Reorder a List by Odd Even Indices in Python

Splits a list into two sublists based on 1-based index parity, then concatenates odd-indexed elements before even-indexed ones.

Easy Python 3.9+ Aug 9, 2026 Algorithms & data structures 18 views 0 copies

list indices reorder algorithm slicing

Python code

16 lines

Python 3.9+

def reorder_by_odd_even(items):
    """Reorders a list so that elements at odd indices come first,
    followed by elements at even indices (1-based).
    
    Example: [0,1,2,3,4,5,6] -> [1,3,5,0,2,4,6]
    """
    odds = [items[i] for i in range(1, len(items), 2)]
    evens = [items[i] for i in range(0, len(items), 2)]
    return odds + evens


if __name__ == "__main__":
    original = [0, 1, 2, 3, 4, 5, 6]
    reordered = reorder_by_odd_even(original)
    print(f"Original: {original}")
    print(f"Reordered (odd indices first, then even): {reordered}")

Output

stdout

Original: [0, 1, 2, 3, 4, 5, 6]
Reordered (odd indices first, then even): [1, 3, 5, 0, 2, 4, 6]

How it works

This function uses two list comprehensions with range(start, len(items), 2) to pick every other element. The first comprehension starts at index 1, collecting all 1-based odd positions; the second starts at index 0, gathering even positions. Concatenating these sublists produces the desired pattern. This approach runs in O(n) time and uses O(n) extra space, making it efficient for most real-world list sizes.

Common mistakes

Confusing 0-based vs 1-based indexing — the pattern expects 1,3,5 first, not 0,2,4.
Using `items[1::2]` and `items[::2]` without realizing slice order still needs odds before evens.
Forgetting that empty lists or single-element lists return a different order than expected.

Variations

Use slicing: `items[1::2] + items[::2]` for a more concise one-liner.
In-place reorder using a loop and swapping elements to avoid extra memory.

Real-world use cases

Rearranging display order in a two-column card layout so natural reading order maps correctly.
Merging two interleaved data streams (e.g., alternate sensor readings) back into grouped blocks.
Preprocessing audio or signal samples where alternating channels need regrouping before analysis.

Reorder a List by Odd Even Indices in Python

Python code

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How it works

Common mistakes

Variations

Real-world use cases

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