Functions

Theory

Functions in C are reusable blocks of code that perform specific tasks. They help organize code, avoid repetition, and make programs easier to debug and maintain.

A function consists of a declaration (prototype) and a definition. The declaration tells the compiler about the function's name, return type, and parameters. The definition contains the actual code.

// Function prototype (declaration)
int add(int a, int b);
 
// Function definition
int add(int a, int b) {
    return a + b;
}

Return types: Functions can return any data type using the return statement. A function that returns nothing uses void.

Parameters:

• Formal parameters — the variables in the function definition
• Actual arguments — the values passed when calling the function

C uses pass-by-value — functions receive copies of the arguments, not the originals. To modify a variable in a calling function, pass its address (using pointers).

Function prototypes (declarations) are typically placed before main() or in header files. They allow the compiler to verify correct usage even if the function definition appears later.

Scope rules:

• Local variables — declared inside a function, accessible only within it
• Global variables — declared outside all functions, accessible everywhere
• Static variables — retain their value between function calls

void counter() {
    static int count = 0;  // initialized once
    count++;
    printf("%d ", count);
}

Recursion occurs when a function calls itself. Every recursive function needs a base case to stop the recursion.

int factorial(int n) {
    if (n <= 1) return 1;      // base case
    return n * factorial(n - 1); // recursive case
}

Inline functions (C99) suggest the compiler to expand the function code at the call site, avoiding function call overhead for small functions:

inline int square(int x) { return x * x; }

The math library (<math.h>) provides functions like sqrt(), pow(), sin(), cos(), abs(). Compile with -lm flag.

Practical Examples

Exercises

#include <stdio.h>\n\n// Function prototypes\n\nint main() {\n  int a = 20, b = 5;\n  // Call and print results\n  return 0;\n}

20 + 5 = 25\n20 - 5 = 15\n20 * 5 = 100\n20 / 5 = 4.00

#include <stdio.h>\n\nint power(int base, int exp) {\n  // Your recursive code here\n}\n\nint main() {\n  // Test with 2^10\n  return 0;\n}

2^10 = 1024\n3^4 = 81\n5^0 = 1

#include <stdio.h>\n\nint uniqueID() {\n  static int id = 1000;\n  // Your code here\n}\n\nint main() {\n  // Call and print 5 IDs\n  return 0;\n}

ID: 1000\nID: 1001\nID: 1002\nID: 1003\nID: 1004

Mini Quiz

Mini Project