ctype.h in C

Example

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


int main() {
    char c = 'A';
    
    if (isalpha(c)) {
        printf("%c is an alphabet.\n", c);
    }
    
    if (isdigit(c)) {
        printf("%c is a digit.\n", c);
    }
    
    if (islower(c)) {
        printf("%c is lowercase.\n", c);
    }
    
    if (isupper(c)) {
        printf("%c is uppercase.\n", c);
    }
    
    char lowercaseChar = tolower(c);
    printf("Lowercase of %c is %c.\n", c, lowercaseChar);
    
    return 0;
}

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Output

A is an alphabet.
A is uppercase.
Lowercase of A is a.

In this example, we include the <ctype.h> header file & use various functions to check the properties of the character 'A'. We use `isalpha()` to check if it's an alphabet, `isdigit()` to check if it's a digit, `islower()` & `isupper()` to check its case, and `tolower()` to convert it to lowercase.

Now, we will discuss the functions which we discussed above in detail like how they are different and their return value: 

Let’s look at a few more examples using these functions: 

1: If you want to check if a user input is a valid digit, you can use the isdigit() function:

char input = getchar();

if (isdigit(input)) {
    printf("Valid digit entered.\n");
} else {
    printf("Invalid input. Please enter a digit.\n");
}

2: Similarly, you can use tolower() & toupper() functions to convert characters between lowercase & uppercase:

char c = 'H';
char lowercaseChar = tolower(c);
char uppercaseChar = toupper(lowercaseChar);

printf("Original: %c, Lowercase: %c, Uppercase: %c\n", c, lowercaseChar, uppercaseChar);

Output:

Original: H, Lowercase: h, Uppercase: H

Frequently Asked Questions

Are the functions in <ctype.h> case-sensitive?

Yes, the functions in <ctype.h> are case-sensitive. For example, 'a' is considered lowercase, while 'A' is considered uppercase.

Can the <ctype.h> functions be used with non-ASCII characters?

The behavior of <ctype.h> functions with non-ASCII characters is implementation-defined & may vary across different platforms.

Are the <ctype.h> functions affected by the current locale settings?

Yes, some of the <ctype.h> functions, such as isalpha() & ispunct(), are affected by the current locale settings.

Conclusion

In this article, we discussed the <ctype.h> header file in C and its various functions for testing & manipulating characters. We learned about commonly used functions like isalpha(), isdigit(), tolower(), & toupper(), along with their descriptions, return values with few examples also.

You can also check out our other blogs on Code360.