Structures, Unions, and Bit Manipulation

CSS Computer Science (Optional) — Paper-I, Section-A, Topic II subtopic. Grouping data under one name, and manipulating data at the bit level.

Structures (struct)

Groups different data types under one name; each member gets its own separate memory, so the struct's total size is (at least) the sum of its members' sizes

struct Student {
    char name[20];
    int age;
    float gpa;
};
struct Student s1 = {"Ali", 20, 3.8};
printf("%s", s1.name);       // dot operator to access members

Members are accessed with the dot operator (.) on a struct variable, or the arrow operator (->) via a pointer to a struct: ptr->name
Nested structures: a struct can contain another struct as a member
Structures can be passed to functions by value (a full copy) or by reference/pointer (to avoid copying and/or allow modification)

Unions

Groups different data types under one name but all members share the same memory location — total size equals the size of the largest member, not the sum

union Data {
    int i;
    float f;
    char c;
};
union Data d;
d.i = 10;     // writing to i overwrites the same memory f and c would use

Useful when only one of several possible types is needed at a time (memory-saving), e.g. representing a value that could be an int OR a float depending on context
Writing to one member and then reading a different member gives meaningless/reinterpreted data — a frequently tested distinction from struct

Struct vs Union

	struct	union
Memory per member	Separate for each	Shared (overlapping)
Total size	Sum of all members (plus padding)	Size of the largest member
Simultaneous access	All members valid at once	Only the most recently written member is valid

Enumerations (enum)

A user-defined type consisting of a set of named integer constants, improving readability over "magic numbers"

enum Day {SUN, MON, TUE, WED, THU, FRI, SAT};   // SUN=0, MON=1, ... by default
enum Day today = WED;   // today holds the integer value 3

Bit Manipulation Operators

Operator	Name	Effect
&	AND	1 only if both bits are 1
		OR
^	XOR	1 if bits differ
~	NOT	Flips every bit (1's complement)
<<	Left shift	Shifts bits left; each shift multiplies by 2
>>	Right shift	Shifts bits right; each shift divides by 2 (for unsigned/positive values)

Example: 6 & 3 → 0110 & 0011 = 0010 = 2; 6 | 3 → 0110 | 0011 = 0111 = 7; 6 ^ 3 → 0101 = 5