Arrays & Strings in C

Arrays — A Row of Lockers

Imagine a row of school lockers, all side by side, each with a number on the door. An array is exactly that:

int marks[5] = {80, 75, 90, 60, 85};

Locker:   [0]  [1]  [2]  [3]  [4]
Value:     80   75   90   60   85
Address:  200  204  208  212  216    (each int = 4 bytes)

Rules: - All elements are the same type and same size - They sit next to each other in memory (contiguous) - The name marks is the address of the first locker (locker 0)

So marks[2] means "go to locker 0's address, skip 2 × 4 = 8 bytes, open that locker" → value 90.

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2D Arrays — A Grid of Lockers

A 2D array is like a grid in a spreadsheet. But in memory, it's stored as one long row (row by row).

int mat[3][4];   // 3 rows, 4 columns

How it looks logically:       How it sits in memory:
Col:  0   1   2   3
Row 0: a  b  c  d            a  b  c  d  e  f  g  h  i  j  k  l
Row 1: e  f  g  h            ↑                ↑                ↑
Row 2: i  j  k  l         mat[0][0]        mat[1][0]        mat[2][0]

This is called row-major order — row 0 comes first, then row 1, then row 2.

Address Formula

To find where mat[i][j] lives:

Address = Base + (i × num_columns + j) × sizeof(element)

Example: mat[2][1] in a 3×4 int matrix, base address = 1000:

Address = 1000 + (2 × 4 + 1) × 4
        = 1000 + (8 + 1) × 4
        = 1000 + 36
        = 1036

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Array Decay — The Sneaky Rule

When you pass an array to a function, C secretly converts it to a pointer to the first element. The function has no idea how big the array is.

void print(int arr[], int n) {
    // arr is actually int* here
    // sizeof(arr) gives sizeof(int*) = 8, NOT the array size!
}

int main() {
    int nums[5] = {1,2,3,4,5};
    print(nums, 5);  // must pass size separately
}

This is why C functions always take a separate n for the array size.

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Strings — Arrays with a Secret Ending

A string in C is just a char array with a special marker at the end: the null character \0 (ASCII value 0).

char name[] = "gate";

Stored as:   g   a   t   e  \0
Index:       0   1   2   3   4
ASCII:      103  97 116 101   0

The null terminator tells string functions "stop here." Without it, functions like printf keep reading memory until they accidentally find a 0 — which causes garbage output or a crash.

char array vs char pointer

char arr[] = "hello";   // copy on stack — YOU OWN this memory, can modify
char *ptr  = "hello";   // pointer to READ-ONLY literal in code segment

arr[0] = 'H';  // OK — modifies your own copy
ptr[0] = 'H';  // UNDEFINED BEHAVIOUR — string literal is read-only

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Common String Functions (string.h)

Function	Example	What it does
strlen(s)	strlen("gate") → 4	Length (does NOT count \0)
strcpy(dst, src)	strcpy(a, "hi")	Copies src into dst
strcat(dst, src)	strcat(a, "!")	Appends src to dst
strcmp(s1, s2)	strcmp("a","b") → negative	0 if equal, <0 if s10 if s1>s2
strchr(s, c)	strchr("gate", 'a') → pointer to 'a'	First occurrence of char

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Step-by-Step Example: 2D Address Calculation

GATE question style: A 2D array B[4][5] is stored in row-major order. Base address is 2000. Each element is 2 bytes. What is the address of B[3][2]?

Step 1: formula = Base + (i × cols + j) × size
Step 2:         = 2000 + (3 × 5 + 2) × 2
Step 3:         = 2000 + (15 + 2) × 2
Step 4:         = 2000 + 17 × 2
Step 5:         = 2000 + 34
Answer:         = 2034

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Quick Check

Q1. What does strlen("\0hello") return? Answer: 0 — strlen stops at the first \0, which is at index 0.

Q2. Array A[6][8] of integers (4 bytes each), base = 1000, row-major. Address of A[2][3]?

= 1000 + (2 × 8 + 3) × 4 = 1000 + 19 × 4 = 1000 + 76 = 1076