CSS Computer Science (Optional) — Paper-I, Section-A, Topic II subtopic. Three closely related, heavily-tested memory concepts.

Arrays

• A fixed-size, contiguous collection of elements of the same type, accessed by index (0-based in C/C++)

int marks[5] = {90, 85, 78, 92, 88};   // marks[0] = 90 ... marks[4] = 88

• Size is fixed at declaration — cannot grow/shrink at runtime (unlike dynamic structures)
• 2D arrays model tables/grids: int matrix[3][4]; — 3 rows, 4 columns, stored row-major in memory (all of row 0, then all of row 1, ...)
• Accessing an index outside the valid range (marks[10] on a 5-element array) is undefined behavior in C/C++ — no automatic bounds checking

Pointers

• A variable that stores the memory address of another variable

int x = 10;
int *p = &x;    // p now holds the address of x
*p = 20;         // dereference: changes x to 20, via p

• & — address-of operator, gets a variable's address
• * — dereference operator, accesses/modifies the value at an address (context-dependent: also used to declare a pointer)
• Pointer arithmetic: p + 1 advances the pointer by sizeof(type) bytes, not by 1 byte — e.g. on an int*, p+1 moves 4 bytes ahead to the next int
• Pointer to pointer: int **pp = &p; — a pointer that stores the address of another pointer, common in dynamic 2D arrays and passing pointers by reference to functions
• NULL pointer: a pointer initialized to NULL/nullptr, pointing to nothing — always check before dereferencing to avoid a crash
• Dangling pointer: a pointer that still holds the address of memory that has been freed/gone out of scope — dereferencing it is undefined behavior

Array–Pointer Relationship

• An array name, in most expressions, decays to a pointer to its first element: marks behaves like &marks[0]
• marks[i] is equivalent to *(marks + i) — array indexing is really pointer arithmetic in disguise
• Key difference: sizeof(marks) gives the whole array's size; sizeof(pointer) gives just the pointer's size (e.g. 8 bytes on a 64-bit system) — a classic exam trap