Substitute string

#include <stdio.h>

int main(void) {
  char str[100] = "Hello, world!";
  char sub = 'l', rep = 'k';

  for(size_t i = 0; str[i] != '\0'; i++) {
    str[i] = str[i] == sub ? rep : str[i];
  }

  printf("%s", str);
  return 0;
}