How do I add items to a C# array so that I have exactly 3 of each item?

Question

I have an int array[] with a length of 12, and i want to fill it with numbers from 0 to 3 randomly, but i want to make sure that there are exactly three of 0, 1, 2, and 3 in the array. Any ideas on how to do this?

What are you actually trying to accomplish with the results of this array? — Thomas, Mar 23 '11 at 23:59
Set the colors of textboxes. [0 is red, 1 is green, 2 is blue, 3 is black...] It could probably be more semantic, but this project is pretty hacky anyway. — awesomeguy, Mar 24 '11 at 00:05

Jim Mischel · Accepted Answer · 2011-03-24T04:17:16.413

5

Fill it non-randomly and then shuffle:

int[] myArray = new int(12);
for (i = 0; i < 12; ++i)
{
    myArray[i] = i/3;
}

Random rnd = new Random();
for (i = 0; i < 12; ++i)
{
    //int swapWith = rnd.Next(12);
    // Corrected to avoid bias.  See comments.
    int swapWith = rnd.Next(i+1);
    int temp = myArray[i];
    myArray[i] = myArray[swapWith];
    myArray[swapWith] = temp;
}

edited Mar 24 '11 at 04:17

answered Mar 23 '11 at 23:59

Jim Mischel

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+1 other answers are correct too, but you wrote the code something like 45 seconds – Joel Lee Mar 24 '11 at 00:03
1

+1 for the principle, but your shuffle implementation isn't a true Fisher-Yates-Durstenfeld shuffle and will exhibit some bias: See the second paragraph of the "Implementation errors" section of http://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle#Implementation_errors and also http://www.codinghorror.com/blog/2007/12/the-danger-of-naivete.html. – LukeH Mar 24 '11 at 02:04
@LukeH: Thanks. I typed that in from (faulty) memory. Fixed now. – Jim Mischel Mar 24 '11 at 04:17

score 2 · Answer 2 · answered Mar 23 '11 at 23:58

2

You can start with an ordered array (such as 0,0,0,1,1,1... etc.) and do a shuffle, like shuffling cards. Go through each index and swap the contents with the contents of another random one.

answered Mar 23 '11 at 23:58

AlbeyAmakiir

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score 1 · Answer 3 · answered Mar 24 '11 at 00:13

Several of the other answers here suggest simply swapping randomly selected elements. That won't yield truly random results. See here for the details why and a better way of randomly sorting: http://www.codinghorror.com/blog/2007/12/the-danger-of-naivete.html

score 0 · Answer 4 · edited May 23 '17 at 11:47

0

Fill the array and then shuffle the numbers. How to do shuffling you can find here: Randomize a List<T>

edited May 23 '17 at 11:47

Community

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answered Mar 24 '11 at 00:01

ChrisWue

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score 0 · Answer 5 · answered Mar 24 '11 at 00:15

0

Random rnd=new Random();
var array = new[] {0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3}
                           .OrderBy(i => rnd.Next() )
                           .ToArray();

answered Mar 24 '11 at 00:15

Bala R

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score 0 · Answer 6 · answered Mar 24 '11 at 00:26

0

Here's yet one more way...

        var rnd = new Random();
        var items = Enumerable.Repeat(Enumerable.Range(0, 4), 3).SelectMany(item => item).OrderBy(item => rnd.Next()).ToArray();

answered Mar 24 '11 at 00:26

Rudy

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How do I add items to a C# array so that I have exactly 3 of each item?

6 Answers6