• Status: Solved
  • Priority: Medium
  • Security: Public
  • Views: 201
  • Last Modified:

Problems with Fischer-Yates shuffle

int Array[3] = {0,1,2}

I want an algorithm that will shuffle the order of this array randomly, so that the following six combinations are all equally likely:
{0,1,2}
{0,2,1}
(1,0,2}
{1,2,0}
{2,0,1}
{2,1,0}

Tbe following Fischer-Yates shuffle I was using yields only the combinations {2,0,1} and {1,2,0}:

for (int l=1,l<3,l++)
{ int r = random(l);
  int temp = Array[l];
  Array[l] = Array[r];
  Array [r] = temp;
}

0
mm162
Asked:
mm162
1 Solution
 
mnewton022700Commented:
I am going to assume that random(i) returns a random greater than or equal to 0 and less than i. If this is not the case the following comment will not be correct.

Your current code will first swap Array[1] with Array[0] to produce {1,0,2} and then swap Array[2] with either Array[0] or Array[1] to produce either {2,0,1} or {1,2,0}.

Try this instead:

for (int i = 2, i > 0 ,i--)
{
  int r = random(i + 1);
  int temp = Array[i];
  Array[i] = Array[r];
  Array[r] = temp;
}

On the first iteration this will produce one of:
{0,1,2} {0,2,1} {2,1,0}

And on the second will produce one of:
{0,1,2} {1,0,2} {0,2,1} {2,0,1} {2,1,0} {1,2,0}

Let me know if my first assumption was wrong and I will fix the algorithm for you.
0
 
ozoCommented:
// or
for (int l=1,l<3,l++)
{ int r = random(l+1);
  int temp = Array[l];
  Array[l] = Array[r];
  Array [r] = temp;
}
0

Featured Post

Hire Technology Freelancers with Gigs

Work with freelancers specializing in everything from database administration to programming, who have proven themselves as experts in their field. Hire the best, collaborate easily, pay securely, and get projects done right.

Tackle projects and never again get stuck behind a technical roadblock.
Join Now