Lesson 11 — Intermediate

Structures & Unions

Ek student ka naam (char), roll (int) aur marks (float) — teen alag type. Inhe ek jagah rakhne ka tarika hai structure. Is lesson mein hum struct banana, pointer ke saath access, typedef/enum, aur union ka "ek hi memory" wala khel dekhenge.

Structure kaise banta hai

#include <stdio.h>
#include <string.h>

struct Student {
    char name[50];
    int roll;
    float marks;
};

int main() {
    struct Student s1;
    strcpy(s1.name, "Rahul");
    s1.roll = 101;
    s1.marks = 88.5;

    printf("%s %d %.1f\n", s1.name, s1.roll, s1.marks);
    return 0;
}

Output: Rahul 101 88.5

• Dot operator (s1.roll): direct member access.
• Declaration: struct Student s1; — C mein struct keyword dobara likhna zaroori hai (C++ mein nahi).
• Initialization: struct Student s1 = {"Rahul", 101, 88.5}; order member jaisa hona chahiye.
• Size: sab members ka size + padding (alignment) — sizeof se check karo.

Array of structs aur -> operator

#include <stdio.h>

struct Student { char name[20]; int roll; float marks; };

int main() {
    struct Student cls[3] = {
        {"Anu", 1, 90.0}, {"Raj", 2, 76.5}, {"Sia", 3, 85.0}
    };

    for (int i = 0; i < 3; i++)
        printf("%s: %.1f\n", cls[i].name, cls[i].marks);

    struct Student *p = &cls[0];       // pointer to struct
    printf("Roll = %d\n", p->roll);     // -> = pointer se member
    return 0;
}

Output:
Anu: 90.0
Raj: 76.5
Sia: 85.0
Roll = 1

Array of structures hi asli duniya hai — class ki attendance list, bank ki account list, cricket scorecard. Har record ek struct, poora list ek array. DSA mein iska advanced roop linked list banega (size fixed nahi chahiye toh).

typedef aur enum — naam ki suvidha

#include <stdio.h>

typedef struct {
    int x, y;
} Point;                 // Point ab struct ka alias hai

typedef unsigned long big;   // big = unsigned long

enum Day { MON, TUE, WED };  // MON=0, TUE=1, WED=2
enum Color { RED = 1, GREEN = 5, BLUE };  // BLUE = 6 (agle se)

int main() {
    Point p; p.x = 10; p.y = 20;
    printf("(%d,%d)\n", p.x, p.y);
    printf("%d %d %d\n", MON, TUE, WED);
    printf("%d %d %d\n", RED, GREEN, BLUE);
    return 0;
}

Output:
(10,20)
0 1 2
1 5 6

• typedef: existing type ko chhota/meaningful naam. struct ke saath sabse zyada — struct Student likhna nahi padta.
• enum: named integer constants. Default 0 se shuru; explicit de sakte ho; agla value = pichla + 1.
• Kahan enum: status codes, weekdays, menu options — int ki jagah padhne layak naam.
• typedef vs #define: typedef ko compiler samajhta hai (scope, type check), macro andha text replace hai.

Union — ek hi memory sabke liye

#include <stdio.h>

struct S { char a; int b; char c; };
union U { char a; int b; char c; };

int main() {
    printf("struct = %zu\n", sizeof(struct S));  // 12 (alag + padding)
    printf("union  = %zu\n", sizeof(union U));   // 4  (sabse bada member)

    union U u;
    u.b = 65;
    printf("%c %d\n", u.a, u.b);   // A (int ka pehla byte = char)
    return 0;
}

Output (typical):
struct = 12
union  = 4

• Struct: har member ki ALAG memory — sab ek saath store.
• Union: sab members EK hi memory share — isliye size = sabse bada member.
• Ek time pe ek: union mein jo member last likha, wahi valid. Doosra likh pehla mita deta hai.
• Fayda: memory bachana — jab pata ho ek time pe ek hi type chahiye (variant data).

Exercise

Common mistakes

• Pointer pe dot operator laga dena: struct *p = &s; mein s1.roll nahi, p->roll chahiye. Dot value ke liye, arrow pointer ke liye.
• Union members ko parallel samajhna: Union mein sab ek dusre ko overwrite karte hain — teeno ek saath valid nahi rehte (struct ki tarah nahi).
• Initialization ka order bhoolna: struct Student s = {101, "Rahul"} — galat. Order member declaration jaisa hona chahiye: {"Rahul", 101}.

struct Aadhar {
    char naam[30];   // Rahul
    long number;     // 1234
    char dob[11];    // 01-01-2000
};                   // teeno ek saath — struct

union Kyc {          // KYC mein ek time pe ek:
    long pan;
    char passport[10];
};                   // shared memory — size = bada wala