Control flow — if - else, switch, while, do-while, for, break, continue, goto

5.1.6 · Coding › C Programming

Control flow kyun exist karta hai

Selection: kaunsa path lena hai yeh decide karna

if / else

if (condition) {
    // runs when condition != 0
} else if (other) {
    // runs when other != 0
} else {
    // fallback
}

Worked example Ek number ko classify karo

int n = -3;
if (n > 0)       printf("positive");
else if (n == 0) printf("zero");
else             printf("negative");   // prints this

Yeh step kyun? Conditions order mein, upar se neeche check hoti hain; pehli true condition jeet jaati hai aur baaki skip ho jaati hain. Kyunki n>0 false hai aur n==0 false hai, hum else tak pahunch jaate hain.

switch

switch (expr) {
    case 1:  /* ... */ break;
    case 2:  /* ... */ break;
    default: /* none matched */
}

Worked example Cases ko deliberately group karna

switch (grade) {
    case 'A':
    case 'B': printf("Pass with distinction"); break;
    case 'C': printf("Pass"); break;
    default:  printf("Fail");
}

Yeh step kyun? 'A' mein koi break nahi hai, isliye woh deliberately 'B' ke code mein fall through karta hai — dono ek hi message print karte hain.

Iteration: kaam repeat karna

while — pehle test karo

int i = 0;            // 1 init
while (i < 3) {       // 2 condition (checked first)
    printf("%d ", i); // 3 body
    i++;              // 4 update
}                     // prints: 0 1 2

do-while — baad mein test karo

int i = 5;
do {
    printf("%d ", i);   // prints 5 once even though 5<3 is false
    i++;
} while (i < 3);

for — compact counting loop

for (int i = 0; i < 3; i++) {
    printf("%d ", i);
}

Figure — Control flow — if - else, switch, while, do-while, for, break, continue, goto

Jumps: break, continue, goto

Worked example break vs continue

for (int i = 0; i < 5; i++) {
    if (i == 2) continue;  // skip printing 2
    if (i == 4) break;     // stop entirely before 4
    printf("%d ", i);
}                          // prints: 0 1 3

Yeh step kyun? i==2 par, continue i++ par jump karta hai (isliye 2 kabhi print nahi hota). i==4 par, break loop se bahar nikal jaata hai, isliye 4 bhi print nahi hota — aur hum 3 pichli iteration mein already print kar chuke hain.

Worked example Sahi goto — nested loops todna

for (int i = 0; i < N; i++)
    for (int j = 0; j < M; j++)
        if (grid[i][j] == target)
            goto found;     // break only exits inner loop; goto escapes both
found:
    printf("done");

Yeh step kyun? Ek akela break sirf inner j loop se exit karta hai. goto dono se ek saath clean bahar nikal jaata hai.

Mnemonic

Feynman check

Recall Ek 12-saal ke bachche ko samjhao

Socho tum ek recipe follow kar rahe ho. if/else = "agar dough sticky hai, toh flour daalo; warna bake karo." switch = ek vending machine: ek button dabaao (A1, A2...) aur tum seedha us snack par pahunch jaate ho. while = "jab tak gaaanth hai tab tak chalate raho hilaana." do-while = "ek baar hilao, phir agar abhi bhi gaaanth hai toh hilate raho." for = "exactly 10 baar hilao." break = "ABHI hilana band karo." continue = "yeh ek hilana skip karo aur agle par jao." goto = ek magic trapdoor jo tumhe recipe mein kisi aur marked jagah teleport kar deta hai — kabhi-kabhi kaam aata hai, aksar confusing hota hai.

Active recall

C mein, ek condition "true" kab hoti hai?

Jab uski value koi bhi non-zero ho (false sirf tab hoti hai jab exactly 0 ho).

if (x = 5) actually kya karta hai?

x mein 5 assign karta hai; expression ki value 5 hoti hai (non-zero) → hamesha true. Bug.

switch mein har case mein break kyun chahiye?

Execution agle case ke code mein fall through hoti rehti hai jab tak break ya block end na aaye.

switch expression kya ho sakta hai?

Ek integer ya char (integral) type jo constant case labels se compare hota hai.

while aur do-while mein kya fark hai?

while pehle test karta hai (0 baar chal sakta hai); do-while baad mein test karta hai (kam se kam ek baar chalta hai).

for≡while equivalence do.

for(A;B;C){S} ≡ A; while(B){ S; C; }

break kya karta hai?

Innermost enclosing loop ya switch se turant exit karta hai.

continue kya karta hai?

Current iteration ka baaki hissa skip karta hai aur loop ke update/condition test par jaata hai.

Hand-written while loop mein continue infinite loops kyun cause kar sakta hai?

Yeh increment statement ko skip kar sakta hai agar update continue ke baad rakha ho.

C mein goto ka main legitimate use kya hai?

Deeply nested loops se bahar nikalna ya centralized error/cleanup handling.

Do nested loops ke andar break kya karta hai?

Sirf innermost loop se exit karta hai, dono se nahi.

for(i=0;i<5;i++){if(i==2)continue; if(i==4)break; printf("%d",i);} ka output kya hai?

0 1 3

Connections

C Operators — ==, =, &&, ||, ! woh conditions produce karte hain jo branches test karti hain.
Loops and Time Complexity — nested loops → $O (n^{2})$ etc.
Functions in C — return ek aur control-flow jump hai; goto function-local hai.
Boolean Logic — conditions mein &&/|| ki short-circuit evaluation.
Recursion — repetition ke liye iteration ka ek alternative.