C ctype.h Functions

The ctype.h library has helpers that check what kind of character you have, such as a letter, digit, or space, and can change its case. It is like a sorter for characters.

In this page:

Checking for Alphabets
Checking for Digits
Checking for Alphanumeric
Case Conversions
Checking for Whitespace

Syntax

#include <ctype.h>

isalpha(character);
isdigit(character);
toupper(character);
tolower(character);

Checking for Alphabets

isalpha() checks whether a character falls in the ranges A-Z or a-z and returns a non-zero value if it does, which is the standard way to validate that user input contains only letters before processing it as a name or word rather than a number or symbol.

Example: Checking for Alphabets

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Include character-test functions (isdigit, toupper, ...)
#include <ctype.h>
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("%d", isalpha('A'));
	// Return 0 to signal that the program finished successfully
	return 0;
}

Checking for Digits

isdigit() checks whether a character is one of the decimal digits 0 through 9, which is useful for validating that a piece of input meant to represent a number doesn't actually contain letters or punctuation before you try to convert it with something like atoi().

Example: Checking for Digits

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Include character-test functions (isdigit, toupper, ...)
#include <ctype.h>
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("%d", isdigit('7'));
	// Return 0 to signal that the program finished successfully
	return 0;
}

Checking for Alphanumeric

isalnum() checks whether a character is either a letter or a digit, combining what isalpha() and isdigit() each check individually, which is convenient when validating things like usernames or identifiers that should allow both letters and numbers but reject punctuation.

Example: Checking for Alphanumeric

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Include character-test functions (isdigit, toupper, ...)
#include <ctype.h>
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("%d %d", isalnum('A'), isalnum('!'));
	// Return 0 to signal that the program finished successfully
	return 0;
}

Case Conversions

tolower() converts an uppercase letter to its lowercase equivalent and leaves any character that isn't an uppercase letter unchanged, while toupper() does the reverse conversion — both are the standard building blocks for case-insensitive string comparisons in C.

Example: Case Conversions

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Include character-test functions (isdigit, toupper, ...)
#include <ctype.h>
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("%c %c", tolower('A'), toupper('b'));
	// Return 0 to signal that the program finished successfully
	return 0;
}

Checking for Whitespace

isspace() checks whether a character is one of the recognized whitespace characters (space, tab, newline, carriage return, and a couple of others), which is commonly used while parsing text to skip over or trim leading and trailing blank characters.

Example: Checking for Whitespace

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Include character-test functions (isdigit, toupper, ...)
#include <ctype.h>
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("%d %d", isspace(' '), isspace('a'));
	// Return 0 to signal that the program finished successfully
	return 0;
}

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