Is there an easy way to generate combination k from n in PARI/GP?

Peter

Suppose I have a vector with n elements in PARI/GP.

I want to generate combination k from n elements.

For example, if the vector is [3,7,11,14,18] and k=3, the output should be

[3,7,11]
[3,7,14]
[3,7,18]
[3,11,14]
[3,11,18]
[3,14,18]
[7,11,14]
[7,11,18]
[7,14,18]
[11,14,18]

Is there a command in PARI/GP to do this or do I have to program the function?

Piotr Semenov

Unfortunately, PARI doesn't have built-in commands to do all the work you need.

Functions forvec(with flag=2) are great for combinatorial problems. So your function can be as follows:

subsets(A, k) = {
   my (lst = List());
   forvec(v = vector(k, i, [1, #A]), listput(lst, vecextract(A, v)), 2);

   Vec(lst)
};

subsets([3,7,11,14,18], 3)
gp> [[3,7,11], [3,7,14], [3,7,18], [3,11,14], [3,11,18], [3,14,18], [7,11,14], [7,11,18],
     [7,14,18], [11,14,18]]

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