Working with Characters: ctype.h

The toolbox <ctype.h> has handy functions for single characters:

The is... functions return a non-zero number for true and 0 for false, so you use them in if:

#include <stdio.h>
#include <ctype.h>

int main() {
    char text[40] = "Room 42, Floor 3!";
    int letters = 0;
    int digits = 0;
    for (int i = 0; text[i] != '\0'; i++) {
        if (isalpha(text[i])) {
            letters++;
        } else if (isdigit(text[i])) {
            digits++;
        }
    }
    printf("Letters: %d, Digits: %d\n", letters, digits);
    return 0;
}

Output

Letters: 9, Digits: 3

Shouting: make everything capital

#include <stdio.h>
#include <ctype.h>

int main() {
    char msg[30] = "please hurry";
    for (int i = 0; msg[i] != '\0'; i++) {
        msg[i] = toupper(msg[i]);
    }
    printf("%s\n", msg);
    return 0;
}

Output

PLEASE HURRY

Characters are numbers

Remember that a char is stored as a number. The letters 'a' to 'z' come one after another, and so do '0' to '9'. That lets you do maths with them:

#include <stdio.h>

int main() {
    char digit = '7';
    int value = digit - '0';      // turns the character '7' into the number 7
    printf("%d\n", value * 2);

    char letter = 'c';
    printf("%c\n", letter + 1);   // the next letter
    return 0;
}

Output

14
d

Tip: c - '0' is the classic trick to turn a digit character into its number.