Return values in C - Mini Project: Build & Apply

Return values

📖 Scenario: You are creating a simple calculator program in C that adds two numbers. You will write a function that returns the sum of two integers.

🎯 Goal: Build a C program with a function called add that takes two integers and returns their sum. Then call this function and print the result.

📋 What You'll Learn

Create a function named add that takes two int parameters

The add function must return the sum of the two integers

Call the add function from main with the values 5 and 7

Print the returned result using printf

💡 Why This Matters

🌍 Real World

Functions that return values are used everywhere in programming to calculate results and pass data between parts of a program.

💼 Career

Understanding return values is essential for writing reusable code and working with APIs, libraries, and complex software systems.

1

Create the add function signature

Write the function signature for a function called add that takes two int parameters named a and b and returns an int. Do not write the function body yet.

C

// Write the function signature for add here
// Your code here

Hint

Function signature includes return type, function name, and parameters.

2

Define the add function body

Write the full definition of the add function that returns the sum of a and b. Use a return statement.

C

int add(int a, int b) {
    // Your code here
}

Hint

Use return a + b; inside the function body.

3

Call add in main and store result

Write the main function. Inside it, call the add function with arguments 5 and 7, and store the result in an int variable named result.

C

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

int main() {
    // Your code here
    return 0;
}

Hint

Call add(5, 7) and assign it to result.

4

Print the result

Add a printf statement in main to print the value of result followed by a newline. Use the format specifier %d.

C

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

int main() {
    int result = add(5, 7);
    // Your code here
    return 0;
}

Hint

Use printf("%d\n", result); to print the sum.

Practice

1. What does the return statement do in a C function?

easy

A. It sends a value back to the place where the function was called.

B. It prints a value on the screen.

C. It declares a new variable inside the function.

D. It pauses the program execution.

Solution

1. Step 1: Understand the purpose of return

   The return statement sends a value from the function back to the caller.

2. Step 2: Differentiate from other actions

   Printing, declaring variables, or pausing are not what return does.

3. Final Answer:

   It sends a value back to the place where the function was called. -> Option A

4. Quick Check:

   Return sends value back [OK]

Hint: Return sends value back to caller, not print or pause [OK]

Common Mistakes:

• Thinking return prints output
• Confusing return with variable declaration
• Assuming return pauses program

2. Which of the following is the correct syntax for a function that returns an integer in C?

easy

A. int myFunction() { return 5; }

B. void myFunction() { return 5; }

C. int myFunction() { print(5); }

D. myFunction() int { return 5; }

Solution

1. Step 1: Check function return type and return statement

   The function declares int return type and returns an integer value 5.

2. Step 2: Identify incorrect options

   void myFunction() { return 5; } returns a value but declares void (no return). int myFunction() { print(5); } uses print which is invalid in C. myFunction() int { return 5; } has wrong syntax.

3. Final Answer:

   int myFunction() { return 5; } -> Option A

4. Quick Check:

   Return type matches returned value [OK]

Hint: Return type must match returned value type [OK]

Common Mistakes:

• Using void but returning a value
• Using print instead of return
• Wrong function syntax

3. What is the output of this C program?

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

int main() {
    int result = add(3, 4);
    printf("%d", result);
    return 0;
}

medium

A. 34

B. 7

C. 0

D. Error

Solution

1. Step 1: Understand the add function

   The function adds two integers and returns their sum.

2. Step 2: Calculate the sum of 3 and 4

   3 + 4 equals 7, which is stored in result and printed.

3. Final Answer:

   7 -> Option B

4. Quick Check:

   3 + 4 = 7 [OK]

Hint: Add returns sum; printed result is sum [OK]

Common Mistakes:

• Concatenating numbers as strings
• Confusing return with print
• Expecting zero or error output

4. Identify the error in this C function:

int multiply(int x, int y) {
    int product = x * y;
}

medium

A. Variables x and y are not declared.

B. Incorrect multiplication operator.

C. Function name should be void.

D. Missing return statement to return the product.

Solution

1. Step 1: Check function return type and body

   The function declares it returns an int but does not return any value.

2. Step 2: Identify missing return statement

   It calculates product but never returns it, causing undefined behavior.

3. Final Answer:

   Missing return statement to return the product. -> Option D

4. Quick Check:

   Return missing in non-void function [OK]

Hint: Non-void functions must return a value [OK]

Common Mistakes:

• Forgetting to return value in non-void function
• Thinking multiplication operator is wrong
• Assuming variables are undeclared

5. You want to write a function that returns the maximum of three integers. Which of the following implementations correctly returns the maximum value?

hard

A. int maxOfThree(int a, int b, int c) {
if (a > b && a > c) return a;
else if (b > a && b > c) return b;
else return c;
}

B. int maxOfThree(int a, int b, int c) {
return a > b > c ? a : b > c ? b : c;
}

C. int maxOfThree(int a, int b, int c) {
int max = a;
if (b > max) max = b;
if (c > max) max = c;
return max;
}

D. int maxOfThree(int a, int b, int c) {
if (a > b) return a;
else if (b > c) return b;
else return c;
}

Solution

1. Step 1: Analyze int maxOfThree(int a, int b, int c) { if (a > b && a > c) return a; else if (b > a && b > c) return b; else return c; } logic

   This version correctly checks if b is greater than both a and c in the else-if condition, fixing the missing cases in the original version.

2. Step 2: Analyze int maxOfThree(int a, int b, int c) { int max = a; if (b > max) max = b; if (c > max) max = c; return max; } logic

   This compares all values step-by-step and returns the correct maximum.

3. Step 3: Analyze options C and D

   int maxOfThree(int a, int b, int c) { return a > b > c ? a : b > c ? b : c; } uses invalid chained comparison. int maxOfThree(int a, int b, int c) { if (a > b) return a; else if (b > c) return b; else return c; } misses some cases like when a is not greater than b but b is not greater than c.

4. Final Answer:

   int maxOfThree(int a, int b, int c) { int max = a; if (b > max) max = b; if (c > max) max = c; return max; } -> Option C

5. Quick Check:

   Stepwise max update returns correct max [OK]

Hint: Compare stepwise and return max variable [OK]

Common Mistakes:

• Using chained comparisons incorrectly
• Missing some comparison cases
• Returning before checking all values