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Structure vs Union in C

Both struct and union are user-defined data types in C that group multiple members together. They differ fundamentally in how memory is allocated and how members are accessed.

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Comparison Table

FeatureStructure (struct)Union (union)
Keywordstructunion
MemoryOwn memory per memberShared memory
SizeSum of all membersSize of largest member
Values heldAll at onceOnly the last-written one
AccessMultiple at onceOne at a time
InitializationMultiple membersOnly the first member
Use caseGroup related fields of different typesStore one of several types at a time

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Memory Illustration

struct S {
    int   a;    // 4 bytes
    float b;    // 4 bytes
    char  c;    // 1 byte
};
// sizeof(struct S) ≈ 12 bytes (with padding)
// Each member at a different address

union U {
    int   a;    // 4 bytes
    float b;    // 4 bytes
    char  c;    // 1 byte
};
// sizeof(union U) = 4 bytes (largest member)
// All members share the same address

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Example: Side by Side

#include <stdio.h>

struct S { int a; float b; };
union  U { int a; float b; };

int main() {
    struct S s;
    union  U u;

    s.a = 10;  s.b = 3.14f;
    u.a = 10;  u.b = 3.14f;

    // struct: both values preserved
    printf("Struct: a=%d, b=%.2f\n", s.a, s.b);

    // union: only b is valid (overwrote a)
    printf("Union : a=%d, b=%.2f\n", u.a, u.b);

    printf("Size of struct S = %lu bytes\n", sizeof(s));
    printf("Size of union  U = %lu bytes\n", sizeof(u));

    return 0;
}
Output: 
Struct: a=10, b=3.14
Union : a=1078523331, b=3.14    ← a is garbage (overwritten)
Size of struct S = 8 bytes
Size of union  U = 4 bytes

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Enum in C

An enum (Enumeration) is a user-defined type that assigns meaningful names to integer constants. It makes code more readable. 
enum Week {
    Monday,     // 0
    Tuesday,    // 1
    Wednesday,  // 2
    Thursday,   // 3
    Friday,     // 4
    Saturday,   // 5
    Sunday      // 6
};
By default, the first constant = 0, and each subsequent one increments by 1. Custom values: 
enum Level {
    LOW    = 1,
    MEDIUM = 5,
    HIGH   = 10
};
Complete example: 
#include <stdio.h>

enum Week { Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday };

int main() {
    enum Week today = Wednesday;
    printf("Day number: %d", today);  // Output: 2
    return 0;
}