How to find permutation of k in a given length?

Question

How can I find the permutations of k in a given length?

For example:

The word cat has 3 letters: How can I find all the permutations of 2 in the

Answer 1

void Prem (char *str, int k, int length) {
    if (k == length-1){
         printf("%s\n",str);
         return;
    } else {
        for (int i = k ; i < length; ++i) {
            char t = str[k];
            str[k] = str[i];
            str[i] = t;
            Prem(str,k+1,length);
            t = str[k];
            str[k] = str[i];
            str[i] = t;
        }
    }
}