Lesson 9 — Intermediate

Functions in C

Function code ka reusable block hai — ek baar likho, hazaar jagah bulao. Is lesson mein hum dekhenge function kaise banate hain, arguments kaise jaate hain (aur kyun original change nahi hota), recursion kya hai, aur static/local/global variables mein kya farak hai.

Function kaise banta hai — prototype aur definition

#include <stdio.h>

int add(int a, int b);        // prototype (declaration)

int main() {
    printf("%d\n", add(5, 3));   // 8
    printf("%d\n", add(10, 20)); // 30
    return 0;
}

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

Output:
8
30

• Return type: int, float, void (kuch nahi lauta), char — function ke baad ka type.
• Parameters: (int a, int b) — input. Call ke waqt asli values (arguments) jaati hain.
• Prototype: definition se pehle likho — compiler ko pehle se pata chal jaata hai, warna implicit declaration warning (C99 se error).
• Ek hi main: lekin functions kitne bhi — same file ya doosri file (link karte waqt jodna).

Pass by value — copy hi jaati hai

#include <stdio.h>
void show(int x) {
    x = 100;             // sirf LOCAL copy badli
}
int main() {
    int a = 10;
    show(a);
    printf("a = %d\n", a);   // a = 10 (change nahi hua!)
    return 0;
}

Output: a = 10

C mein arguments pass by value hain — function ko asli variable nahi, uski copy milti hai. Function apni copy pe chahe kare, bahar wali variable jaisi thi waisi hi rehti hai.

// Value lautani ho — return use karo
int max(int a, int b) {
    return (a > b) ? a : b;
}
int main() {
    int m = max(10, 30);    // 30 assign ho gaya
    printf("%d\n", m);
}
// Output: 30

Recursion — function khud ko bulaye

#include <stdio.h>

int fact(int n) {
    if (n == 0) return 1;        // base case — yahan rukna hai
    return n * fact(n - 1);      // khud ko bulaya
}

int fib(int n) {
    if (n <= 1) return n;        // fib(0)=0, fib(1)=1
    return fib(n - 1) + fib(n - 2);
}

int main() {
    printf("fact(5) = %d\n", fact(5));  // 120
    printf("fib(6) = %d\n", fib(6));    // 8
    return 0;
}

Output:
fact(5) = 120
fib(6) = 8

fact(5) = 5 × fact(4) = 5 × 4 × fact(3) ... = 5 × 4 × 3 × 2 × 1 × 1 = 120. Har call apna frame stack pe rakhta hai — isliye base case ke bina stack overflow.

• Do zaroori cheezein: base case (rukne ki shart) + recursive case (chhoti problem pe call).
• Fibonacci tree: fib(6) ke liye bahut saare dohra calls — O(2^n) = dheema. Memoization/iteration se behtar hota hai.
• Fayda: tree, sorting, divide-and-conquer problems recursion mein aasaan likhe jaate hain.

Scope — local, global aur static

#include <stdio.h>
int count = 0;                 // GLOBAL — poori file mein

void counter(void) {
    static int s = 0;          // STATIC — ek baar init, phir yaad
    s++;
    count++;
    printf("static=%d global=%d\n", s, count);
}
int main() {
    counter();   // static=1 global=1
    counter();   // static=2 global=2
    counter();   // static=3 global=3
    return 0;
}

• Local: function ke andar — call khatam, value gayi. Sabse safe.
• Global: file ke top pe — sab functions dekh sakte hain. Zyada use = code mushkil debug (koi bhi badal deta hai).
• Static: local dikhata hai par function calls ke beech VALUE YAAD rakhta hai (ek baar init hoti hai).
• Shadowing: local aur global ka naam same ho toh function ke andar wala jeet-ta hai — confusion ka source.

Exercise

Common mistakes

• Function ke andar original variable badalne ki koshish: Pass by value hai — copy milti hai. Change chahiye toh pointer bhejo (&x) ya return karwao.
• Base case bhool jaana recursion mein: Base case nahi toh call stack badhta hi jaayega = stack overflow = program crash. Pehle rukne ki shart likho.
• Prototype aur definition ka signature alag rakhna: Prototype mein int add(int, int) aur definition mein float add(float...) = link error/undefined behaviour. Dono match karne chahiye.

#include <stdio.h>
void bill(void) {
    static int total = 0;   // memory mein bana rehta hai
    total += 50;
    printf("Bill total = %d\n", total);
}
int main() {
    bill();   // Bill total = 50
    bill();   // Bill total = 100
    bill();   // Bill total = 150
    return 0;
}