C Academy · Lesson

The ctype Library

isdigit, isalpha, toupper.

Lesson 2 of 413 steps

The ctype Library is a free C Academy lesson on CoddyKit. This is lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C Academy learning path, and your progress syncs across the web and the CoddyKit app. The C Academy course includes 4 lessons in total.

What Is ctype.h

The <ctype.h> header provides functions to classify and convert characters.

It saves you from memorizing ASCII ranges and writing manual comparisons.

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

int main(void) {
    printf("Is 'A' alpha? %d\n", isalpha('A'));
    return 0;
}

isdigit

isdigit(c) returns a nonzero value if c is a digit character '0'-'9', and 0 otherwise.

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

int main(void) {
    printf("isdigit('7') = %d\n", isdigit('7'));
    printf("isdigit('x') = %d\n", isdigit('x'));
    return 0;
}

isalpha

isalpha(c) returns nonzero if c is a letter (either uppercase or lowercase).

Digits and symbols return 0.

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

int main(void) {
    printf("isalpha('G') = %d\n", isalpha('G'));
    printf("isalpha('3') = %d\n", isalpha('3'));
    return 0;
}

isalnum and isspace

isalnum(c) is true for letters or digits. isspace(c) is true for whitespace such as space, tab, and newline.

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

int main(void) {
    printf("isalnum('9') = %d\n", isalnum('9'));
    printf("isspace(' ') = %d\n", isspace(' '));
    printf("isspace('a') = %d\n", isspace('a'));
    return 0;
}

isupper and islower

isupper(c) tests for uppercase letters, islower(c) for lowercase.

These are handy before deciding whether to convert case.

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

int main(void) {
    printf("isupper('A') = %d\n", isupper('A'));
    printf("islower('A') = %d\n", islower('A'));
    return 0;
}

ispunct

ispunct(c) returns nonzero for printable punctuation characters like !, ?, and , that are not letters, digits, or spaces.

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

int main(void) {
    printf("ispunct('!') = %d\n", ispunct('!'));
    printf("ispunct('a') = %d\n", ispunct('a'));
    return 0;
}

toupper

toupper(c) returns the uppercase version of a letter. If c is not a lowercase letter, it returns c unchanged.

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

int main(void) {
    printf("%c\n", toupper('h'));
    printf("%c\n", toupper('5'));
    return 0;
}

tolower

tolower(c) returns the lowercase version of an uppercase letter, or the character unchanged if it is not uppercase.

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

int main(void) {
    printf("%c\n", tolower('H'));
    printf("%c\n", tolower('z'));
    return 0;
}

Classifying a String

Loop over a string and use ctype functions to count digits and letters.

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

int main(void) {
    char *s = "abc123";
    int letters = 0, digits = 0;
    for (int i = 0; s[i]; i++) {
        if (isalpha(s[i])) letters++;
        if (isdigit(s[i])) digits++;
    }
    printf("letters=%d digits=%d\n", letters, digits);
    return 0;
}

Uppercasing a Whole String

Combine toupper with a loop to convert an entire string to uppercase, character by character.

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

int main(void) {
    char s[] = "Hello";
    for (int i = 0; s[i]; i++) {
        s[i] = toupper(s[i]);
    }
    printf("%s\n", s);
    return 0;
}

Pass int, Not Negative char

ctype functions take an int that must be representable as unsigned char or be EOF.

Passing a negative char directly is undefined behavior. Cast to unsigned char when in doubt.

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

int main(void) {
    char c = 'Q';
    int result = isalpha((unsigned char)c);
    printf("safe isalpha = %d\n", result);
    return 0;
}

Quick Check

Test your knowledge of the ctype library.

Answer it in the interactive lesson

Recap

The <ctype.h> library lets you:

Classify with isdigit, isalpha, isalnum, isspace, ispunct, isupper, islower.
Convert with toupper and tolower.
Always pass values cast to unsigned char to avoid undefined behavior.

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Frequently Asked Questions

Is the “The ctype Library” lesson free?

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isdigit, isalpha, toupper. You practise C Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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