Python copy array element

How to copy elements from one list to another in Python [Easy Way]

There are many ways to copy a list to another memory location. But are you aware of how these lists are copied under the hood?

Just assigning the old list to a new empty list won’t copy the content from the old list to the new list. When a list (which has elements) is assigned to a new list like, list_two = list_one , where list_two is a new empty list and list_one has a few elements, it doesn’t really create two new lists with the same data. It just copies the reference from list_one to list_two , i.e list_two would contain the reference to the memory location to which list_one also refers to.

To actually, truly copy content from list_one to list_two , many methods can be used, one of which is copy.deepcopy() , which we have covered in this tutorial.

How to copy elements from one list to another in Python

1. Using the built-in copy method

This copy method is available in Python beginning from the Python 3.3 version. A shallow copy is made when the copy method is used. For example,

list_two = [] list_one = [1,2,4,4,7] list_two = list_one.copy() print(list_two)

2. List slicing for copying contents from an old list to a new one

Below is the code example to show how list slicing can be used to copy elements of one list to another list.

list_two = [] list_one = [1,2,4,4,7] list_two = list_one[:] print(list_two)

3. Using the built-in list method

The list method is used to create a new list.

list_two = [] list_one = [1,2,4,4,7] list_two = list(list_one) print(list_two)

4. Using the generic copy.copy method

This is comparatively slower than the built-in list method. This is because the generic copy method is required to find out the type of the list which has to be copied to the new list.

import copy list_two = [] list_one = [1,2,4,4,7] list_two = copy.copy(list_one) print(list_two) # if we change list_two, list_one is also changed list_two[1] = 500 print("List two", list_two) print("List one", list_one)

[1, 2, 4, 4, 7] List two [1, 500, 4, 4, 7] List one [1, 500, 4, 4, 7]

When we updated the list_two , the list_one also got changed because they both point to the same list in the memory. Hence, if one changes any list elements, those changes are reflected in the other list too.

5. Using the copy.deepcopy method

This method is used when we want a new copy of the list to be created. This method is extremely slow, but sometimes it has to be inevitably used as using this method a deep copy of list elements is created which means a separate copy of the list elements.

In the code example below, when we copy the list a to list b , then the list elements are actually copied and a new list is created in the memory. While in all the other ways of copy that we covered above, only provide the reference of the existing list to the new list, not actually a copy of the list elements.

import copy a = ['a', 'b', 1, 2, 3] b = copy.deepcopy(a) # if we change b elements, a is not changed b[2] = '10' print(a) print(b)

6. Using the * (unpacking) operator

This operator is known as the unpacking operator although it has no official name. It was introduced in Python 3.5+.

list_two = [] list_one = [1,2,4,4,7] list_two = [*list_one] print(list_two)

This works with simple lists only, not nested lists.

Don’t confuse the above code with the code below. The code below simply means multiply by 1, and it will repeat all the list elements of list_one once, if you change 1 with 2, the complete list will be repeated twice. This is not the unpacking operator, but using the * operator with 1, also means you are copying the list_one to list_two . It is similar to using a simple assignment operator i.e. list_one = list_two .

list_two = [] list_one = [1,2,4,4,7] list_two = list_one * 1 print(list_two) list_two = list_one * 2 print(list_two)

[1, 2, 4, 4, 7] [1, 2, 4, 4, 7, 1, 2, 4, 4, 7]

Conclusion

Copying elements from one list to another is a fundamental operation in Python, and mastering different copying techniques is crucial for efficient list manipulation. In this article, we explored various methods, including shallow copying, deep copying, and copying subsets of elements, providing you with a comprehensive understanding of list copying in Python.

Now armed with this knowledge, you can confidently handle list operations, create backups, extract specific elements, and even modify elements during the copying process. Whether you’re a beginner or an experienced Pythonista, these techniques will serve you well in your programming journey.

Frequently Asked Questions(FAQs)

1. In Python, can I transfer items from one list to another?
Yes, you can transfer components from one list to another using techniques such as list slicing, list comprehension, and the built-in copy() function.

2. Is a loop required to transfer items from one list to another?
A: No, using a loop is not required. To duplicate components without using a loop, use list slicing or the built-in copy() method.

3. Is it possible to transfer components between lists of varying lengths?
Yes, you can copy components from a list of any length to another list, independent of the length of the target list.

4. Will transferring components from one list to another cause the original to change?
It depends on the method you use. Some methods, like list slicing, will not modify the original list, while others, like the built-in copy() method, will create a copy of the original list.

5. How can I make a shallow copy of a list in Python?

To make a shallow copy of a list, you can use the slicing technique: new_list = original_list[:] or the copy() method: new_list = original_list.copy() .

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Python Program to copy all the elements of an array to another array

In this simple python program, we have to copy the elements of an array to another array. It’s an intermediate-level python program.

To understand this example, you should have knowledge of the following Python programming topics:

What is an Array or List?

In this python program example, we use the concept of Arrays in python. An array is a set of elements of the same data type which are stored in sequential memory locations, that can be accessed by an index. Array elements must be the same type. The array is denoted by a[i] where ‘a’ is the array name and ‘1’ is the subscript to point the element, a[0] will be the first element of the array ‘a’.

How to copy elements of an array to another array in python?

In the python language, there are no arrays here we use the list as arrays which is the same as arrays. In this array python program, we are going to copy the elements of one array to another array, to achieve that we have to initialize an array arr1 with some predefined elements in it. Create another array arr2 of the length of the first array. Then we use a for loop in python language to copy each element of the first array arr1[i] to the second array arr2[i]. Finally, we display both arrays.

ALGORITHM

STEP 1: Initialize the first array which we need to copy using the array concept in python language.

STEP 2: Create another array of lengths of the first array using len() method.

STEP 3: Use a for loop starting from zero to the length of the first array.

STEP 4: Assign each value of the first array to the second array in the for loop using arr2[i] = arr1[i];

STEP 5: Use a for loop from zero to the length of the first array and print the elements of the first array using print in the python programming language

STEP 6: Use another for loop from zero tho length of the second array to display the elements in the second copied array using the print statement.

Python Source Code

arr1 = [1, 2, 3, 4, 5]; # define the first array with elements arr2 = [None] * len(arr1); # create a second array using the len() for i in range(0, len(arr1)): # Use for loop to assign first array elements to second array arr2[i] = arr1[i]; print("Elements of first array: "); # display first array for i in range(0, len(arr1)): print(arr1[i]), print(); print("Elements of second array: "); # display the second array for i in range(0, len(arr2)): print(arr2[i]), 

OUTPUT

Elements of first array: 1 2 3 4 5 Elements of second array: 1 2 3 4 5

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Create array from another array in Python (Copy array elements)

In this tutorial, you will learn how to create an array from another (existing) array in Python. In simpler words, you will learn to copy the array elements into another array.

You must be familiar with what an array is and its uses.
To recap, an array is a data structure that stores multiple elements (values) in a single variable.

Using Python NumPy library’s method-copy ()

On execution, the above statement returns a new array – array2, which contains exactly the same elements as array1.
Here,
array1 is the n-dimensional array to be copied.
array2 is the new array to be created to contain elements of array1.

import numpy as np array1=np.array([1,2,3]) print("array 1",array1) array2=array1.copy() print("array 2",array2)

array 1 [1 2 3] array 2 [1 2 3]

It is important to note that first, we are creating a new array instance. Then, we are copying the contents of the original array into the new one.
Thus, any changes you, later on, make to the first array will not be reflected in the copied array.

import numpy as np array1=np.array([1,2,3]) array2=array1.copy() array1[1]=7 print("array 1",array1) print("array 2",array2)

array 1 [1 7 3] array 2 [1 2 3]

Well, what happens if you use the assignment operator (=) to copy the array elements?

It not only copies the elements but also assigns them as equals. So, any changes made to array1 will automatically be reflected in array2 as shown.

import numpy as np array1=np.array([1,2,3]) array2=array1 array1[1]=7 print("array 1",array1) print("array 2",array2)

array 1 [1 7 3] array 2 [1 7 3]

In better words, you are not creating a new object, but in fact, creating a reference to the original object. For better understanding, observe the code below:

import numpy as np array1=np.array([1,2,3]) array2=array1 array1[1]=7 print(id(array1)) print("array 1",array1) print(id(array2)) print("array 2",array2)

1924624603936 array 1 [1 7 3] 1924624603936 array 2 [1 7 3]

If you have observed, when you were using the copy () method, you were creating a new array object and not just a reference instance for the original object.

Copying the array elements into a new array by looping through in Python

1. Create a new array with the same length as that of the one to be copied
2. Loop through the two arrays, copying the elements from the first and then assigning them to the second respectively.

Note:
Even in this case, you are copying the elements into another array. So, any changes made to array1 will not be reflected in array2.

import numpy as np array1=np.array([1,2,3]) print("array 1",array1) array2=[None]*len(array1) for i in range(0,len(array1)): array2[i]=array1[i] print("array 2",array2)

array 1 [1 2 3] array 2 [1, 2, 3]

To learn more about Python arrays, Python Array Module

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