Storing strings in an array - c

Question

The code given below prints all the possible combination of a given string. It produces strings recursively . Now i want to store each combination in an array using an array of pointer to each string . How do i initialize the pointer so that it points to string.
The code is :-
Input ABC
Output
ABC in b[0]
ACB in b[1]
BAC
BCA
CAB
CBA
and so on .
thanks :)
void permute(char *a, int i, int n)
{
int k=0;
char *b[100];
int j;
if (i == n)
{
// *b[k]=a;
printf("%s\n", a);
i++;
}
else
{
for (j = i; j <= n; j++)
{
swap((a+i), (a+j));
permute(a, i+1, n);
swap((a+i), (a+j)); //backtrack
}
}
}

Answer 1

There is no point in pointing elements of b to a, since is a variable string (that is, it keeps changing). Possible output of such code would be that all elements of b would be last permutation of the string a.
You need dynamic allocation of string each time you find a new permutation.
That you can do manually using malloc(). Or can use strdup() to create duplicate strings for you (Remember to free() them at the end ofcourse).
/*Sample Code*/
if (i == n)
{
b[k] = strdup(a);
...
}
Remember, that you also need to pass k as the argument to the function permute(), since k is an automatic variable, newly created with value = 0, each time function permute() is called. There are other possibilities, make k global or static variable.

Answer 2

You could dynamically allocate an array that would hold individual char arrays (or C strings) representing each permutation. One of the things that would make this code generic is to find the value of total_permutations in the main() for a given string with strlen N, which would actually be factorial(N). Here:
void swap(char* A, char* B) {
char t;
t = *A;
*A = *B;
*B = t;
}
int permute(char **arr_of_chars, int count, char *a, int i, int n)
{
int k=0;
char *b[100];
int j;
if (i == n) {
// *b[k]=a;
printf("%s\n", a);
memcpy(arr_of_chars[count], a, strlen(a));
count++;
i++;
} else {
for (j = i; j <= n; j++) {
swap((a+i), (a+j));
count = permute(arr_of_chars, count, a, i+1, n);
swap((a+i), (a+j)); //backtrack
}
}
return count;
}
int main() {
char str[] = "";
char **arr_of_str = NULL;
int len_str = strlen(str);
int i = len_str;
int total_permutations = 1;
while (i > 0) { /* Get all the combinations */
total_permutations *= i;
i--;
}
arr_of_str = (char **) malloc(total_permutations * sizeof(char*));
for (i=0; i <total_permutations; i++) {
arr_of_str[i] = (char *) malloc(sizeof(char) * len_str);
}
permute(arr_of_str, 0, str, 0, (len_str-1));
for (i=0; i <total_permutations; i++) {
printf("%s \n", arr_of_str[i]);
free(arr_of_str[i]);
}
free(arr_of_str);
}