How do you change the position of a list in python?

How to change the position of an element in an array (list... in python)?

Suppose I've a list and now I want to change the position of one of it's elements. For instance, I want to change the position of A[i] to j. How can I do it efficiently (with less runtime and memory)? #if you are non-pythonistas then please share the algorithm **not swapping ex. [1,2,3,4,5] > [1,4,2,3,5] #moving 4 from position 3 to 1 **its INSERTION **I've got one: A.insert(j, A.pop(i)) #It is not an efficient approach, because pop is deleting an element, and reducing the size of the list, and again insert is adding an element, and increasing the size of list. So, the length of the array is changing for 2 times, and change of length is expensive for any type of array. [Updated on March 5, 2021] Hello everyone! For the last few days I was trying to solve the above problem in the fastest way possible. Here goes my implementation: https://code.sololearn.com/cr6MgZwo82P1/?ref=app . If anyone has got some other idea to solve the same, then please let us know. Thanks!

Table of Contents

• How to change the position of an element in an array (list... in python)?
• Python | Move element to end of the list
• Steps to Modify an Item Within a List in Python
• Step 1: Create a List
• Step 2: Modify an Item within the list
• Python Replace Item in List
• Replace an Item in a Python List at a Particular Index
• Inserting an element in list at specific index using list.insert()

arrays python3 list algorithm insertion

Md. Faheem Hossain

David Ashton Lothar Input : [1,2,3,4,5] Let i = 3, j = 1 Output : [1,4,2,3,5] My code : A.insert(j, A.pop(i)) #i gave this in the question and i repeat that i dont know why, but its not that of a great use. I dont know its algorithm, but probably its running in a very bad runtime; please can you say why its bad and how i can increase its efficiency like one given by David Ashton 's one, but keeping the concept like one i exampled above.

Md. Faheem Hossain

Python | Move element to end of the list

The manipulation of lists is quite common in day-day programming. One can come across various issues where one wishes to perform using just one-liners. One such problem can be of moving a list element to the rear ( end of list ). Let’s discuss certain ways in which this can be done.

Method #1 : Using append() + pop() + index()
This particular functionality can be performed in one line by combining these functions. The append function adds the element removed by pop function using the index provided by index function.

# Python3 code to demonstrate

# moving element to end

# using append() + pop() + index()

# initializing list

test_list = ['3', '5', '7', '9', '11']

# printing original list

print ("The original list is : " + str(test_list))

# using append() + pop() + index()

# moving element to end

test_list.append(test_list.pop(test_list.index(5)))

# printing result

print ("The modified element moved list is : " + str(test_list))

Output : The original list is : ['3', '5', '7', '9', '11'] The modified element moved list is : ['3', '7', '9', '11', '5']

Method #2 : Using sort() + key = (__eq__)
The sort method can also be used to achieve this particular task in which we provide the key as equal to the string we wish to shift so that it is moved to the end.

# Python3 code to demonstrate

# moving element to end

# using sort() + key = (__eq__)

# initializing list

test_list = ['3', '5', '7', '9', '11']

# printing original list

print ("The original list is : " + str(test_list))

# using sort() + key = (__eq__)

# moving element to end

test_list.sort(key = '5'.__eq__)

# printing result

print ("The modified element moved list is : " + str(test_list))

Output : The original list is : ['3', '5', '7', '9', '11'] The modified element moved list is : ['3', '7', '9', '11', '5']

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Steps to Modify an Item Within a List in Python

Step 1: Create a List

To start, create a list in Python. For demonstration purposes, the following list of names will be created:

Names = ['Jon', 'Bill', 'Maria', 'Jenny', 'Jack'] print(Names)

Run the code in Python, and you’ll get this list:

['Jon', 'Bill', 'Maria', 'Jenny', 'Jack']

Step 2: Modify an Item within the list

You can modify an item within a list in Python by referring to the item’s index.

What does it mean an “item’s index”?

Each item within a list has an index number associated with that item (starting from zero). So the first item has an index of 0, the second item has an index of 1, the third item has an index of 2, and so on.

In our example:

• The first item in the list is ‘Jon.’ This item has an index of 0
• ‘Bill’ has an index of 1
• ‘Maria’ has an index of 2
• ‘Jenny’ has an index of 3
• ‘Jack’ has an index of 4

Let’s say that you want to change the third item in the list from ‘Maria’ to ‘Mona.’ In that case, the third item in the list has an index of 2.

You can then use this template to modify an item within a list in Python:

ListName[Index of the item to be modified] = New value for the item

And for our example, you’ll need to add this syntax:

Names[2] = 'Mona'

So the complete Python code to change the third item from Maria to Mona is:

Names = ['Jon', 'Bill', 'Maria', 'Jenny', 'Jack'] #modify Names[2] = 'Mona' print(Names)

When you run the code, you’ll get the modified list with the new name:

['Jon', 'Bill', 'Mona', 'Jenny', 'Jack']

Python Replace Item in List

You can replace an item in a Python list using a for loop, list indexing, or a list comprehension. The first two methods modify an existing list whereas a list comprehension creates a new list with the specified changes.

Let’s summarize each method:

• List indexing: We use the index number of a list item to modify the value associated with that item. The equals sign is used to change a value in a list.
• List comprehension: The list comprehension syntax creates a new list from an existing one. You can specify conditions in your list comprehension to determine the values in the new list.
• For Loop: The loop iterates over the items in a list. You use indexing to make the actual change to the list. We use the enumerate() method to create two lists of index numbers and values over which we can iterate.

In this guide, we walk through each of these methods. We’ll refer to an example of each to help you get started.

Replace an Item in a Python List at a Particular Index

Python lists are ordered, meaning that we can access (and modify) items when we know their index position. Python list indices start at 0 and go all the way to the length of the list minus 1.

You can also access items from their negative index. The negative index begins at -1 for the last item and goes from there. To learn more about Python list indexing, check out my in-depth overview here.

If we want to replace a list item at a particular index, we can simply directly assign new values to those indices.

Let’s take a look at what that looks like:

# Replace an item at a particular index in a Python list a_list = [1, 2, 3, 4, 5, 6, 7, 8, 9] # Mofidy first item a_list[0] = 10 print(a_list) # Returns: [10, 2, 3, 4, 5, 6, 7, 8, 9] # Modify last item a_list[-1] = 99 print(a_list) # Returns: [10, 2, 3, 4, 5, 6, 7, 8, 99]

In the next section, you’ll learn how to replace a particular value in a Python list using a for loop.

Want to learn how to use the Python zip() function to iterate over two lists? This tutorial teaches you exactly what the zip() function does and shows you some creative ways to use the function.

Inserting an element in list at specific index using list.insert()

In python list provides a member function insert() i.e.

list.insert(position, element)
It accepts a position and an element and inserts the element at given position in the list.

Let’s see an example,

Suppose we have a list of strings i.e.

# List of string list1 = ['Hi' , 'hello', 'at', 'this', 'there', 'from']
Now let insert ‘why’ at 3rd position in the list i.e.
# Add an element at 3rd position in the list list1.insert(3, 'why')
Index will start from 0 in list. So, element will be inserted at 3rd position i.e. after 0,1 & 2.

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So, list contents will be now,

['Hi', 'hello', 'at', 'why', 'this', 'there', 'from']

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