Chapter 30Beginner

C ctype.h Library Reference

Complete reference for ctype.h - character classification (isalpha, isdigit) and conversion (toupper, tolower).

12 min readUpdated 2024-12-16
ctype.hisalphaisdigitisspacetouppertolower

What You Will Learn

  • ✓Check character types
  • ✓Convert character case
  • ✓Validate user input

01Introduction to ctype.h

🔤 What is ctype.h?

<ctype.h> provides functions to classify and convert characters. Perfect for input validation and text processing!

CategoryFunctions
Classificationisalpha, isdigit, isalnum, isspace, ispunct
Case Checkisupper, islower
Conversiontoupper, tolower
Otherisprint, isgraph, iscntrl, isxdigit

02Character Classification

FunctionPrototypeReturns True If
isalpha()int isalpha(int c)Letter (A-Z, a-z)
isdigit()int isdigit(int c)Digit (0-9)
isalnum()int isalnum(int c)Letter OR digit
isspace()int isspace(int c)Whitespace (space, tab, newline)
ispunct()int ispunct(int c)Punctuation (!@#$...)
isxdigit()int isxdigit(int c)Hex digit (0-9, A-F, a-f)
isprint()int isprint(int c)Printable (including space)
isgraph()int isgraph(int c)Printable (excluding space)
iscntrl()int iscntrl(int c)Control character
classification.c
C
1#include <stdio.h>
2#include <ctype.h>
3
4int main() {
5    char ch = 'A';
6    
7    // Test single character
8    printf("'%c' isalpha: %d\n", ch, isalpha(ch));  // 1 (true)
9    printf("'%c' isdigit: %d\n", ch, isdigit(ch));  // 0 (false)
10    printf("'%c' isalnum: %d\n", ch, isalnum(ch));  // 1 (true)
11    
12    // Test different characters
13    printf("'5' isdigit: %d\n", isdigit('5'));      // 1
14    printf("' ' isspace: %d\n", isspace(' '));      // 1
15    printf("'!' ispunct: %d\n", ispunct('!'));      // 1
16    printf("'F' isxdigit: %d\n", isxdigit('F'));    // 1
17    
18    return 0;
19}

💡 Return Values

These functions return non-zero (true) or 0 (false). Use them in conditions directly!

03Case Check and Conversion

FunctionPrototypeDescription
isupper()int isupper(int c)Check if uppercase (A-Z)
islower()int islower(int c)Check if lowercase (a-z)
toupper()int toupper(int c)Convert to uppercase
tolower()int tolower(int c)Convert to lowercase
case_functions.c
C
1#include <stdio.h>
2#include <ctype.h>
3
4int main() {
5    char ch = 'a';
6    
7    // Case checking
8    printf("'%c' isupper: %d\n", ch, isupper(ch));  // 0
9    printf("'%c' islower: %d\n", ch, islower(ch));  // 1
10    
11    // Case conversion
12    printf("toupper('a') = '%c'\n", toupper('a'));  // 'A'
13    printf("tolower('Z') = '%c'\n", tolower('Z'));  // 'z'
14    
15    // Non-letters are unchanged
16    printf("toupper('5') = '%c'\n", toupper('5'));  // '5'
17    printf("tolower('!') = '%c'\n", tolower('!'));  // '!'
18    
19    return 0;
20}

04Practical Examples

Convert String to Uppercase

main.c
C
void toUpperStr(char *str) {
    while (*str) {
        *str = toupper(*str);
        str++;
    }
}
int main() {
    char text[] = "Hello World!";
    toUpperStr(text);
    printf("%s\n", text);  // HELLO WORLD!
    return 0;
}

Count Letters and Digits

main.c
C
void countChars(const char *str) {
    int letters = 0, digits = 0, spaces = 0;
    
    while (*str) {
        if (isalpha(*str)) letters++;
        else if (isdigit(*str)) digits++;
        else if (isspace(*str)) spaces++;
        str++;
    }
    
    printf("Letters: %d, Digits: %d, Spaces: %d\n", 
           letters, digits, spaces);
}
int main() {
    countChars("Hello 123 World");
    // Output: Letters: 10, Digits: 3, Spaces: 2
    return 0;
}

Validate Alphanumeric Username

main.c
C
int isValidUsername(const char *username) {
    if (!*username) return 0;  // Empty string
    
    // First char must be letter
    if (!isalpha(*username)) return 0;
    
    // Rest must be alphanumeric
    while (*username) {
        if (!isalnum(*username)) return 0;
        username++;
    }
    return 1;
}
int main() {
    printf("user123: %s\n", isValidUsername("user123") ? "Valid" : "Invalid");
    printf("123user: %s\n", isValidUsername("123user") ? "Valid" : "Invalid");
    printf("user@123: %s\n", isValidUsername("user@123") ? "Valid" : "Invalid");
    // user123: Valid
    // 123user: Invalid
    // user@123: Invalid
    return 0;
}

Capitalize First Letter of Each Word

main.c
C
void capitalizeWords(char *str) {
    int newWord = 1;  // Start with new word
    
    while (*str) {
        if (isspace(*str)) {
            newWord = 1;
        } else if (newWord) {
            *str = toupper(*str);
            newWord = 0;
        }
        str++;
    }
}
int main() {
    char text[] = "hello world from c";
    capitalizeWords(text);
    printf("%s\n", text);  // Hello World From C
    return 0;
}

05Character Classification Chart

Charalphadigitalnumspacepunctupperlower
'A'✓✗✓✗✗✓✗
'a'✓✗✓✗✗✗✓
'5'✗✓✓✗✗✗✗
' '✗✗✗✓✗✗✗
'!'✗✗✗✗✓✗✗

06Summary

🎯 Key Functions

Classification:

isalpha, isdigit, isalnum, isspace, ispunct

Case Check:

isupper, islower

Conversion:

toupper, tolower

Other:

isprint, isxdigit, iscntrl

08Next Steps

Learn about date and time functions:

Next: time.h→Review: math.h