Simple Calculator in C: Complete Tutorial with CLI Interface

Learn How to Create a Simple Calculator in C: Step-by-Step Guide for Beginners

This comprehensive simple calculator in C tutorial teaches you how to build a fully functional menu-driven calculator with a command-line interface from scratch. Perfect for beginner C programmers, this project demonstrates essential programming concepts including function organization, switch statement control flow, input validation, and error handling. Learn to create a C calculator program that performs five arithmetic operations—addition, subtraction, multiplication, division, and modulus—with professional input validation and zero-division protection. This step-by-step guide includes complete source code, compilation instructions, common mistake solutions, and practice exercises to strengthen your C programming foundation.

Source Code
Common Mistakes and Debugging Tips
Practice Exercise
Conclusion

Implementation Details

This project is perfect for beginners learning C programming as it covers essential concepts like functions, switch statements, input validation, and user interaction.

What This Project Does

Our calculator will perform five basic mathematical operations:

➕ Addition
➖ Subtraction
✖️ Multiplication
➗ Division (with zero-division protection)
📊 Modulus (remainder operation)

Why This Project Is Useful

Practical Application: Demonstrates real-world programming concepts.
Function Usage: Shows how to organize code with modular functions.
Input Validation: Teaches error handling and user input validation.
Menu Systems: Introduces menu-driven programming patterns.
Foundation Building: Perfect stepping stone to more complex projects.

Program Flow and Logic

Here's how our calculator works step by step:

        START
          |
    ┌─────▼─────┐
    │  Display  │
    │   Menu    │
    └─────┬─────┘
          │
    ┌─────▼─────┐
    │   Get     │
    │User Choice│
    └─────┬─────┘
          │
    ┌─────▼─────┐     NO
    │Choice = 6?├────────┐
    │  (Exit)   │        │
    └─────┬─────┘        │
          │ YES          │
    ┌─────▼─────┐        │
    │    END    │        │
    └───────────┘        │
                         │
           ┌─────────────┘
           │
           ▼
    ┌─────────────┐
    │   Get Two   │
    │   Numbers   │
    └─────┬───────┘
          │
    ┌─────▼─────┐
    │  Perform  │
    │ Operation │
    │  (Switch) │
    └─────┬─────┘
          │
    ┌─────▼─────┐
    │  Display  │
    │  Result   │
    └─────┬─────┘
          │
    ┌─────▼─────┐
    │Continue or│
    │   Exit?   │
    └─────┬─────┘
          │
          └──────────────┐
                         │
                    ┌────▼────┐
                    │  LOOP   │
                    │  BACK   │
                    └─────────┘

Key Programming Concepts Used:

Functions: Each operation has its own function.
Switch Statement: Menu selection handling.
Do-While Loop: Continues until user exits.
Input Validation: Prevents errors and crashes.
Modular Design: Clean, organized code structure

Complete Source Code

Calculator.c

/*
===============================================================================
                        SIMPLE CALCULATOR - CLI VERSION
===============================================================================
Description: A beginner-friendly calculator program in C with menu interface
Author: C Programming Tutorial
Features: Addition, Subtraction, Multiplication, Division, Modulus
Compilation: gcc -Wall -Wextra -std=c99 -o calculator calculator.c
===============================================================================
*/

#include <stdio.h>
#include <stdlib.h>

/* ===============================================================================
                                FUNCTION PROTOTYPES
=============================================================================== */

// Mathematical operation functions
double add(double a, double b);
double subtract(double a, double b);
double multiply(double a, double b);
double divide(double a, double b);
int modulus(int a, int b);

// Utility functions
void displayMenu(void);
void clearInputBuffer(void);
int getValidChoice(void);
double getValidNumber(const char* prompt);
void displayResult(double result, const char* operation);
void pressEnterToContinue(void);

/* ===============================================================================
                                    MAIN FUNCTION
=============================================================================== */

int main(void) {
    int choice;
    double num1, num2, result;

    printf("\n");
    printf("===============================================================================\n");
    printf("                    WELCOME TO SIMPLE CALCULATOR v1.0\n");
    printf("===============================================================================\n");
    printf("This calculator performs basic mathematical operations.\n");
    printf("Follow the menu prompts to perform calculations.\n");

    // Main program loop - continues until user chooses to exit
    do {
        displayMenu();
        choice = getValidChoice();

        // Handle exit option
        if (choice == 6) {
            printf("\n");
            printf("===============================================================================\n");
            printf("                    THANK YOU FOR USING THE CALCULATOR!\n");
            printf("===============================================================================\n");
            break;
        }

        // Get input numbers for calculation
        printf("\n");
        num1 = getValidNumber("Enter first number: ");
        num2 = getValidNumber("Enter second number: ");

        printf("\n");
        printf("===============================================================================\n");

        // Perform calculation based on user choice
        switch (choice) {
            case 1: // Addition
                result = add(num1, num2);
                displayResult(result, "Addition");
                printf("%.2f + %.2f = %.2f\n", num1, num2, result);
                break;

            case 2: // Subtraction
                result = subtract(num1, num2);
                displayResult(result, "Subtraction");
                printf("%.2f - %.2f = %.2f\n", num1, num2, result);
                break;

            case 3: // Multiplication
                result = multiply(num1, num2);
                displayResult(result, "Multiplication");
                printf("%.2f × %.2f = %.2f\n", num1, num2, result);
                break;

            case 4: // Division
                if (num2 == 0) {
                    printf("                              ERROR: DIVISION BY ZERO\n");
                    printf("===============================================================================\n");
                    printf("❌ Cannot divide by zero! Please try again with a non-zero divisor.\n");
                } else {
                    result = divide(num1, num2);
                    displayResult(result, "Division");
                    printf("%.2f ÷ %.2f = %.2f\n", num1, num2, result);
                }
                break;

            case 5: // Modulus
                // Modulus only works with integers
                if (num2 == 0) {
                    printf("                              ERROR: MODULUS BY ZERO\n");
                    printf("===============================================================================\n");
                    printf("❌ Cannot perform modulus by zero! Please try again.\n");
                } else {
                    int intNum1 = (int)num1;
                    int intNum2 = (int)num2;
                    int modResult = modulus(intNum1, intNum2);
                    printf("                               MODULUS RESULT\n");
                    printf("===============================================================================\n");
                    printf("✓ %d %% %d = %d\n", intNum1, intNum2, modResult);
                }
                break;

            default:
                printf("                                 INVALID CHOICE\n");
                printf("===============================================================================\n");
                printf("❌ Please select a valid option (1-6).\n");
        }

        pressEnterToContinue();

    } while (choice != 6);

    return 0;
}

/* ===============================================================================
                            MATHEMATICAL OPERATION FUNCTIONS
=============================================================================== */

/**
 * Addition function
 * @param a: First number
 * @param b: Second number
 * @return: Sum of a and b
 */
double add(double a, double b) {
    return a + b;
}

/**
 * Subtraction function
 * @param a: First number (minuend)
 * @param b: Second number (subtrahend)
 * @return: Difference of a and b
 */
double subtract(double a, double b) {
    return a - b;
}

/**
 * Multiplication function
 * @param a: First number
 * @param b: Second number
 * @return: Product of a and b
 */
double multiply(double a, double b) {
    return a * b;
}

/**
 * Division function
 * @param a: Dividend
 * @param b: Divisor (should not be zero)
 * @return: Quotient of a divided by b
 */
double divide(double a, double b) {
    return a / b;
}

/**
 * Modulus function (remainder after division)
 * @param a: Dividend (integer)
 * @param b: Divisor (integer, should not be zero)
 * @return: Remainder of a divided by b
 */
int modulus(int a, int b) {
    return a % b;
}

/* ===============================================================================
                                UTILITY FUNCTIONS
=============================================================================== */

/**
 * Display the main menu options
 */
void displayMenu(void) {
    printf("\n");
    printf("===============================================================================\n");
    printf("                               CALCULATOR MENU\n");
    printf("===============================================================================\n");
    printf("Choose an operation:\n");
    printf("  1. ➕  Addition       (a + b)\n");
    printf("  2. ➖  Subtraction    (a - b)\n");
    printf("  3. ✖️   Multiplication (a × b)\n");
    printf("  4. ➗  Division       (a ÷ b)\n");
    printf("  5. 📊  Modulus        (a %% b)\n");
    printf("  6. 🚪  Exit Program\n");
    printf("===============================================================================\n");
}

/**
 * Clear input buffer to handle invalid input
 */
void clearInputBuffer(void) {
    int c;
    while ((c = getchar()) != '\n' && c != EOF) {
        // Clear the input buffer
    }
}

/**
 * Get valid menu choice from user
 * @return: Valid choice (1-6)
 */
int getValidChoice(void) {
    int choice;

    while (1) {
        printf("Enter your choice (1-6): ");

        if (scanf("%d", &choice) == 1) {
            clearInputBuffer();
            if (choice >= 1 && choice <= 6) {
                return choice;
            } else {
                printf("❌ Invalid choice! Please enter a number between 1 and 6.\n\n");
            }
        } else {
            clearInputBuffer();
            printf("❌ Invalid input! Please enter a numeric choice (1-6).\n\n");
        }
    }
}

/**
 * Get valid number from user with custom prompt
 * @param prompt: Message to display to user
 * @return: Valid floating-point number
 */
double getValidNumber(const char* prompt) {
    double number;

    while (1) {
        printf("%s", prompt);

        if (scanf("%lf", &number) == 1) {
            clearInputBuffer();
            return number;
        } else {
            clearInputBuffer();
            printf("❌ Invalid input! Please enter a valid number.\n");
        }
    }
}

/**
 * Display formatted result header
 * @param result: The calculated result
 * @param operation: Name of the operation performed
 */
void displayResult(double result, const char* operation) {
    printf("                               %s RESULT\n", operation);
    printf("===============================================================================\n");
    printf("✓ ");
}

/**
 * Pause program and wait for user to press Enter
 */
void pressEnterToContinue(void) {
    printf("\n📝 Press Enter to continue...");
    getchar();
}

/* ===============================================================================
                                END OF PROGRAM
=============================================================================== */

Compilation and Execution

gcc -Wall -Wextra -std=c99 -o calculator calculator.c

How to Run:

./calculator    # Linux/Mac
calculator.exe  # Windows

Sample Input and Output

Here's what you'll see when running the program:

===============================================================================
WELCOME TO SIMPLE CALCULATOR v1.0
===============================================================================
This calculator performs basic mathematical operations.
Follow the menu prompts to perform calculations.

===============================================================================
CALCULATOR MENU
===============================================================================
Choose an operation:
1. ➕ Addition (a + b)
2. ➖ Subtraction (a - b)
3. ✖️ Multiplication (a × b)
4. ➗ Division (a ÷ b)
5. 📊 Modulus (a % b)
6. 🚪 Exit Program
===============================================================================
Enter your choice (1-6): 1

Enter first number: 15.5
Enter second number: 7.3

===============================================================================
Addition RESULT
===============================================================================
✓ 15.50 + 7.30 = 22.80

📝 Press Enter to continue...

===============================================================================
CALCULATOR MENU
===============================================================================
Choose an operation:
1. ➕ Addition (a + b)
2. ➖ Subtraction (a - b)
3. ✖️ Multiplication (a × b)
4. ➗ Division (a ÷ b)
5. 📊 Modulus (a % b)
6. 🚪 Exit Program
===============================================================================
Enter your choice (1-6): 4

Enter first number: 100
Enter second number: 0

===============================================================================
ERROR: DIVISION BY ZERO
===============================================================================
❌ Cannot divide by zero! Please try again with a non-zero divisor.

📝 Press Enter to continue...

Common Mistakes and Debugging Tips

1. Forgetting break in Switch Statements

❌ Wrong:

switch (choice) {
    case 1:
        result = add(num1, num2);
        // Missing break - will continue to case 2!
    case 2:
        result = subtract(num1, num2);
        break;
}

✅ Correct:

switch (choice) {
    case 1:
        result = add(num1, num2);
        break;  // Essential!
    case 2:
        result = subtract(num1, num2);
        break;
}

2. Not Handling Division by Zero

❌ Wrong:

double divide(double a, double b) {
    return a / b; // Will cause runtime error if b = 0
}

✅ Correct:

// In main function:
if (num2 == 0) {
    printf("Error: Cannot divide by zero!\n");
} else {
    result = divide(num1, num2);
    printf("%.2f ÷ %.2f = %.2f\n", num1, num2, result);
}

3. Input Buffer Problems

❌ Problem: When scanf fails, invalid input stays in buffer

scanf("%d", &choice);  // If user enters "abc", it stays in buffer

✅ Solution: Clear the buffer after each input

if (scanf("%d", &choice) == 1) {
    clearInputBuffer();  // Clear any remaining input
    // Process valid input
} else {
    clearInputBuffer();  // Clear invalid input
    // Handle error
}

4. Infinite Loops in Input Validation

❌ Wrong:

do {
    printf("Enter choice: ");
    scanf("%d", &choice);
} while (choice < 1 || choice > 6);  // No handling of invalid input

✅ Correct:

while (1) {
    printf("Enter choice: ");
    if (scanf("%d", &choice) == 1 && choice >= 1 && choice <= 6) {
        clearInputBuffer();
        break;  // Valid input received
    } else {
        clearInputBuffer();
        printf("Invalid input! Try again.\n");
    }
}

Practice Exercises

Exercise 1: Add Power Operation

Challenge: Add a power operation (a^b) to the calculator.

Add option 7 to the menu.
Create a power() function.
Add a new case in the switch statement.
You can use a loop or include <math.h> and use pow() function.

// Add this include at the top
#include 

// Add this function
double power(double base, double exponent) {
    return pow(base, exponent);
}

// Update the menu display
void displayMenu(void) {
    printf("\n");
    printf("===============================================================================\n");
    printf("                               CALCULATOR MENU\n");
    printf("===============================================================================\n");
    printf("Choose an operation:\n");
    printf("  1. ➕  Addition       (a + b)\n");
    printf("  2. ➖  Subtraction    (a - b)\n");
    printf("  3. ✖️   Multiplication (a × b)\n");
    printf("  4. ➗  Division       (a ÷ b)\n");
    printf("  5. 📊  Modulus        (a %% b)\n");
    printf("  6. ⚡  Power          (a ^ b)\n");  // New option
    printf("  7. 🚪  Exit Program\n");              // Updated exit option
    printf("===============================================================================\n");
}

// Add this case in the switch statement (before case 7)
case 6: // Power
    result = power(num1, num2);
    displayResult(result, "Power");
    printf("%.2f ^ %.2f = %.2f\n", num1, num2, result);
    break;

// Update exit condition and validation to use 7 instead of 6

Exercise 2: Add Square Root Operation

Challenge: Add square root functionality that works on a single number.

Add square root option to menu.
Only ask for one number (not two).
Validate that the number is non-negative.
Use sqrt() from <math.h>function.

// Add this function
double squareRoot(double number) {
    return sqrt(number);
}

// Add to menu (option 7, making exit option 8)
printf("  7. √   Square Root    (√a)\n");
printf("  8. 🚪  Exit Program\n");

// Add this case in switch statement
case 7: // Square Root
    if (num1 < 0) {
        printf("                              ERROR: NEGATIVE NUMBER\n");
        printf("===============================================================================\n");
        printf("❌ Cannot calculate square root of negative number!\n");
    } else {
        result = squareRoot(num1);
        displayResult(result, "Square Root");
        printf("√%.2f = %.2f\n", num1, result);
    }
    break;

// Note: For square root, you might want to modify the input logic
// to only ask for one number when choice == 7

Exercise 2: Add Memory Functions

Challenge: Implement basic memory functions (M+, M-, MR, MC) like a real calculator.

Add global memory variable.
Add menu options for Memory Add, Memory Subtract, Memory Recall, Memory Clear.
Allow using memory value as input for calculationse.

// Add global variable
double memory = 0.0;

// Add these functions
void memoryAdd(double value) {
    memory += value;
    printf("✓ Memory: %.2f (Added %.2f)\n", memory, value);
}

void memorySubtract(double value) {
    memory -= value;
    printf("✓ Memory: %.2f (Subtracted %.2f)\n", memory, value);
}

void memoryRecall(void) {
    printf("✓ Memory Recall: %.2f\n", memory);
}

void memoryClear(void) {
    memory = 0.0;
    printf("✓ Memory Cleared\n");
}

// Update menu with memory options (9-12)
printf("  9. M+  Memory Add\n");
printf(" 10. M-  Memory Subtract\n");
printf(" 11. MR  Memory Recall\n");
printf(" 12. MC  Memory Clear\n");
printf(" 13. 🚪  Exit Program\n");

// Add cases in switch statement
case 9: // Memory Add
    num1 = getValidNumber("Enter number to add to memory: ");
    memoryAdd(num1);
    break;
case 10: // Memory Subtract
    num1 = getValidNumber("Enter number to subtract from memory: ");
    memorySubtract(num1);
    break;
case 11: // Memory Recall
    memoryRecall();
    break;
case 12: // Memory Clear
    memoryClear();
    break;

Conclusion

Congratulations! 🎉 You've successfully created a fully functional calculator in C with:

The simple calculator tutorial in C provides an excellent introduction to using basic programming concepts such as variables, user input, conditional statements (if-else or switch-case), and arithmetic operations. By building this project step-by-step, beginners learn how to structure a console application that performs addition, subtraction, multiplication, and division based on user input. This foundational exercise helps new programmers develop their logic and C coding skills while gaining confidence in building interactive command-line tools.

View GitHub

Next Steps:

Try the exercises above to enhance your calculator.
Add more operations like factorial, trigonometric functions.
Create a GUI version using libraries like GTK or ncurses.
Implement expression parsing to evaluate complex expressions like "2+3*4".
Add history functionality to remember previous calculations.

Key Programming Skills Learned:

Function definition and organization.
Switch statement control flow.
Input validation techniques.
Error handling patterns.
User interface design in CLI.
Modular programming principles.

Keep practicing and building more projects to strengthen your C programming foundation! 🚀

Happy Coding!

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Simple Calculator in C: Complete Tutorial with CLI Interface

Table of Contents

Implementation Details

What This Project Does

Why This Project Is Useful

Program Flow and Logic

Key Programming Concepts Used:

Complete Source Code

Compilation and Execution

Sample Input and Output

Common Mistakes and Debugging Tips

1. Forgetting break in Switch Statements

2. Not Handling Division by Zero

3. Input Buffer Problems

4. Infinite Loops in Input Validation

Practice Exercises

Conclusion

Next Steps:

Key Programming Skills Learned:

Other Projects

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To-Do CLI App

Weather App

Team Card App

Password Strength Checker

VPN Connectivity verification in C