Example: Letter to Phone Number Converter (if-else if) in C

// Letter to Phone Number Converter (if-else if)
#include <stdio.h>
#include <string.h> // For strlen()
#include <ctype.h>  // For toupper()

void convertLettersToPhoneNumber(char *str) {
    for (int i = 0; str[i] != '\0'; i++) {
        char ch = toupper(str[i]); // Convert to uppercase to handle both cases

        if (ch >= 'A' && ch <= 'C') {
            str[i] = '2';
        } else if (ch >= 'D' && ch <= 'F') {
            str[i] = '3';
        } else if (ch >= 'G' && ch <= 'I') {
            str[i] = '4';
        } else if (ch >= 'J' && ch <= 'L') {
            str[i] = '5';
        } else if (ch >= 'M' && ch <= 'O') {
            str[i] = '6';
        } else if (ch >= 'P' && ch <= 'S') { // P, Q, R, S
            str[i] = '7';
        } else if (ch >= 'T' && ch <= 'V') {
            str[i] = '8';
        } else if (ch >= 'W' && ch <= 'Z') { // W, X, Y, Z
            str[i] = '9';
        }
        // If it's a digit or another character, it remains unchanged
    }
}

int main() {
    // Step 1: Declare character arrays for input and output.
    char phoneNumber1[50] = "1-800-COLLECT";
    char phoneNumber2[50] = "HELLO-WORLD";
    char phoneNumber3[50] = "MyNumberIs123";

    // Step 2: Print original strings.
    printf("Original: %s\n", phoneNumber1);
    printf("Original: %s\n", phoneNumber2);
    printf("Original: %s\n", phoneNumber3);

    // Step 3: Convert letters to phone number digits.
    convertLettersToPhoneNumber(phoneNumber1);
    convertLettersToPhoneNumber(phoneNumber2);
    convertLettersToPhoneNumber(phoneNumber3);

    // Step 4: Print converted strings.
    printf("Converted: %s\n\n", phoneNumber1);
    printf("Converted: %s\n\n", phoneNumber2);
    printf("Converted: %s\n\n", phoneNumber3);

    return 0;
}