Permute a grouped vector

Function RANDPERG extends Matlab's RANDPERM by considering permutations of a grouped vector: if x is an n*1 data vector, and c is an n*1 vector of group assignments - so that x(i) belongs to group c(i) - randperg(x,c) returns an n*1 vector y where each c group retains the indexes it had in x [permutation is thus 'partial'], but its individual elements are randomly rearranged. See the function's help header for details.