Histogram of string in C

Question

I need to find how many times letters appear in string. But I need to ignore the difference between lowercase and uppercase letters.

#include <stdio.h>
#include <string.h>
void stringHistogram(char *s) {
  int i, len = 0, counts[256] = {0};
  len = strlen(s);
  for (i = 0; i < len; i++) {
    counts[(s[i])]++;
  }
  for (i = 65; i < 123; i++) {
      if(i>=91&&i<=96)continue;
    if (counts[i] > 0)
       printf("%c occurs %d times.\n", i, counts[i]);
  }
}
int main() {
  char s[] = "!!!!!PrOgraMMing -------iN C Is easy....";
  stringHistogram(s);

  return 0;
}

For example in this string "I" appears once and "i" appears twice. I want to add these two numbers and print number of them as one character. Program output should be "I appears 3 times." Could you help me modify these code?

Answer 1

You do not need to use strlen and then iterate the string again. Use the correct type for sizes.

void stringHistogram(const char *s) 
{
    size_t counts[256] = {0};

    while(*s) counts[toupper((unsigned char)*s++)] += 1;
    
    for(size_t i = 0; i < sizeof(counts) / sizeof(counts[0]); i++)
        if (counts[i] > 0)
            printf("%3zu (`%c`) occurs %zu times.\n", i, isprint((unsigned char)i) ? (char)i : '.', counts[i]);
}