Generate array of unique characters in C - c

Question

I'm trying to generate an array of 10 random but unique characters. Sometimes when I run the code, characters are not unique.
I would appreciate any help. Thank you.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main() {
srand(time(NULL));
char letters[10];
for (int i = 0; i < 10; i++) {
letters[i] = 97 + rand() % (122-97);
for (int j = 1; j < i; j++) {
if (letters[i]==letters[j]) {
letters[i] = 97 + rand() % (122-97); // continue
}
}
printf("%c\n", letters[i]);
}
}

Answer 1

The problem is that random can give the same number. It doesn't know you want a different number every time.
This is how to solve this, I am telling you the way, you will need to program it.
Decide what characters are participating, then create an array that holds these characters.
Then roll the dice to give a number within the array range. The number you get is an index to the array, you then take the character in that index.
Then you take the last characters in the array and put in that index, and the next time you roll the dice you limit the max number to one less the original size of the array.
You continue with this algorithm until you get all the required amount of characters.
This algorithm ensures that you get different characters.

Answer 2

Your code checks only if the character you just generated is different from its predecessor in your array ; you should try with a flag like that :
srand(time(NULL));
char letters[10];
int is_unique = 1, i=0,j ;
while (i<10){
is_unique = true;
letters[i] = 'a' + rand() % 26;
for(j=0;j<i;j++){
if (letters[i]==letters[j])
is_unique = false;
}
if (is_unique)
i++;
}
BTW, it's better to replace rand() % (122-97) by simply 26, it's more clear for anyone reading your code.
Another way would be :
srand(time(NULL));
char letters[10];
int j;
for (int i = 0; i < 10; i++)
{
letters[i] = 'a' + rand() % 26;
j=0;
while (j<i) {
if (letters[i]==letters[j]){
letters[i] = 'a' + rand() % 26;
j = 1;
} else
j++;
}
printf("%c\n", letters[i]);
}