String manipulation (variable output with the same input) - c

Question

Maybe some property is passing unnoticed to me, but when 'i' is 1 it just freeze. When i input whatever string, 'j' variable goes to 700 or 2000 in different executions. The code goal is to output repetitive letters if you input "cheese" the output should be "eee". What am i doing wrong?
#include <stdio.h>
char * repeticoes(char *s) {
int index = 0;
for (int i = 0;( s[i] != '\0'); i++) //problem starts when i is > 0
{
for (int j = 0; ( s[j] != '\0'); j++)
{
if (s[i] == s[j])
{
printf("%c == %c\ni %d j %d\n", s[i], s[j],i,j);
s[index++] = s[i];
}
else
{
printf("not happening %c != %c\ni %d j %d\n", s[i],s[j],i,j);
}
}
}
s[++index] = '\0';
return s;
}
main() {
char input[21];
printf("str 1\n");
fgets(input, 20, stdin);
repeticoes(input);
printf("duplicated letters %s\n", input);
}

Answer 1

You need to start the inner loop at the next character after the one being processed in the outer loop, otherwise you'll process the same pair of characters twice, as well as testing a character against itself when i == j.
You should also break out of the inner loop as soon as you find a match. You'll find later matches in a future iteration of the outer loop. Otherwise you'll process the same pair twice again.
And you shouldn't increment index before assigning the null character after the loop. It was already incremented when adding the repetition.
#include <stdio.h>
char * repeticoes(char *s) {
int index = 0;
for (int i = 0;( s[i] != '\0'); i++) //problem starts when i is > 0
{
for (int j = i+1; ( s[j] != '\0'); j++)
{
if (s[i] == s[j])
{
printf("%c == %c\ni %d j %d\n", s[i], s[j],i,j);
s[index++] = s[i];
break;
}
else
{
printf("not happening %c != %c\ni %d j %d\n", s[i],s[j],i,j);
}
}
}
s[index] = '\0';
return s;
}
int main() {
char input[21];
printf("str 1\n");
fgets(input, 20, stdin);
repeticoes(input);
printf("duplicated letters %s\n", input);
}

Answer 2

With an array of letters to count and an array of counts. Once the letters have been counted, loop through the input again and set the letters that have a count greater than one.
#include <stdio.h>
void repeticoes ( char *s) {
char tocount[] = "abcdefghijklmnopqrstuvwxyz";
size_t len = sizeof tocount;
size_t index = 0;
int count[len];
for (int j = 0; j < len; j++) {
count[j] = 0;
}
for (int i = 0;( s[i] != '\0'); i++) {
for (int j = 0; j < len; j++) {
if ( s[i] == tocount[j]) {
count[j]++;
}
}
}
for (int i = 0;( s[i] != '\0'); i++) {
for (int j = 0; j < len; j++) {
if ( s[i] == tocount[j] && 1 < count[j]) {
s[index] = s[i];
index++;
}
}
}
s[index] = '\0';
}
int main ( void) {
char input[21];
printf("str 1\n");
fgets(input, 20, stdin);
repeticoes(input);
printf("duplicated letters %s\n", input);
return 0;
}