Square-ish matrix
Input
transpose([[1,2,3],[4,5,6]])
Output
[[1, 4], [2, 5], [3, 6]]
Standard transpose: columns become rows.
Full lesson preview
Transpose a 2D list
Write a transpose function for 2D lists that handles regular and irregular rows by filling missing entries with None.
Python practice15 minList & Dictionary PatternsIntermediateLast updated March 20, 2026
Problem statement
Given a 2D list (a list of rows), return its transpose (columns become rows). Rows may have different lengths; when a row is missing a value for a transposed column, the corresponding position should be filled with None. An empty input should return an empty list.
Examples:
- [[1,2,3],[4,5,6]] -> [[1,4],[2,5],[3,6]]
- [[1,2],[3]] -> [[1,3],[2,None]]
Task
Implement a transpose function that returns the matrix transpose. If rows have unequal lengths, missing elements should be represented by None in the transposed result.
Examples
Input format
A list of lists representing rows (matrix).
Output format
A list of lists representing the transposed matrix; missing values padded with None.
Constraints
- Use only built-in Python modules. - Aim for O(r * c) where r = number of rows and c = length of the longest row.
Samples
Sample 1
Input
transpose([[1,2],[3]])
Output
[[1, 3], [2, None]]
Second row is shorter, so the missing entry in the second column is None.