Function calling in C++ - Mini Project: Build & Apply

Function calling

📖 Scenario: Imagine you are creating a simple calculator program that can add two numbers. You will write a function to add numbers and then call it to get the result.

🎯 Goal: Build a program that defines a function to add two integers and calls this function to display the sum.

📋 What You'll Learn

Define a function named add that takes two int parameters and returns their sum.

Create two integer variables with exact values 5 and 7.

Call the add function with these two variables.

Print the result of the function call.

💡 Why This Matters

🌍 Real World

Functions help organize code into reusable blocks, like a calculator that can add many pairs of numbers without rewriting code.

💼 Career

Understanding function calling is essential for writing clean, modular code in any software development job.

1

Create two integer variables

Create two integer variables called num1 and num2 with values 5 and 7 respectively.

C++

// Your code here

Hint

Use int to declare variables and assign the exact values.

2

Define the add function

Define a function named add that takes two int parameters named a and b and returns their sum as an int.

C++

int num1 = 5;
int num2 = 7;

// Your code here

Hint

Write the function header with two parameters and return their sum inside the function body.

3

Call the add function and store result

Call the add function with num1 and num2 as arguments and store the result in an integer variable called sum.

C++

int num1 = 5;
int num2 = 7;

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

// Your code here

Hint

Use the function name add with the two variables inside parentheses and assign the result to sum.

4

Print the sum

Print the value of the variable sum using std::cout.

C++

int num1 = 5;
int num2 = 7;

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

int sum = add(num1, num2);

// Your code here

Hint

Use std::cout << sum << std::endl; inside main() to print the sum.

Practice

1. What does calling a function in C++ do?

easy

A. Runs the code inside the function

B. Deletes the function

C. Creates a new function

D. Changes the function name

Solution

1. Step 1: Understand what a function call means

   Calling a function means telling the program to run the code inside that function.

2. Step 2: Identify the effect of calling a function

   When called, the function's instructions execute, performing its task.

3. Final Answer:

   Runs the code inside the function -> Option A

4. Quick Check:

   Function call = run function code [OK]

Hint: Function call runs its code block immediately [OK]

Common Mistakes:

• Thinking calling creates or deletes functions
• Confusing function call with function definition
• Assuming function call changes function name

2. Which of the following is the correct way to call a function named printMessage in C++?

easy

A. printMessage();

B. printMessage;

C. call printMessage();

D. function printMessage();

Solution

1. Step 1: Recall the syntax for calling a function

   In C++, to call a function, write its name followed by parentheses and a semicolon.

2. Step 2: Check each option

   printMessage(); uses the correct syntax: printMessage();. Others are incorrect or invalid syntax.

3. Final Answer:

   printMessage(); -> Option A

4. Quick Check:

   Function call syntax = name + () + ; [OK]

Hint: Function calls always end with parentheses and semicolon [OK]

Common Mistakes:

• Omitting parentheses when calling functions
• Adding keywords like 'call' or 'function' incorrectly
• Missing semicolon at the end

3. What is the output of this C++ code?

void greet() {
    std::cout << "Hello!" << std::endl;
}

int main() {
    greet();
    return 0;
}

medium

A. Compilation error

B. greet()

C. No output

D. Hello!

Solution

1. Step 1: Understand what the function greet() does

   The function prints "Hello!" followed by a new line.

2. Step 2: See what main() does

   Main calls greet(), so "Hello!" is printed to the screen.

3. Final Answer:

   Hello! -> Option D

4. Quick Check:

   Function call prints "Hello!" [OK]

Hint: Function call runs its print statements [OK]

Common Mistakes:

• Expecting function name printed instead of output
• Thinking no output because no return value
• Assuming compilation error without syntax issues

4. Identify the error in this code snippet:

void sayHi() {
    std::cout << "Hi!" << std::endl;
}

int main() {
    sayHi;
    return 0;
}

medium

A. Function sayHi is not defined

B. Missing parentheses when calling sayHi

C. Missing semicolon after sayHi

D. Incorrect return type in main

Solution

1. Step 1: Check how sayHi is called

   In main(), sayHi is written without parentheses, so it is not called.

2. Step 2: Understand correct function call syntax

   To call sayHi, it must be sayHi(); with parentheses.

3. Final Answer:

   Missing parentheses when calling sayHi -> Option B

4. Quick Check:

   Function call needs parentheses [OK]

Hint: Always use () to call a function [OK]

Common Mistakes:

• Forgetting parentheses on function calls
• Thinking function name alone calls it
• Ignoring compiler errors about function usage

5. Given this function:

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

What is the correct way to call add to get the sum of 5 and 3 and store it in result?

hard

A. int result = add(5 + 3);

B. int result = add(5);

C. int result = add(5, 3);

D. int result = add();

Solution

1. Step 1: Check the function parameters

   The function add requires two integers as input: a and b.

2. Step 2: Match the call with two arguments

   int result = add(5, 3); passes both 5 and 3 correctly. Others pass wrong number or format of arguments.

3. Final Answer:

   int result = add(5, 3); -> Option C

4. Quick Check:

   Function call needs correct number of arguments [OK]

Hint: Match function parameters count exactly when calling [OK]

Common Mistakes:

• Passing fewer or more arguments than function needs
• Passing expressions without commas for multiple args
• Calling function without any arguments when required