Randomize an array while leaving specific elements in place?

Question

Quick question, something in my logic here isn't adding up. Basically I want to shuffle or randomize an array, but keep specific elements in place. Anything over 900 should stay in the same position. This looks like it should work, but apparently I'm missing something.

public static int[] ShuffleArray(int[] data)
{
    for (int t = 0; t < data.Length; t++)
    {
        int tmp = data[t];
        if (tmp < 900)
        {
            int r;
            do
            {
                r = (int)RandomBetween(t, data.Length);
            } while (r > 900);

            data[t] = data[r];
            data[r] = tmp;
        }
    }

    return data;
}

Inputting this:

0,0,1,1,2,2,3,
3,999,4,5,1,999,6,
0,0,1,999,2,2,3,
3,4,4,5,5,6,6,
0,999,1,1,999,999,3,
3,4,4,5,5,6,6,
0,0,1,1,2,2,3,

Getting this:

3,5,6,2,6,1,0,
999,999,3,3,5,999,6,
2,4,999,1,999,2,4,
5,2,0,999,2,1,3,
0,1,5,6,2,0,4,
0,0,4,3,6,0,3,
1,1,5,4,3,1,1,

I thought I caught it with the <900 and the do while.. what am I missing here? The 999's need to stay in place.

Not knowing the language or libraries in use, can't verify that RandomBetween returns a value greater-than-or-equal to the first parameter and strictly less than the second, but you need it to. — Tony Delroy
– Tony Delroy, Commented May 2, 2015 at 6:43
Sorry about not tagging the C#.. apparently I'm more tired than I realized. — SquigglyFrog
– SquigglyFrog, Commented May 2, 2015 at 6:47

barak manos · Accepted Answer · 2015-05-02 06:33:53Z

2

Your logic is incorrect because you are using r as an index.

Change this:

while (r > 900)

To this:

while (data[r] > 900)

Please note that your algorithm by itself is extremely inefficient.

answered May 2, 2015 at 6:33

barak manos

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1 Comment

SquigglyFrog Over a year ago

Oh good gravy... I must be more tired than I thought.. thank you for pointing out the obvious.. Sorry about not tagging the language, didn't think it made much difference in this case.. C# for the record though.. I'm sure there's a lot that inefficient, this was mostly a first pass just to make it work.. I would love to see a more optimized version though if you have time! thanks!

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