Understanding Functions in C — Part 2: The Magic of Function Parameters and Return Types

 

In the previous post, we explored why functions exist — how they make our code modular, readable, and reusable.
Now, it’s time to peek under the hood and see how data actually travels in and out of a function.

🧩 1. The Function Flow — From Call to Return

When you call a function in C, three things happen:

1. Control jumps from the caller to the function definition.
2. Values are sent as arguments (inputs).
3. A result may or may not come back.

Think of it like ordering a pizza:

• You send parameters (toppings, size, crust type).
• The pizza shop (the function) prepares it.
• You get a return value (your pizza 🍕).

🧠 2. Parameters and Arguments — What’s the Difference?

• Parameters are placeholders in the function definition.
• Arguments are the actual values you pass when calling the function.

Example:

#include <stdio.h>

void greetUser(char name[]) {     // 'name' is a parameter
printf("Hello, %s! Welcome to ProgVeda!\n", name);
}

int main() {
greetUser("Sanchayeta");      // "Sanchayeta" is an argument
    return 0;
}

Output:

Hello, Sanchayeta! Welcome to ProgVeda!

So, parameters are like variable names in the function — they receive values sent from the caller.

🔁 3. Return Type — How Functions Send Data Back

Some functions only do things (like printing), while others calculate and return something.

Example:

#include <stdio.h>

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

int main() {
    int result = add(5, 7);
printf("Sum = %d\n", result);
    return 0;
}

Output:

Sum = 12

Here,

• int add(int a, int b) means the function returns an int.
• The return statement sends the result back.

⚙️ 4. Function Categories Based on Parameters & Return Types

C functions fall into four classic types:

Function Type	Parameters	Return Value	Example
Type 1	No	No	void greet();
Type 2	Yes	No	void greetUser(char name[]);
Type 3	No	Yes	int getLuckyNumber();
Type 4	Yes	Yes	int add(int a, int b);

Example for each:

#include <stdio.h>

// Type 1: No parameter, no return
void greet() {
printf("Hello, C Learner!\n");
}

// Type 2: With parameter, no return
void greetUser(char name[]) {
printf("Hi %s, keep learning C!\n", name);
}

// Type 3: No parameter, with return
int getLuckyNumber() {
    return 7;
}

// Type 4: With parameter, with return
int add(int a, int b) {
    return a + b;
}

int main() {
greet();
greetUser("Ananya");
printf("Lucky Number = %d\n", getLuckyNumber());
printf("Sum = %d\n", add(3, 4));
    return 0;
}

🧪 5. Memory Game — Call by Value in C

When you pass variables to a function in C, only copies are sent.
The original values remain unchanged. This is called Call by Value.

Example:

#include <stdio.h>

void modify(int x) {
    x = 100;
}

int main() {
    int num = 5;
    modify(num);
printf("Value of num = %d\n", num);
    return 0;
}

Output:

Value of num = 5

Even though modify() changed x, it didn’t affect num — because it was just a copy.

💡 Key Takeaways

• Parameters act as placeholders for input data.
• Return values send results back to the caller.
• Every function in C has a return type — void means “no return”.
• Call by Value ensures your original data stays safe.

🧭 Assignment / Lab Challenges

Challenge 1:
Write a function square(int n) that returns the square of a number.
Input: 5 → Output: 25

Challenge 2:
Create a function convertToFahrenheit(float celsius) that returns the equivalent temperature in Fahrenheit.
Formula: F = (C * 9/5) + 32

Challenge 3:
Design a function sumOfDigits(int n) that returns the sum of digits of a number.
Example: Input 123 → Output 6

Challenge 4 (Thinking Corner 🧠):
Why is “Call by Reference” not the default in C?
Try predicting what would happen if it were!

🌱 Coming Up Next (Post 3 Preview)

“Functions Revisited — Local vs Global Variables, Scope, and Storage Classes”
We’ll uncover how C remembers variables inside and outside functions (and how it forgets them too).