Simple Calculator

Welcome to Part 3! It’s time to start building bigger things. This lesson is our first real PROJECT: a simple calculator that works from the command line.

THE GOAL: We want to be able to run our program like this: ./16_simple_calculator 10 + 22 And have it print the result: 10.00 + 22.00 = 32.00

SKILLS WE WILL USE:

1. Command-Line Arguments (argc, argv): To get the numbers and the operator from the user when they run the program.
2. Data Conversion: The arguments in argv are STRINGS. We need to convert the number strings (“10”, “22”) into actual numbers (double) so we can do math with them. We’ll use the atof() function for this.
3. Conditional Logic (if, switch): To check if the user provided the correct input and to decide which mathematical operation to perform.
4. Error Handling: We will provide a helpful USAGE MESSAGE if the user makes a mistake and handle potential math errors like division by zero.

Full Source

/**
 * @file 16_simple_calculator.c
 * @brief Part 3, Lesson 16: Simple Calculator
 * @author dunamismax
 * @date 06-15-2025
 *
 * This file is our first full PROJECT. It combines several concepts we've
 * learned--command-line arguments, data type conversion, and conditional
 * logic--to build a complete, useful command-line tool.
 */

/*
 * =====================================================================================
 * |                                   - LESSON START -                                  |
 * =====================================================================================
 *
 * Welcome to Part 3! It's time to start building bigger things. This lesson is
 * our first real PROJECT: a simple calculator that works from the command line.
 *
 * THE GOAL:
 * We want to be able to run our program like this:
 * `./16_simple_calculator 10 + 22`
 * And have it print the result: `10.00 + 22.00 = 32.00`
 *
 * SKILLS WE WILL USE:
 * 1. Command-Line Arguments (`argc`, `argv`): To get the numbers and the
 *    operator from the user when they run the program.
 * 2. Data Conversion: The arguments in `argv` are STRINGS. We need to convert
 *    the number strings ("10", "22") into actual numbers (`double`) so we can
 *    do math with them. We'll use the `atof()` function for this.
 * 3. Conditional Logic (`if`, `switch`): To check if the user provided the
 *    correct input and to decide which mathematical operation to perform.
 * 4. Error Handling: We will provide a helpful USAGE MESSAGE if the user makes a
 *    mistake and handle potential math errors like division by zero.
 */

#include <stdio.h>
#include <stdlib.h> // Required for atof() and exit()
#include <string.h> // Required for strcmp()

// We need argc and argv to get input from the command line.
int main(int argc, char *argv[])
{
    double num1, num2, result;
    char operator;

    // --- Step 1: Validate the Input ---
    // We need exactly four arguments:
    // argv[0]: program name
    // argv[1]: first number
    // argv[2]: operator
    // argv[3]: second number
    if (argc != 4)
    {
        // If the count is wrong, print a helpful usage message to standard error.
        fprintf(stderr, "Usage: %s <number1> <operator> <number2>\n", argv[0]);
        fprintf(stderr, "Example: %s 10.5 / 2\n", argv[0]);
        exit(1); // Exit with an error code.
    }

    // --- Step 2: Convert and Store Arguments ---

    // `atof` (ASCII to float) converts a string to a double.
    num1 = atof(argv[1]);
    num2 = atof(argv[3]);

    // The operator is a string like "+". We only need the first character.
    // We check if the string is longer than one character, which would be invalid.
    if (strlen(argv[2]) != 1)
    {
        fprintf(stderr, "Error: Invalid operator '%s'. Operator must be a single character.\n", argv[2]);
        exit(1);
    }
    operator = argv[2][0];

    // --- Step 3: Perform the Calculation ---
    // A `switch` statement is perfect for checking the operator and deciding
    // which calculation to perform.
    switch (operator)
    {
    case '+':
        result = num1 + num2;
        break; // `break` exits the switch statement.

    case '-':
        result = num1 - num2;
        break;

    case '*':
        // Some shells interpret * as a wildcard. Users may need to type '*'
        // to pass it correctly.
        result = num1 * num2;
        break;

    case '/':
        // This is an important EDGE CASE. We cannot divide by zero.
        if (num2 == 0)
        {
            fprintf(stderr, "Error: Division by zero is not allowed.\n");
            exit(1);
        }
        result = num1 / num2;
        break;

    default:
        // If the operator is not one of the above, it's invalid.
        fprintf(stderr, "Error: Unknown operator '%c'.\n", operator);
        fprintf(stderr, "Supported operators are: + - * /\n");
        exit(1);
    }

    // --- Step 4: Display the Result ---
    // If we get here, the calculation was successful.
    // We use %.2f to format the numbers to two decimal places for clean output.
    printf("%.2f %c %.2f = %.2f\n", num1, operator, num2, result);

    return 0; // Success!
}

/*
 * =====================================================================================
 * |                                    - LESSON END -                                   |
 * =====================================================================================
 *
 * Congratulations! You've just built your first complete, functional command-line
 * tool. You've seen how to take user input, validate it, process it, and
 * produce a result, all while handling potential errors gracefully.
 *
 * Key Achievements in this Project:
 * - Combined multiple C features into a working application.
 * - Used `argc` and `argv` for real-world program input.
 * - Converted string data to numeric data for calculations (`atof`).
 * - Implemented robust error checking for both user input and math rules.
 *
 * HOW TO COMPILE AND RUN THIS CODE:
 *
 * 1. Open a terminal or command prompt.
 * 2. Navigate to the directory where you saved this file.
 * 3. Use the GCC compiler to create an executable file:
 *    `gcc -Wall -Wextra -std=c11 -o 16_simple_calculator 16_simple_calculator.c`
 *
 * 4. Run the executable with different arguments:
 *
 *    Addition:
 *    ./16_simple_calculator 5 + 3
 *
 *    Subtraction with decimals:
 *    ./16_simple_calculator 100.5 - 50.2
 *
 *    Multiplication (you might need quotes around '*' in some terminals like bash):
 *    ./16_simple_calculator 7 '*' 6
 *
 *    Division:
 *    ./16_simple_calculator 20 / 4
 *
 *    Division by zero (this will trigger our error message):
 *    ./16_simple_calculator 10 / 0
 *
 *    Incorrect usage (this will trigger our usage message):
 *    ./16_simple_calculator 10 plus 5
 */

How to Compile and Run

cc -Wall -Wextra -std=c11 -o 16_simple_calculator 16_simple_calculator.c
./16_simple_calculator