1. What is an Array?

• An array is a collection of elements of the same data type stored in contiguous memory locations.

• Instead of declaring many variables, we can group them together.

👉 Example:
Instead of writing

int marks1, marks2, marks3, marks4, marks5;

We can write

int marks[5];

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2. Declaration of Array

data_type array_name[size];

✅ Example:

int numbers[10];   // array of 10 integers
float prices[5];   // array of 5 floats
char grade[3];     // array of 3 characters

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3. Initialization of Array

a) Compile-time Initialization

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

b) Partial Initialization

int arr[5] = {10, 20};   // Remaining will be 0 → {10, 20, 0, 0, 0}

c) Without size

int arr[] = {1, 2, 3, 4};   // Size = 4 (auto-calculated)

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4. Accessing Array Elements

• Array index starts from 0.

• arr[0] → first element

• arr[1] → second element

👉 Example:

#include <stdio.h>
int main() {
    int arr[5] = {10, 20, 30, 40, 50};
    printf("First element = %d\n", arr[0]);
    printf("Third element = %d\n", arr[2]);
    printf("Last element = %d\n", arr[4]);
    return 0;
}

✅ Output:

First element = 10
Third element = 30
Last element = 50

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5. Input & Output using Arrays

👉 Example:

#include <stdio.h>
int main() {
    int marks[5];
    int i;

    printf("Enter 5 marks:\n");
    for(i = 0; i < 5; i++) {
        scanf("%d", &marks[i]);
    }

    printf("\nYou entered:\n");
    for(i = 0; i < 5; i++) {
        printf("%d ", marks[i]);
    }
    return 0;
}

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6. Memory Representation

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

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

• Stored in contiguous memory.

• If base address = 1000, and int = 4 bytes:

arr[0] → 1000
arr[1] → 1004
arr[2] → 1008
...

📘 Arrays and Pointers in C

1. Relationship between Arrays and Pointers

• In C, the name of the array is a pointer to its first element.

• Example:

    int arr[5] = {10, 20, 30, 40, 50};
    printf("%p\n", arr);     // address of arr[0]
    printf("%p\n", &arr[0]); // also address of arr[0]
  

✅ Both will print the same address.

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2. Accessing Array Elements using Pointers

👉 Normal way:

arr[2];   // third element

👉 Pointer way:

*(arr + 2);   // third element

💡 Rule:
arr[i] == *(arr + i)

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3. Example Program

#include <stdio.h>
int main() {
    int arr[5] = {10, 20, 30, 40, 50};
    int *ptr = arr;   // pointer points to first element

    printf("Using array indexing:\n");
    for(int i = 0; i < 5; i++) {
        printf("%d ", arr[i]);
    }

    printf("\nUsing pointer arithmetic:\n");
    for(int i = 0; i < 5; i++) {
        printf("%d ", *(ptr + i));
    }
    return 0;
}

✅ Output:

Using array indexing:
10 20 30 40 50
Using pointer arithmetic:
10 20 30 40 50

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4. Pointer Arithmetic with Arrays

If ptr is pointing to arr[0]:

• ptr + 1 → address of arr[1]

• ptr + 2 → address of arr[2]

• *(ptr + 2) → value at arr[2]

👉 Example:

#include <stdio.h>
int main() {
    int arr[3] = {5, 10, 15};
    int *ptr = arr;

    printf("Value at arr[0] = %d\n", *ptr);
    printf("Value at arr[1] = %d\n", *(ptr + 1));
    printf("Value at arr[2] = %d\n", *(ptr + 2));
    return 0;
}

✅ Output:

Value at arr[0] = 5
Value at arr[1] = 10
Value at arr[2] = 15

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5. Arrays of Pointers

You can also create an array of pointers.
👉 Example:

#include <stdio.h>
int main() {
    const char *names[3] = {"Amit", "Rahul", "Sneha"};

    for(int i = 0; i < 3; i++) {
        printf("%s\n", names[i]);
    }
    return 0;
}

✅ Output:

Amit
Rahul
Sneha