Shuffle an ArrayList

Date Published: 27 January 2005

Shuffle an ArrayList

I have a need to randomly order the contents of a collection which has as its underlying container an ArrayList. Googling took more effort than I expected to come up with anything useful, but did yield this link. Working from there, I came up with this method, which differs in a few ways:

public void ShuffleInPlace()
{
ArrayList source = this.InnerList;
Random rnd = new Random();
for (int inx = source.Count-1; inx > 0; inx–)
{
int position = rnd.Next(inx+1);
object temp = source[inx];
source[inx] = source[position];
source[position] = temp;
}
}

The main improvements are these:

  1. The original used ArrayList.Length which wasn’t defined, so I converted to ArrayList.Count.
  2. The original was a static and took an ArrayList as a parameter (named source) — I refactored it to work as an instance method within the collection.
  3. The original had a bug in it where it generated the random number. I corrected it by adding +1 to the rnd.Next() call. You can reproduce the bug by shuffling a 3-element array with the original algorithm. The item that begins in position 2 will never be in position 2 after shuffling (and I think the same is true for positions 1 and 3, but now that I’ve fixed it I don’t care enough to verify that).

Steve Smith

About Ardalis

Software Engineer

Steve is an experienced software architect and trainer, focusing currently on ASP.NET Core and Domain-Driven Design.

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