Question

How to use the isalnum function to check if a character is alphanumeric in C

Answer 1

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

/**
 * Checks whether a given character is alphanumeric.
 * 
 * @param c The character to evaluate.
 * @return Non-zero (true) if alphanumeric, 0 (false) otherwise.
 */
int check_alphanumeric(char c) {
    // Cast to unsigned char to avoid undefined behavior with signed chars
    return isalnum((unsigned char)c) != 0;
}

int main(void) {
    // Test cases containing letters, digits, and special characters
    char test_characters[] = {'G', '5', 'a', '#', ' ', '\n'};
    int total_chars = sizeof(test_characters) / sizeof(test_characters[0]);

    for (int i = 0; i < total_chars; i++) {
        char current = test_characters[i];
        
        // Using our custom wrapper function
        if (check_alphanumeric(current)) {
            printf("Character '%c' (ASCII: %d) is alphanumeric.\n", current, current);
        } else {
            printf("Character '%c' (ASCII: %d) is NOT alphanumeric.\n", current, current);
        }
    }

    return 0;
}


/*
run:

Character 'G' (ASCII: 71) is alphanumeric.
Character '5' (ASCII: 53) is alphanumeric.
Character 'a' (ASCII: 97) is alphanumeric.
Character '#' (ASCII: 35) is NOT alphanumeric.
Character ' ' (ASCII: 32) is NOT alphanumeric.
Character '
' (ASCII: 10) is NOT alphanumeric.

*/

 

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