraycopy()

Sorry about that.

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Array Copy in Java

The assignment procedure would result in both variables pointing to the same array, arrays in Java cannot be copied using the = operator. Making copies of primitive types is straightforward, but you must take into account whether the copy is deep or shallow when making copies of objects or references. Java has several methods for array copying.

Introduction

Java Copy array operation will be covered in this section. Java array copy can be done using many methods provided by Java. Arrays in Java, as we know, can include items of primitive types, objects, or references. Making copies of primitive types is a simple process, but when it comes to objects or references, you must consider whether the copy is deep or shallow

In this example, we are going to check how to copy an array in java using for loop.

Code Explanation

In the above example, we have created an array with elements 1,2,3 . Then created another element and assign elements to another array. This will create a copy of array. To check if a copy is created we checked it using the equals() method this method returns false means both arrays have different references.

Problem with Simple Assignment Operation

We are essentially assigning a reference to the array when we write array2 = array1 . Because both array1 and array2 refer to the same location, any changes we make to one array will also affect the other arrays. We can also verify it with the code as shown below as follows:

In theabovee code, we have created an array with elements 1,2,3 . Created another array and assign the previous array to the new array. As the assignment operator will assign a reference to the previous array so equals() method will return true.

Java Copy Array Various Methods

There are many methods of copying an array we are going to discuss each one of them in detail.

Manual Copying Using For Loop

Copying one element at a time while iterating through every element of the original array provided. Using this method ensures that any changes to b won't affect the initial array a, as seen in the example below follows this example we, are going to check how to copy an array in java using for loop

In above example, we have created an array with elements 1,2,3 . Then created another element and assign elements to another array. This will create a copy of array. To check if ia f copy is created we checked it usinthe g equals() method this method returns false m ns both arrays have a differenreferencesce.

Using clone() Method

The act of making an exact duplicate of an object is referred to as object cloning. It makes a new instance of the current object's class and fills all of its fields exactly with the data found in the corresponding fields of the current object.

In this example we are going to check how to cop an array in java usinthe g clone method.

In this example, we have created an array with element 1,2,3,4,5,6 . We have used the clone() method to create a copy of array array1.

Using arraycopy() Method

We can also use the System.arraycopy() Method. The system is present in `java.lang package.

This method starts the copy action from the source position to the target position till the specified length.

The target array's length is equal to the number of elements copied there. It offers a simple method for copying a section of an array to another.

A NullPointerException is thrown if any of the array arguments are null. An IndexOutOfBoundException is thrown if any of the integer inputs are negative or outside of the acceptable range. Syntax of arraycopy() method is as follows

Parameters :

• srcPos index from which copying starts.
• dest stands for the destination array.
• destPos is the index from which the copied elements are added to the destination arra.
• length is the size of the copied subarray.

In this example, we are going to check how to copy an array in java using arraycopy() method.

In the above code f,first, we have created an an array withelementst 1,2,3,4,5,6,7 . We have used arraycopy() method with the following parameters src:a , dest:b , srcpos:0 , destpos :0 , and length as 3 .

Using copyOf() Method of Arrays class

The function java.util.Arrays.copyOf(int[] original, int newLength) duplicates the provided array, trimming or padding with zeros (if needed) to make the copy the desired length. The two arrays will have the same values for all indices that are valid in both the original array and the duplicate. The copy will contain 0 for any indices that are accurate in the copy but not the original. Only if the given length is larger than the length of the original array will such indices be present.

original − array to be copied. newLength − length of the copy to be returned.

This method gives back a copy of the original array that has been padded with zeros or trimmed to the desired length.

In this example, we are going to discuss how to copy an array in java using the copyof() method

Code Output

In the above example we have created an array with elements 1,2,3,4,5,6 and created a copyanof an array using the Arrays.copyOf() method with original arrays parameters as a and length of the array as newlength parameter.

Using copyOfRange() Method of Arrays Class

This function creates a new array by copying the desired range of the supplied array.

In this section, we are going to check how to copy an array in java using copyofRange() .

Parameters:

• original: array from which to copy a range in the original
• from: the range's starting index to be copied
• to: exclusive final index of the range to be copied

In the above example, we have created an array with elements 1 8 3 5 9 10 and created a copy of an array using the Arrays.copyOfRange() method fthe from 2 indexthe to 6 index.

Copying 2d Arrays Using Loop

We may use the for loop to copy the 2-dimensional array like how we clone a single-dimensional array.

In this example, we are going to check how to copy an array in java of type 2D.

In the above example we have created a copy of a 2D array using for loop similar top 1D array.

• Because the assignment procedure would result in both variables pointing to the same array, arrays in Java cannot be copied using the = operator.
• There arevariouss methods to copy an Object.
• Manual Copying using for loop: copying one element at a time while iterating through every element of the original array provided
• using clone() method
• using arraycopy() method
• using copyOf() method
• using copyOfRange() method

How to clone an array in JavaScript

JavaScript has many ways to do anything. I’ve written on 10 Ways to Write pipe/compose in JavaScript , and now we’re doing arrays.

Here's an interactive scrim that shows various ways to clone arrays in JavaScript:

1. spread operator (shallow copy).

Ever since ES6 dropped, this has been the most popular method. It’s a brief syntax and you’ll find it incredibly useful when using libraries like React and Redux.

Note: This doesn’t safely copy multi-dimensional arrays. Array/object values are copied by reference instead of by value .

This is fine

This is not fine

2. Good Old for() Loop (Shallow copy)

I imagine this approach is the least popular, given how trendy functional programming’s become in our circles.

Pure or impure, declarative or imperative, it gets the job done!

Note: This doesn’t safely copy multi-dimensional arrays. Since you’re using the = operator, it’ll assign objects/arrays by reference instead of by value .

3. Good Old while() Loop (Shallow copy)

Same as for —impure, imperative, blah, blah, blah…it works! ?

Note: This also assigns objects/arrays by reference instead of by value .

4. Array.map (Shallow copy)

Back in modern territory, we’ll find the map function. Rooted in mathematics , map is the concept of transforming a set into another type of set, while preserving structure.

In English, that means Array.map returns an array of the same length every single time.

To double a list of numbers, use map with a double function.

What about cloning??

True, this article’s about cloning arrays. To duplicate an array, just return the element in your map call.

If you’d like to be a bit more mathematical, (x) => x is called identity . It returns whatever parameter it’s been given.

map(identity) clones a list.

5. Array.filter (Shallow copy)

This function returns an array, just like map , but it’s not guaranteed to be the same length.

What if you’re filtering for even numbers?

The input array length was 3, but the resulting length is 1.

If your filter 's predicate always returns true , however, you get a duplicate!

Every element passes the test, so it gets returned.

6. Array.reduce (Shallow copy)

I almost feel bad using reduce to clone an array, because it’s so much more powerful than that. But here we go…

reduce transforms an initial value as it loops through a list.

Here the initial value is an empty array, and we’re filling it with each element as we go. That array must be returned from the function to be used in the next iteration.

7. Array.slice (Shallow copy)

slice returns a shallow copy of an array based on the provided start/end index you provide.

If we want the first 3 elements:

If we want all the elements, don’t give any parameters

Note: This is a shallow copy, so it also assigns objects/arrays by reference instead of by value .

8. JSON.parse and JSON.stringify (Deep copy)

JSON.stringify turns an object into a string.

JSON.parse turns a string into an object.

Combining them can turn an object into a string, and then reverse the process to create a brand new data structure.

Note: This one safely copies deeply nested objects/arrays !

9. Array.concat (Shallow copy)

concat combines arrays with values or other arrays.

If you give nothing or an empty array, a shallow copy’s returned.

10. Array.from (Shallow copy)

This can turn any iterable object into an array. Giving an array returns a shallow copy.

Well, this was fun ?

I tried to clone using just 1 step. You’ll find many more ways if you employ multiple methods and techniques.

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Program to copy array in C

This program shall help you learn one of basics of arrays. Copying an array involves index-by-index copying. For this to work we shall know the length of array in advance, which we shall use in iteration. Another array of same length shall be required, to which the array will be copied.

Let's first see what should be the step-by-step procedure of this program −

Let's now see the pseudocode of this algorithm −

Implementation

The implementation of the above derived pseudocode is as follows −

The output should look like this −

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Copying Array Content to Another Array in Assembly

I'm looking to copy some elements of an array to another one in Assembly. Both arrays are accessed via pointers which are stored in registers. So, edx would be pointing to one array and eax would point to another. Basically, edx points to an array of character read in from a text file, and I'd like eax to contain only 32 of the characters. Here's what I'm attempting to do:

I386 Assembly using NASM

Again, i'd like this to place the 32 characters after the first eight to be placed in the second array. The problem is that I'm stumped on how to do this.. Any help is very much appreciated.

• cpu-registers

3 Answers 3

You can't do direct memory-to-memory moves in x86. You need to use another scratch register:

Or something like that...

• You get a segfault when you use ecx because it overwrites your loop counter ( cl is the low 8 bits of ecx ). You need to use a different register for the copy: ebx seems the logical choice. –  Stephen Canon Mar 8, 2011 at 23:27
• @Stephen Canon Actually, I was using ebx. The seg fault was due to mov cl, 32 being commented out. –  Gavin Mar 8, 2011 at 23:30
• Actually, you can do "direct memory-to-memory moves in x86". –  DigitalRoss Mar 8, 2011 at 23:57
• @DigitalRoss: You mean in a modern intel processor? and How? –  The Mask Feb 25, 2013 at 21:31
• @TheMask, he's referring to the string copy instructions. See his answer. –  Carl Norum Feb 25, 2013 at 21:34

Both ia32 and ia64 do contain a memory-to-memory string move instruction that can move bytes, "words", and "doublewords".

The source address is specified in ESI and the destination in EDI. 1 By itself, it moves one byte, word, or doubleword. If the rep prefix is used, then ECX will contain a count and the instruction will move an entire string of values.

1. I think these instructions are the reason that the ESI and EDI registers are so named. (Source Index and Destination Index.)

The simple solution is to just do:

Be aware that under many platform's ABIs, ebx is a callee-save register, so you will need to save and restore its value in your function.

The simpler solution is to link against the standard library and call memcpy , which is perfectly acceptable in assembly, and will usually be substantially faster than writing your own loop.

• Thanks, Stephen. I'll have to remember that if I ever need an alternative to the above. –  Gavin Mar 8, 2011 at 23:34
• Or you could use rep movs to copy from [esi] to [edi] . –  ninjalj Mar 8, 2011 at 23:52
• @ninjalj: you could, but calling memcpy will be substantially faster on at least some architectures. –  Stephen Canon Mar 9, 2011 at 0:24

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