Arrays

ARRAYS IN C 

An array is a collection of multiple values of the same data type, stored in contiguous (continuous) memory locations.

It allows storing many values under one variable name.

Example

int marks[5];

This creates space for 5 integers.

Array Notation and Representation

Every array has:

• Name: identifier
• Index: starts from 0
• Elements: values stored inside
• Size: number of elements

Example

int a[5] = {10, 20, 30, 40, 50};

Index mapping:

Index	Value
0	10
1	20
2	30
3	40
4	50

Memory is stored continuously.

Declaring a One-Dimensional Array

Syntax

data_type array_name[size];

Example

int num[10];
float price[50];
char grade[5];

Size must be a positive integer.

Initializing Arrays

You can assign initial values at the time of declaration.

A. Full Initialization

int a[5] = {1, 2, 3, 4, 5};

B. Partial Initialization

Remaining values become 0.

int a[5] = {10, 20};

C. Without specifying size

Compiler determines size.

int a[] = {1, 2, 3, 4};

D. String Initialization

Char arrays behave like strings.

char name[] = "Hello";

Accessing Array Elements

Use the index number.

Example

a[0] = 10;      // store
printf("%d", a[0]);  // access

Manipulating Array Elements

You can update, modify, or process array values using loops.

Example: Adding 5 to every element

for (int i = 0; i < 5; i++) {
    a[i] += 5;
}

Example: Searching for an element

for (int i = 0; i < n; i++) {
    if (a[i] == key) {
        printf("Found");
    }
}

Arrays of Unknown or Varying Size

A. Using user input

Size is given by user.

int n;
scanf("%d", &n);
int arr[n];   // allowed in C99 as Variable-Length Array (VLA)

B. Using dynamic memory (malloc)

For very large or flexible arrays.

int *arr = malloc(n * sizeof(int));

Two-Dimensional Arrays (2D Arrays)

2D arrays are like tables with rows and columns.

Syntax

data_type array_name[rows][columns];

Example

int matrix[3][3];

Initialization

int matrix[3][3] = {
    {1,2,3},
    {4,5,6},
    {7,8,9}
};

Accessing Elements

printf("%d", matrix[1][2]);   // prints 6

Multidimensional Arrays

Arrays with more than 2 dimensions (3D, 4D...).

Example of 3D Array

int box[2][3][4];

This means:

• 2 tables
• each table has 3 rows
• each row has 4 columns

Accessing elements

box[1][2][3];

Difference Between 1D, 2D & Multi-D Arrays

Type	Structure	Example
1D	Single row	int a[5]
2D	Table (rows × columns)	int b[3][3]
3D & above	Multiple tables	int c[2][3][4]

Real-Life Uses of Arrays

Area	Use
Data science	Storing datasets
Matrices	Mathematical operations
Image processing	Pixel representation (2D/3D arrays)
Games	Storing maps, scores
Banking	Customer records

Example Program (2D Array Sum)

#include <stdio.h>

int main() {
    int a[2][2] = {{1,2},{3,4}};
    int sum = 0;

    for (int i = 0; i < 2; i++) {
        for (int j = 0; j < 2; j++) {
            sum += a[i][j];
        }
    }

    printf("Sum = %d", sum);
    return 0;
}

SUMMARY TABLE

Concept	Explanation
Array	Collection of similar data
1D Array	Linear structure
2D Array	Row-column matrix
Initialization	Assigning values
Accessing	Using index
Manipulation	Updating array values
VLA	Variable Length Arrays
Multidimensional	3D, 4D… arrays

POINTERS IN C 

Introduction to Pointers

A pointer is a variable that stores the memory address of another variable.

Example:

int a = 10;
int *p = &a;

Here:

• a → stores value 10
• p → stores address of a
• *p → gives value of a (dereferencing)

Why pointers are important?

• Memory management
• Dynamic allocation
• Call by reference
• Efficient array handling
• Structures, linked lists, trees
• Function pointers

Characteristics of Pointers

1. Pointers store address, not value
2. They consume fixed memory (usually 4 or 8 bytes)
3. Pointer type must match the variable type
4. Pointer arithmetic is possible (+, -, ++, --)
5. Pointers are powerful but require careful usage

& and * Operators

& (Address-of operator)

Used to get the address of a variable.

int x = 5;
printf("%p", &x);   // prints address of x

* (Dereference operator)

Used to access the value stored at the address.

int x = 5;
int *p = &x;
printf("%d", *p);   // prints 5

Pointer Type Declaration and Assignment

Declaration

int *p;
float *q;
char *c;

Assignment

int a = 20;
p = &a;   // pointer stores address of a

Pointers must be assigned addresses, not values.

Pointer Arithmetic

Valid operations for pointers:

Operation	Meaning
p++	Move to next memory location
p--	Move to previous location
p + n	Jump forward n elements
p - n	Move backward n elements

Example

int arr[3] = {10, 20, 30};
int *p = arr;   // arr = &arr[0]

printf("%d", *(p+1));  // prints 20

Pointer arithmetic jumps based on data type size.

For int (4 bytes):

• p+1 → next 4-byte block

Call By Reference Using Pointers

C does not support call-by-reference directly.
But we can simulate it using pointers.

Example

void change(int *n) {
    *n = 50;
}

int main() {
    int a = 10;
    change(&a);
    printf("%d", a); // 50
}

Passing Pointers to Functions

Pointers allow functions to modify actual variables.

Example

void update(int *x) {
    *x += 10;
}

Calling:

int n = 5;
update(&n);

Array and Pointers

Arrays and pointers are closely related.

Example

int a[5] = {1,2,3,4,5};
int *p = a;   // same as &a[0]

• p[0] = 1
• *(p+2) = 3

Array of Pointers

An array that stores addresses.

Example: array of strings

char *names[] = {"Ram", "Shyam", "Mohan"};

Example: array of integer pointers

int a = 10, b = 20, c = 30;
int *arr[] = {&a, &b, &c};

Pointer to Pointer (Double Pointer)

A pointer that stores the address of another pointer.

Syntax

int **pp;

Example

int x = 5;
int *p = &x;
int **pp = &p;

printf("%d", **pp);  // prints 5

Pointer to Function

A pointer that stores the address of a function.

Syntax

return_type (*pointer_name)(parameter_list);

Example

int add(int x, int y) {
    return x + y;
}

int (*fptr)(int, int) = add;
printf("%d", fptr(5, 10)); // 15

Uses:

• Callbacks
• Event handling
• Dynamic function selection

Array of Function Pointers

Stores multiple function addresses.

Example

int add(int a, int b) { return a + b; }
int sub(int a, int b) { return a - b; }

int (*op[2])(int, int) = {add, sub};

printf("%d", op[0](5, 3));  // add → 8
printf("%d", op[1](5, 3));  // sub → 2

Summary Table

Concept	Meaning
Pointer	Stores address
&	Address-of
*	Value-at-address
Pointer arithmetic	Move to next/previous element
Passing pointer	Allows modification in functions
Array of pointers	Stores multiple addresses
Pointer to pointer	Double-level addressing
Function pointer	Holds function address

STRINGS IN C 

A string in C is a collection of characters stored in a character array and terminated with a null character \0.

Example:

char name[] = "India";

Internally stored as:

| I | n | d | i | a | \0 |

Important:
C does not have a built-in "string" data type.
Strings are handled using character arrays or char pointers.

Initializing Strings

A. Initialization using character array

char country[6] = {'I','n','d','i','a','\0'};

B. Initialization using string literal (easier)

char country[] = "India";

C. Initialize an empty string

char name[20] = "";

D. Without specifying size

Compiler decides size automatically.

char city[] = "Delhi";

Accessing String Elements

You use index numbers (0,1,2…) like with arrays.

Example:

char name[] = "Ram";
printf("%c", name[1]);   // prints 'a'

You can also modify characters:

name[2] = 'n';   // "Ran"

Array of Strings

An array of strings means multiple strings stored together.

Method 1: Using 2D array

char cities[3][10] = {"Delhi", "Mumbai", "Goa"};

Memory representation:

| Delhi | Mumbai | Goa |

Method 2: Using array of char pointers

(More memory efficient)

char *cities[] = {"Delhi", "Mumbai", "Goa"};

Passing Strings to Functions

Strings are passed as character pointers because arrays decay to pointers.

Example:

void display(char str[]) {
    printf("%s", str);
}

int main() {
    char name[] = "Jay";
    display(name);
}

You can also use:

void display(char *str);

Common String Functions in C (string.h)

Include header:

#include <string.h>

1. strlen() — length of string

strlen("Hello");  // returns 5

2. strcpy() — copy string

strcpy(dest, src);

3. strcat() — concatenate two strings

strcat(str1, str2);

4. strcmp() — compare two strings

Returns:

• 0 → strings are equal
• <0 → str1 < str2
• >0 → str1 > str2

strcmp("abc","abc"); // 0

5. strrev() — reverse a string

(Not available in all compilers)

strrev(str);

6. strlwr() / strupr() — lowercase / uppercase

Compiler dependent.

Example Program: Count Vowels in a String

#include <stdio.h>

int main() {
    char str[50];
    int i, count = 0;

    printf("Enter a string: ");
    gets(str);

    for (i = 0; str[i] != '\0'; i++) {
        if (str[i]=='a'||str[i]=='e'||str[i]=='i'||str[i]=='o'||str[i]=='u'||
            str[i]=='A'||str[i]=='E'||str[i]=='I'||str[i]=='O'||str[i]=='U') {
            count++;
        }
    }

    printf("Vowels = %d", count);
    return 0;
}

Example Program: Reverse a String Using Function

void reverse(char str[]) {
    int len = strlen(str);
    for(int i=0; i<len/2; i++) {
        char temp = str[i];
        str[i] = str[len-i-1];
        str[len-i-1] = temp;
    }
}

int main() {
    char s[20] = "hello";
    reverse(s);
    printf("%s", s);  // olleh
}

Summary Table

Topic	Explanation
String	Character array ending with \0
Initialization	Literal or char array
Accessing	Using index
Array of strings	2D array or pointer array
Passing to function	Passed as pointer
String functions	strlen, strcpy, strcat, strcmp, strrev