C Program To Convert Letters Into A Phone Number

In this article, you will learn how to write C programs that convert alphabetic letters into their corresponding digits, mimicking the layout of traditional phone keypads. This process is useful for various applications where textual input needs to be translated into a numeric format.

Problem Statement

The challenge lies in translating a given string of letters (and potentially other characters) into a phone number format, where each letter maps to a specific digit as found on a standard 12-button phone keypad. For example, 'A', 'B', 'C' map to '2'; 'D', 'E', 'F' map to '3', and so on. Non-alphabetic characters like hyphens or digits should remain unchanged.

Example

Let's consider the input string "HELLO-WORLD". The expected output, after conversion to a phone number, would be "43556-96753".

Background & Knowledge Prerequisites

To effectively understand this article, readers should have a basic understanding of:

• C Programming Basics: Variables, data types, input/output operations.
• Character Manipulation: Concepts of characters, ASCII values, and functions like toupper() or tolower().
• Conditional Statements: if-else if and switch statements for decision making.
• Loops: for or while loops for iterating through strings.
• Strings in C: How characters arrays are used to represent strings.

Use Cases or Case Studies

Converting letters into phone numbers has several practical applications:

• Vanity Phone Numbers: Allowing users to input memorable words (e.g., "CALL-NOW") and convert them to the corresponding phone number (2255-6669).
• Data Normalization: Standardizing phone number inputs where users might accidentally type letters instead of digits.
• Spelling Out Numbers: In systems where numeric input is cumbersome, letters can be used to represent digits.
• Educational Tools: Demonstrating character processing and mapping logic in C programming.
• Basic Text Processing: As a component in larger text processing or data conversion utilities.

Solution Approaches

We will explore two common approaches to solve this problem: using a series of if-else if statements and using a switch statement.

Approach 1: Using if-else if Statements

This approach uses a sequence of if-else if conditions to check the character and convert it to its corresponding digit.

One-line summary: Iterate through the input string, checking each character against ranges of letters using if-else if to assign the correct digit.

// 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;
}

Run

Sample output:

Original: 1-800-COLLECT
Original: HELLO-WORLD
Original: MyNumberIs123
Converted: 1-800-2655328

Converted: 43556-96753

Converted: 6968623747123

Stepwise explanation:

1. Include Headers: stdio.h for input/output, string.h for strlen (though not explicitly used in this main function for loop, ctype.h for toupper).
2. convertLettersToPhoneNumber Function: This function takes a character array (string) as input.
3. Loop Through String: A for loop iterates over each character of the string until the null terminator (\0) is encountered.
4. Convert to Uppercase: toupper(str[i]) converts the current character to its uppercase equivalent. This simplifies the comparison logic, as we only need to check for uppercase letter ranges.
5. Conditional Checks: A series of if-else if statements checks which range the uppercase character falls into (e.g., 'A' through 'C').
6. Replace Character: If a match is found, the character in the original string str[i] is replaced with its corresponding digit character (e.g., '2', '3').
7. Handle Non-Letters: If the character is not a letter within the defined ranges (e.g., a digit, hyphen, space), it remains unchanged as no if condition will be met to alter it.
8. main Function: Demonstrates the usage with a few example strings.

Approach 2: Using a switch Statement

This approach uses a switch statement, often considered cleaner for multiple specific value checks, although character ranges are less direct.

One-line summary: Use a switch statement on each character (after converting to uppercase) to map it to its corresponding phone digit using case groups.

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

void convertLettersToPhoneNumberSwitch(char *str) {
    for (int i = 0; str[i] != '\\0'; i++) {
        char ch = toupper(str[i]); // Convert to uppercase for consistent checking

        switch (ch) {
            case 'A': case 'B': case 'C':
                str[i] = '2';
                break;
            case 'D': case 'E': case 'F':
                str[i] = '3';
                break;
            case 'G': case 'H': case 'I':
                str[i] = '4';
                break;
            case 'J': case 'K': case 'L':
                str[i] = '5';
                break;
            case 'M': case 'N': case 'O':
                str[i] = '6';
                break;
            case 'P': case 'Q': case 'R': case 'S':
                str[i] = '7';
                break;
            case 'T': case 'U': case 'V':
                str[i] = '8';
                break;
            case 'W': case 'X': case 'Y': case 'Z':
                str[i] = '9';
                break;
            default:
                // Non-alphabetic characters (digits, hyphens, etc.)
                // are left unchanged by default
                break;
        }
    }
}

int main() {
    // Step 1: Declare character arrays for input.
    char phoneNumber1[50] = "1-800-C-A-L-L-N-O-W";
    char phoneNumber2[50] = "Programming-isfun";

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

    // Step 3: Convert letters to phone number digits using the switch approach.
    convertLettersToPhoneNumberSwitch(phoneNumber1);
    convertLettersToPhoneNumberSwitch(phoneNumber2);

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

    return 0;
}

Run

Sample output:

Original: 1-800-C-A-L-L-N-O-W
Original: Programming-isfun
Converted: 1-800-2-2-5-5-6-6-9

Converted: 77647266464-47386

Stepwise explanation:

1. Include Headers: Same as the previous approach.
2. convertLettersToPhoneNumberSwitch Function: Similar structure to the if-else if version.
3. Loop and Convert to Uppercase: Iterates through the string and converts each character to uppercase.
4. switch Statement: The switch statement evaluates the uppercase character ch.
5. Grouped case Labels: Multiple case labels (e.g., case 'A': case 'B': case 'C':) are grouped together. If ch matches any of these, the code block immediately following (before the next break;) is executed.
6. Replace Character: Inside the case block, str[i] is updated to its corresponding digit character.
7. break Statement: Each case block ends with break; to exit the switch statement and prevent "fall-through" to subsequent case blocks.
8. default Case: The default case handles any characters that do not match the alphabetic case labels (digits, symbols, spaces), leaving them unchanged.
9. main Function: Provides examples to test this switch-based conversion.

Conclusion

Converting letters into phone number digits is a straightforward yet practical task in C programming. Both the if-else if and switch statement approaches effectively solve this problem by mapping character ranges or specific characters to their numeric equivalents. The choice between them often comes down to personal preference or specific readability requirements for the codebase, with switch being potentially cleaner for discrete values and if-else if for ranges.

Summary

• Problem: Translate alphabetic letters in a string to corresponding phone keypad digits (e.g., A,B,C -> 2).
• Tools: Basic C programming, character functions (toupper), loops (for), and conditional statements (if-else if or switch).
• Approach 1 (if-else if): Uses sequential if-else if conditions to check character ranges and replace them with digits.
• Approach 2 (switch): Employs a switch statement with grouped case labels to map individual letters to digits.
• Flexibility: Both methods handle uppercase/lowercase letters and preserve non-alphabetic characters.
• Applications: Useful for vanity numbers, data normalization, and general text processing.