how to generate random isotropic vectors using R?

Generate vectors using R

• I would like to ask,if some of You dont know any simple way to solve this kind of problem: I need to generate all combinations of A numbers taken from a set B (0,1,2...B), with their sum = C. ie if A=2, B=3, C=2: Solution in this case: (1,1);(0,2);(2,0) So the vectors are length 2 (A), sum of all its items is 2 (C), possible values for each of vectors elements come from the set {0,1,2,3} (maximum is B).

• Answer:

  z <- expand.grid(0:3,0:3) z[rowSums(z)==2, ] Var1 Var2 3 2 0 5 1 1 7 0 2 If you wanted to do the expand grid programmatically this would work: z <- expand.grid( rep( list(C), A) ) You need to expand as a list so that the items remain separate. rep(0:3, 3) would not return 3 separate sequences. So for A=3: > z <- expand.grid(rep(list(0:3), 3)) > z[rowSums(z)==2, ] Var1 Var2 Var3 3 2 0 0 6 1 1 0 9 0 2 0 18 1 0 1 21 0 1 1 33 0 0 2