Tokenise a string in C by whitespaces.

tokenise-string.c

#include <stdio.h>
#include <string.h>

// Check if a character is a digit
char is_digit(char curr) {
    return (curr >= '0' && curr <= '9');
}

// Check if a character is a letter
char is_letter(char curr) {
    return ((curr >= 'A' && curr <= 'Z') || (curr >= 'a' && curr <= 'z'));
}

// Check if a character is a whitespace
char is_whitespace(char curr) {
    return (curr == ' ' || curr == '\n' || curr == '\r' || curr == '\t');
}

// tokenise and print a string
void tokenise_string(char* string) {
    char token[128] = "\0";
    unsigned long int token_length = 0;
  
    // Loop over the string character-by-character
    for (unsigned long int i = 0; i < strlen(string); ++i) {
        // If the current character is a letter
        if (is_letter(string[i])) {
            // And if it's the first character in a token, print "IDENT: "
            if (token_length == 0) { printf("IDENT: "); }
            
            // Append the character to the token and increment the token's length
            token[token_length++] = string[i];
        }
        
        // If the current character is a digit
        if (is_digit(string[i])) {
            // And it's the first character in a token, print "NUMBER: ");
            if (token_length == 0) { printf("NUMBER: "); }
            
            // Append the character to the token and increment the token's length
            token[token_length++] = string[i];
        }
        
        // If the current character is a whitespace
        if (is_whitespace(string[i])) {
            // And the token's length is not 0
            if (token_length != 0) {
                // Add a NULL character to the token to close the string
                token[token_length++] = '\0';
                
                // Print the token
                printf("%s\n", token);
            }
            
            // NULL-ify the token
            token[0] = '\0';
            
            // Reset the token's length to 0
            token_length = 0;
        }
    }
    
    // If the token's length is not 0, there's still a token that needs to be printed, so let's do that.
    if (token_length != 0) {
        // Add a NULL character to finalise the string
        token[token_length++] = '\0';
        
        // Print the token
        printf("%s\n", token);
        
        // NULL-ify the token
        token[0] = '\0';
        
        // Reset the token's length to 0
        token_length = 0;
    }
}

int main() {
    char* string = "5 plus 6 equals 11";
    printf("Splitting string:\n%s\n", string);
    tokenise_string(string);
    return 0;
}