Determine if a word is an isogram, array subscription error

Question

I wrote a function that determines if a word is an isogram or not. I am close to the end but I have got few errors which I am not able to understand, because one of my compiler understands it but another doesn't.

The errors are:

solution.c:23:9: warning: array subscript is of type 'char' [-Wchar-subscripts]
        isogram[c]=isogram[c]+1;
               ^~
solution.c:23:20: warning: array subscript is of type 'char' [-Wchar-subscripts]
        isogram[c]=isogram[c]+1;
                      ^~
2 warnings generated.

And the code:

bool IsIsogram(char *str)
{
  int isogram[256];
  int i,len_str,cpt,occs;
  char c;

  for (i = 0; str[i] != '\0'; i++) {
      len_str=i;
  }

  for (i=0; i<=256;i++){isogram[i]=0;}
  for(i=0; i <= len_str; i++){
      c = str[i];
      c = tolower(c);
      str[i] = c;
  }
 cpt=0;
 for (i = 0; i < len_str; i++) {
    c = str[i];
    isogram[c]=isogram[c]+1;
  }

  for (i = 0; i < 256; i++) {
  occs = isogram[i];
  if (occs > 0) {
    cpt++;
  }
  }
  if (cpt==len_str) {
      return true;
  }
  else{
  return false;
  }
}

Answer 1

Type char is a bad decision to subscript an array. It can be signed or unsigned. You can fix the warning using unsigned char. You should also change the range of your for loop.

bool IsIsogram(char *str)
{
  int isogram[256];
  int i,len_str,cpt,occs;
  unsigned char c;

  for (i = 0; str[i] != '\0'; i++) {
      len_str=i;
  }

  for (i=0; i<256;i++){isogram[i]=0;} // i <=256 goes out of bounds
  for(i=0; i <= len_str; i++){
      c = str[i];
      c = tolower(c);
      str[i] = c;
  }
 cpt=0;
 for (i = 0; i < len_str; i++) {
    c = str[i];
    isogram[c]=isogram[c]+1;
  }

  for (i = 0; i < 256; i++) {
  occs = isogram[i];
  if (occs > 0) {
    cpt++;
  }
  }
  if (cpt==len_str) {
      return true;
  }
  else{
  return false;
  }
}