how to pick the pair of numbers?

I have a=[1 2 3 4 5 ]. I want to pick any type of pair as [Eg: p1=1,p2=2 p3=3 p4=2 . . . . . .] I want to get 5 pairs,upto p10 but the restriction is any pair should not be like this p1=1 p2=1 or 22 or 33 or 44 or 55.and another restriction is if 1pair is like p1=1 p2=4 then another pair should not contain p3=4 p4=1.

Answer 1

[Joseph Cheng]

You can use the function nchoosek() (example: nchoosek(1:5,2))) so it has your array 1 through 5 and it'll supply the unique pairs.