Unshuffled string is different from original string after shuffling and unshuffling - c

Question

I'm actually implementing a shuffling function with an un-shuffling function. I'm using the Fisher-Yates algorithm with pseudo random number generator with a fixed seed. The unshuffled string doesn't look like the initial string, however.
I've checked if my random array is the same in the shuffling function and the un-shuffling function. The unshuffling function is the same as the shuffling function but in reverse.
Here is my shuffling function :
void shuffle(char * phrase)
{
int size_phrase = strlen(phrase);
srand(seed);
int * rdm_array = (int*)malloc(sizeof(int)*size_phrase);
int i;
for(i = 0; i < size_phrase; i++)
{
rdm_array[i] = rand()%size_phrase;
//printf("%d", rdm_array[i]);
}
//begin shuffle here
int j;
int k = 0;
for(j = size_phrase -1 ; j > 0 ; j-- , k++)
{
int rdm_nb = rdm_array[k];
char temp = phrase[j];
phrase[j] = phrase[rdm_nb];
phrase[rdm_nb] = temp;
}
free(rdm_array);
}
And here is my unshuffling function :
void unshuffle(char * phrase)
{
int size_phrase = strlen(phrase);
srand(seed);
int * rdm_array = (int*)malloc(sizeof(int)*size_phrase);
int i;
for(i = 0; i < size_phrase; i++)
{
rdm_array[i] = rand()%size_phrase;
//printf("%i", rdm_array[i]);
}
//On commence le mélange ici
int j;
int k = size_phrase-1;
for(j = 0 ; j < size_phrase ; j++ , k--)
{
int rdm_nb = rdm_array[k];
char temp = phrase[j];
phrase[j] = phrase[rdm_nb];
phrase[rdm_nb] = temp;
}
free(rdm_array);
}
And here is my Output :
It looks like it's missing one loop or something like that.

Answer 1

Add a printf to show what gets exchanged in both shuffle and unshuffle like this
/* after this line */
int rdm_nb = rdm_array[k];
/* insert debug output */
printf("%d <-> %d\n", j, rdm_nb);
You will see that in function shuffle variable j counts from size_phrase -1 to 1 while in unshuffle it counts from 0 to size_phrase -1.
Probably you should change the for loop in shuffle to
for(j = size_phrase -1 ; j >= 0 ; j-- , k++)