Lesson 6 — Beginner

Loops (for/while/do-while)

Do line ka kaam 100 baar? Loop. Is lesson mein hum teeno loops seekhenge — for (count), while (condition), do-while (kam se kam ek baar) — aur break/continue se unhe control karna. Sath mein factorial, prime jaise asli programs bhi banayenge.

for aur while — do baap ek beta

// for — count known ho
for (int i = 1; i <= 5; i++) {
    printf("%d ", i);
}
// Output: 1 2 3 4 5

// while — count unknown, condition chalaye
int n = 5;
while (n > 0) {
    printf("%d ", n);
    n--;
}
// Output: 5 4 3 2 1

For ke teen hisse: shuruaat (init) → condition (check) → update (++). Iska order yaad rakho: pehle init ek baar, phir har baar check → body → update. While mein init bahar, update andar likhna padta hai.

do-while — pehle kaam phir poochho

int i = 10;
do {
    printf("%d\n", i);
    i++;
} while (i < 5);
// Output: 10   (condition pehle se FALSE phir bhi ek baar chala!)

• Fark sirf ek hai: do-while body PEHLE chalti hai, condition BAAD mein check hoti hai — isliye kam se kam ek iteration pakka.
• Kahan use ho: menu programs — pehle option dikhao, phir poochho, invalid ho toh dobara.
• Syntax dhyan: body ke baad ) while (...); — semicolon bhoolna = compile error.
• Kab for/while: 99% jagah. Do-while sirf "ek baar chahiye hi chahiye" wale case mein.

// Menu wala classic do-while
int choice;
do {
    printf("1. Chai  2. Coffee  0. Exit\n");
    scanf("%d", &choice);
    if (choice == 1) printf("Chai bana raha...\n");
    else if (choice == 2) printf("Coffee bana raha...\n");
} while (choice != 0);

break, continue aur infinite loops

for (int i = 1; i <= 10; i++) {
    if (i == 5) continue;   // 5 CHHOD do — age badho
    if (i == 8) break;      // 8 pe POORA loop band
    printf("%d ", i);
}
// Output: 1 2 3 4 6 7

while (1) {              // infinite loop...
    printf("spin ");
    break;                 // ...tabhi bahar nikalta hai
}
// Output: spin

• break: sabse andar wale loop se turant bahar. Nested loop mein sirf ek level chhodta hai.
• continue: current iteration chhod kar agle iteration pe seedha.
• Infinite loop: while (1), for (;;) — menu/search jahan tak condition khud decide ho.
• Trap: update statement likhna bhool gaye (i++ nahi) toh condition hamesha true = infinite. CPU 100%.

Asli programs — factorial aur prime

#include <stdio.h>
int main() {
    // Factorial: 5! = 120
    int n = 5, f = 1;
    for (int i = 1; i <= n; i++) f *= i;
    printf("Factorial = %d\n", f);

    // Prime check: 29 prime hai
    int num = 29, flag = 0;
    for (int i = 2; i * i <= num; i++)
        if (num % i == 0) { flag = 1; break; }
    printf("%d is %s\n", num, flag ? "Prime" : "Not Prime");
    return 0;
}

Output:
Factorial = 120
29 is Prime

i * i <= num ka matlab hai half tak check karne ki zarurat nahi — square root tak kaafi hai. Isse 10^9 tak ka number bhi tez check ho jaata hai. Ye optimization interview mein poochhi jaati hai.

Exercise

Common mistakes

• Update statement bhool jaana: for (i = 0; i < n;) jaisi line = infinite loop. Condition mein ya body mein update likhna zaroori hai.
• Loop ki condition mein <= aur < ka farak: i <= n mein n bhi chalega, i < n mein nahi. Array mein hamesha i < size — kyunki index 0 se hai. Off-by-one error ka main source.
• break aur continue ulta samajhna: break = poora loop band, continue = sirf ye wala round chhod. Output questions mein dono ka farak exam mein test hota hai.

do {
    order = next_customer();   // ek kaam
    make_chai(order);
} while (order != NULL);      // jab tak customer

Infinite day: while (1) { work(); } — break = ghar jaana