Java array question..

Question

I have a 3x4 array that I want to rotate left once, so that it becomes a 4x3. Imagine a box of values, and just rotate that box left. Here's the function I wrote:

public static int[][] rotLeft(int[][] source) {
    int[][] newImage=new int[source[0].length][source.length];
    for (int i=0;i<source.length;i++)
        for (int j=0;j<source[0].length;j++)
            newImage[source.length-j][i]=source[i][j];
        return newImage;
}

This code should be working, as it passes my handwritten test (when I run it on paper), but when I try to run it through the grader that my teacher wrote, it fails the test. Whats wrong with this code?

Answer 1

Matt Ball is absolutely correct, writing your own tests is the way to go. Many different tests.

One problem I see is that you have confusion between length and width in source.length-j expression. I.e., j is the second index of array and may be > source.length. Run it on 1x3 matrix and you'll get some very funny error.

Answer 2

I do think you shouldn't test code without a computer. However to fix it, these simple steps might help. Try naming i and j, something like x and y and call the bounds width and height. Are the bounds always satisfied? And off course x only goes with a width in an expression.