Characters in C

char_test.c

//Detect if a character is uppercase

int ALPHA_DIFF = 'a' - 'A';

//Determine if a character is uppercase, lowercase, or non-alphabetic.
int isUppercase(char a) {
  if (a >= 'a' && a <= 'z') {
    //a is lowercase
    return 0;
  } else if (a >= 'A' && a <= 'Z') {
    //a is uppercase
    return 1;
  } else {
    //a is not alphabetic
    return -1;
  }
}

//Convert a character to uppercase if it's already a lowercase.
char convertToUpper(char a) {
  if (isUppercase(a)) {
    return a; //Already uppercase
  } else if (isUppercase(a) == 0) {
    return a + ALPHA_DIFF;
  } else {
    return NULL;
  }
}

char convertLower(char a) {
   if (isUppercase(a)) {
    return a - ALPHA_DIFF;
  } else if (isUppercase(a) == 0) {
    return a; //Already lowercase
  } else {
    return NULL;
  }
}