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Putting it together

Combine all parts into a working calculator.

Putting it together

In the final step we'll add multiply and divide options to the menu. The calculator will then be a complete app supporting all four operations:

  • case 1: addition
  • case 2: subtraction
  • case 3: multiplication
  • case 4: division

We'll keep the existing case 1 and case 2 branches and add case 3 and case 4.

Practice Add case 3 and case 4

Add case 3: printf("%d\n", multiply(5, 3)); break; and case 4: printf("%d\n", divide(6, 3)); break; branches to the menu below. The calculator will then support all four operations.

Starter code

main.c
#include <stdio.h>

int add(int a, int b) {
    return a + b;
}

int subtract(int a, int b) {
    return a - b;
}

int multiply(int a, int b) {
    return a * b;
}

int divide(int a, int b) {
    if (b == 0) return 0;
    return a / b;
}

int main(void) {
    printf("Calculator\n");
    int choice = 1;
    while (choice != 0) {
        switch (choice) {
            case 1: printf("%d\n", add(5, 3)); break;
            case 2: printf("%d\n", subtract(5, 3)); break;
            default: break;
        }
        choice = 0;
    }
    return 0;
}

Sample solution

main.c
#include <stdio.h>

int add(int a, int b) {
    return a + b;
}

int subtract(int a, int b) {
    return a - b;
}

int multiply(int a, int b) {
    return a * b;
}

int divide(int a, int b) {
    if (b == 0) return 0;
    return a / b;
}

int main(void) {
    printf("Calculator\n");
    int choice = 1;
    while (choice != 0) {
        switch (choice) {
            case 1: printf("%d\n", add(5, 3)); break;
            case 2: printf("%d\n", subtract(5, 3)); break;
            case 3: printf("%d\n", multiply(5, 3)); break;
            case 4: printf("%d\n", divide(6, 3)); break;
            default: break;
        }
        choice = 0;
    }
    return 0;
}

Expected output

Calculator
8

Project complete

Congratulations! You've reached the end of the five-lesson project. You wrote a complete calculator app:

  • Four functions: add, subtract, multiply, divide
  • A divide-by-zero check
  • A menu with a while loop and switch

Next you can add real user input with scanf, error handling and more operations. Everything you build in C from now on will rest on these foundations.

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 case label

Question 
switch (choice) {
    ____ printf("%d", add(5, 3)); break;
}
Answer 
switch (choice) {
    case 1: printf("%d", add(5, 3)); break;
}

Explanation: The case 1: label runs when choice is 1.

2 Why use switch in a menu?

  • For multi-branch selection Correct answer
  • For loops
  • For computation
  • For output

Explanation: switch picks a branch based on a value; ideal for a menu.

3 Match all concepts

Term Meaning
add Addition
subtract Subtraction
multiply Multiplication
divide Division

All lessons in Beginner

1 C basics

  1. What is C?
  2. Your first program
  3. Compile and run
  4. Comments
  5. printf and escapes
  6. Summary - C basics

2 Variables

  1. Declaring variables
  2. Variable assignment
  3. const
  4. Variable scope
  5. typedef
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. char
  4. sizeof
  5. Type conversion
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Menu loop
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Bitwise operators
  6. Summary - Operators

6 I/O

  1. printf formatting
  2. scanf
  3. Multiple inputs
  4. getchar and putchar
  5. puts and fputs
  6. Summary - I/O

7 Conditionals

  1. The if statement
  2. else and else if
  3. The switch statement
  4. The ternary operator
  5. Nested conditionals
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. break and continue
  5. Nested loops
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding products
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. Array size
  4. Multidimensional arrays
  5. Arrays and loops
  6. Summary - Arrays

11 Strings

  1. String basics
  2. String literals
  3. strlen
  4. strcpy and strcat
  5. strcmp
  6. Summary - Strings

12 Functions

  1. Defining functions
  2. Return values
  3. Parameters
  4. Function prototypes
  5. Recursion
  6. Summary - Functions

13 Pointer basics

  1. What is a pointer?
  2. The & address operator
  3. Pointer dereference
  4. NULL
  5. Pointer arithmetic
  6. Summary - Pointer basics

14 Arrays and pointers

  1. Array name as pointer
  2. Pointer to array
  3. Passing arrays to functions
  4. Pointer loop
  5. Multidim array pointer
  6. Summary - Arrays and pointers

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 String functions

  1. strchr
  2. strstr
  3. strtok
  4. sprintf
  5. sscanf
  6. Summary - String functions

17 Structs

  1. Defining structs
  2. Using structs
  3. typedef struct
  4. Array of structs
  5. Nested structs
  6. Summary - Structs

18 Memory and stack

  1. Stack and heap
  2. Automatic storage
  3. malloc
  4. free
  5. calloc and realloc
  6. Summary - Memory and stack

19 Preprocessor

  1. #define
  2. #include
  3. Conditional compilation
  4. Macro functions
  5. Header files
  6. Summary - Preprocessor

20 Beginner final

  1. Project design
  2. Data model
  3. Functions
  4. Memory management
  5. Putting it together