Lesson 2 — Beginner

Variables & Data Types

Variable ek aisi jagah hai jahan data save hota hai — jaise dukaan mein har saman ka dabba. Is lesson mein hum dekhenge variable kaise banate hain, kaunsa data type kab use hota hai, aur int/float ke beech ki conversion traps kaise bachein.

Variable kya hai aur kaise banta hai

int age = 20;          // poora number
float price = 99.5f;    // decimal
char grade = 'A';       // ek character
double pi = 3.14159265; // bada decimal (15-16 digits)

int a, b, c;           // teeno ek saath declare
a = 10; b = 20;        // ab value daali

• Naming rules: sirf letters, digits aur underscore (_). Shuruaat letter ya underscore se — digit se nahi.
• Case-sensitive: Age aur age do alag variables hain.
• Keywords mat use: int, return, if — ye reserved hain naam ke liye.
• No spaces: student_name chalega, student name nahi.

Core data types aur size

Type      Size*      Format    Example
int       4 bytes   %d        int age = 20;
float     4 bytes   %f        float pi = 3.14f;
double    8 bytes   %f        double big = 1.234567;
char      1 byte    %c        char ch = 'A';
long int  4/8 bytes %ld       long big = 1000000L;
short     2 bytes   %hd       short s = 100;

* typical 32/64-bit compiler pe — hamesha sizeof se check karo

#include <stdio.h>
int main() {
    printf("int=%zu float=%zu char=%zu double=%zu\n",
           sizeof(int), sizeof(float), sizeof(char), sizeof(double));
    return 0;
}

Output (typical):
int=4 float=4 char=1 double=8

Qualifiers — const, signed, unsigned

const float PI = 3.14159;   // fix value — badal nahi sakte
// PI = 3.0;                // ERROR: assignment to constant

unsigned int x = 4000000000; // sirf positive — range double
int y = -5;                  // signed by default (negative allowed)

int a = 10;   // variable — badalta rehta hai
const int b = 20; // constant — pura program mein same

• signed vs unsigned: signed mein ek bit sign ke liye (negative + positive), unsigned mein saari bits value ke liye — isliye unsigned ka positive range dugna.
• const: initialize karte waqt do aur phir kabhi mat chhua. Configuration values ke liye best.
• char ka dhoka: char signed ya unsigned ho sakta hai compiler pe — isliye 200 jaisi values store karne mein overflow ho sakta hai.
• Literal suffix: 3.14f (float), 100L (long), 'A' (char) — compiler ko extra hint.

Type conversion — integer division ka trap

Jab do alag type ke values ka operation hota hai, C chhoti type ko badi type mein convert kar deta hai — ye implicit conversion (automatic promotion) hai. Aur tum khud convert karo toh wo explicit casting.

#include <stdio.h>
int main() {
    int a = 7, b = 2;
    printf("%d\n", a / b);          // 3  — dono int, fraction gaya!
    printf("%f\n", (float)a / b);   // 3.500000 — cast kiya tab sahi

    int x = 5.9;                    // truncation → 5
    float f = 43;                   // promotion  → 43.000000
    printf("%d %f\n", x, f);
    return 0;
}

Output:
3
3.500000
5 43.000000

Exercise

Common mistakes

• Bina declare kiye variable use kar lena: C mein pehle declaration phir use. Compiler error aayega "undeclared" — pehle type batao, tabhi memory milegi.
• 7/2 = 3.5 samajhna: Do int ka division int hi hai — 3.5 ke liye (float) cast lagao. Output questions mein ye sabse common trap hai.
• Float ko exactly 0.3 samajhna: Floating point decimal ko hamesha exact nahi rakh paata (0.1 + 0.2 = 0.300000...). Isliye float ko == se compare mat karo — tolerance use karo.

#include <stdio.h>
int main() {
    float a = 0.1, b = 0.2;
    printf("%.10f\n", a + b);
    if (a + b == 0.3) printf("exact 0.3\n");
    else printf("0.3 NAHI hai!\n");
    return 0;
}
Output: 0.3000000119
        0.3 NAHI hai!
Fix: integer paise rakho (20 + 30 = 50 paise) ya tolerance do.