Lesson 5 — Beginner

Conditionals (if/else/switch)

Program ko decision lena aata hi nahi toh wo sirf ek line repeat kar sakta hai. Is lesson mein hum seekhenge if/else se conditions, else-if ladder se grades, aur switch se menu-style decisions kaise likhte hain — real programs ke saath.

if-else basics

#include <stdio.h>
int main() {
    int marks = 75;
    if (marks >= 33) {
        printf("Pass\n");
    } else {
        printf("Fail\n");
    }
    return 0;
}

Output: Pass

• Condition ek expression hai: x > 5, x != 0, ch, a && b — sab chalenge.
• else optional hai: sirf if bhi chalega jab "kuch nahi karna" ho.
• Ek statement ho toh braces chhod sakte ho — lekin hamesha lagao (dangling else se bachaa).
• if (x = 5) — assignment, hamesha true! Comparison mein ==.

else-if ladder — grades wala program

#include <stdio.h>
int main() {
    int marks = 82;
    char grade;

    if (marks >= 90)      grade = 'A';
    else if (marks >= 75) grade = 'B';
    else if (marks >= 60) grade = 'C';
    else if (marks >= 33) grade = 'D';
    else                  grade = 'F';

    printf("Marks=%d Grade=%c\n", marks, grade);
    return 0;
}

Output: Marks=82 Grade=B

Order matter karta hai. Agar pehle (marks >= 33) likh dete toh 82 pe hi D mil jaata. Hamesha strict condition (90) se shuru karo, dheere-dheere chhoti karo.

switch-case — menu style decisions

#include <stdio.h>
int main() {
    int day = 3;
    switch (day) {
        case 1: printf("Monday\n");   break;
        case 2: printf("Tuesday\n");  break;
        case 3: printf("Wednesday\n"); break;
        case 4: printf("Thursday\n");  break;
        default: printf("Invalid day\n");
    }
    return 0;
}

Output: Wednesday

• Switch value integral honi chahiye: int, char, enum — float/double nahi.
• break zaroori hai: warna ek case ka baad agla case bhi chalta hai (fall-through).
• default optional hai — kuch match na ho tab chalta hai (aakhri mein rakho).
• Case values constant honi chahiye (5, 'A') — variable nahi. Aur duplicate case = compile error.

// Fall-through ka maza (bina break):
int n = 2;
switch (n) {
    case 1: printf("one ");
    case 2: printf("two ");   // break nahi — ghusta rehta
    case 3: printf("three ");
}
// Output: two three   (2 se shuru, 3 tak chala!)

Ternary, nesting aur safe patterns

int a = 10, b = 20;
int max = (a > b) ? a : b;          // 20
printf("%s", (a == 10) ? "dus" : "aur"); // dus

// Nested if — chhota example
if (a > 0) {
    if (a > 10) printf("bada positive\n");
    else        printf("chhota positive\n");
}

// Nested ternary (padhna mushkil — kam use karo)
int r = (x > 90) ? 4 : (x > 75) ? 3 : 2;

• Operator precedence check: a > b ? a : b chalega, par && ke saath brackets lagao — (a > 0 && b > 0) ? ... warna && pehle chal jaata hai.
• Best practice: chhoti condition = ternary, lambi logic = if-else. Readability sabse upar.
• goto se door raho: jump karta hai par flow kharab karta hai — aaj kal sirf error cleanup (out of loop) mein limited use.
• if-else if chain: teen se zyada choices ho toh ladder hi best hai.

Exercise

Common mistakes

• Break bhool jaana switch mein: Bina break ke fall-through — ek case ke baad saare agle case chalte rehte hain. Output questions mein ye sabse common trap hai.
• Ladder ka order ulta rakhna: if (marks >= 33) pehle likh diya toh 90 wale ko bhi D milega. Hamesha sabse strict condition pehle.
• Dangling else: Braces ke bina else kisi aur if se jud jaata hai (nearest se). Nesting mein hamesha braces likho — bug chhupta hai.

#include <stdio.h>
int main() {
    int item = 2;
    switch (item) {
        case 1: printf("Chai = Rs 10\n"); break;
        case 2: printf("Samosa = Rs 20\n"); break;
        case 3: printf("Idli = Rs 30\n"); break;
        default: printf("Item nahi hai\n");
    }
    return 0;
}
Output: Samosa = Rs 20