Python: Start with a list of integers, keep every other element in place and otherwise sort the list

Last update on March 22 2025 13:48:52 (UTC/GMT +8 hours)

Sort Keeping Every Other Element Fixed

Write a Python program to start with a list of integers, keep every other element in place and otherwise sort the list.

Input: 
[2, 5, 6, 3, 1, 4, 34]
Output:
[1, 5, 2, 3, 6, 4, 34]

Input:
[8, 0, 7, 2, 9, 4, 1, 2, 8, 3]
Output:
[1, 0, 7, 2, 8, 4, 8, 2, 9, 3]

Sample Solution:

Python Code:

# Define a function named 'test' that takes a list of numbers 'nums' as a parameter
def test(nums):
    # Create a copy of the original list to avoid modifying the input list
    li = nums.copy()    
    
    # Iterate through the elements of the list with even indices
    for i in range(len(li)):
        # Check if the current index is even
        if i % 2 == 0: 
            # Iterate through the elements with even indices after the current index
            for j in range(i + 2, len(li), 2):
                # Check if the element at index 'j' is less than the element at index 'i'
                if li[j] < li[i]:
                    # Call the 'swap' function to swap elements at indices 'i' and 'j'
                    swap(li, i, j)
    
    # Return the modified list
    return li

# Define a function named 'swap' that swaps elements at indices 'i' and 'j' in the given list 'li'
def swap(li, i, j):
    # Temporary variable to store the value at index 'i'
    temp = li[i]
    # Swap the values at indices 'i' and 'j'
    li[i] = li[j]
    li[j] = temp

# Example 1
nums1 = [2, 5, 6, 3, 1, 4, 34]
print("Original list of numbers:")
print(nums1)
print("Keep every other element in place and otherwise sort the list:")
print(test(nums1))

# Example 2
nums2 = [8, 0, 7, 2, 9, 4, 1, 2, 8, 3]
print("\nOriginal list of numbers:")
print(nums2)
print("Keep every other element in place and otherwise sort the list:")
print(test(nums2))

Sample Output:

Original list (triple) of lists:
[2, 5, 6, 3, 1, 4, 34]
In the said list, keep every other element in place and otherwise sort the list.:
[1, 5, 2, 3, 6, 4, 34]

Original list (triple) of lists:
[8, 0, 7, 2, 9, 4, 1, 2, 8, 3]
In the said list, keep every other element in place and otherwise sort the list.:
[1, 0, 7, 2, 8, 4, 8, 2, 9, 3]

Flowchart:

For more Practice: Solve these Related Problems:

• Write a Python program to sort a list of integers while keeping the elements at even indices fixed in their original positions.
• Write a Python program to separate the elements at odd indices, sort them, and then reinsert them into the original list.
• Write a Python program to use slicing to extract elements that are not fixed, sort them, and merge with fixed elements.
• Write a Python program to implement an alternating sort that leaves every other element in place and sorts the remainder.

Python Code Editor :

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