How do you get all pairs without double counting?

Given, say, 10 numbers, how do you get all possible pairs between the numbers without double counting? Order doesn't matter, so 1-3 is the same as 3-1. How would you do this with for loops vs. vectorized? Is there a difference in performance between the two?

Answer 1

[Voss]

Here's one way:

ten_numbers = 1:10;
all_pairs = nchoosek(ten_numbers,2);

disp(all_pairs);

     1     2
     1     3
     1     4
     1     5
     1     6
     1     7
     1     8
     1     9
     1    10
     2     3
     2     4
     2     5
     2     6
     2     7
     2     8
     2     9
     2    10
     3     4
     3     5
     3     6
     3     7
     3     8
     3     9
     3    10
     4     5
     4     6
     4     7
     4     8
     4     9
     4    10
     5     6
     5     7
     5     8
     5     9
     5    10
     6     7
     6     8
     6     9
     6    10
     7     8
     7     9
     7    10
     8     9
     8    10
     9    10