Programming Praxis

Today’s exercise appears regularly on lists of interview questions. We’ve done something similar in the past, but since it’s so common we’ll do it again.

Given a matrix, print a list of elements of the matrix. Start in the top-right corner of the matrix, move left across the top row, then down the left column, then across the bottom row, then up the right column to the element below the top row, then left, then down, then right, and so on, collecting the elements of the matrix as it goes. For instance, with this matrix

     1  2  3  4
     5  6  7  8
     9 10 11 12
    13 14 15 16
    17 18 19 20

the elements are collected in order 4, 3, 2, 1, 5, 9, 13, 17, 18, 19, 20, 16, 12, 8, 7, 6, 10, 14, 15, 11.

Your task is to write a program to unwrap the elements of a matrix in the indicated spiral. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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