Arrays – Storing Data in Blocks of Memory

So far, we’ve been working with single variables:

int age = 20;
int score = 85;

That works fine when you only need one value.

But what if you need to store:

• 5 test scores?

• 10 student IDs?

• 100 numbers?

Creating 100 separate variables would be messy.

This is where arrays come in.

1. What Is an Array?

An array is:

A collection of values of the same type stored next to each other in memory.

The important parts are:

• Same data type

• Stored in consecutive memory locations

• Accessed using an index

2. Declaring an Array

Basic syntax:

data_type array_name[size];

Example:

int scores[5];

This creates:

• An array named scores

• That can hold 5 integers

• Stored in continuous memory

Right now, the array is created but not initialized.

3. Initializing an Array

You can assign values when declaring:

int scores[5] = {85, 90, 78, 92, 88};

Now the array contains:

scores[0] = 85
scores[1] = 90
scores[2] = 78
scores[3] = 92
scores[4] = 88

4. Indexing (Very Important)

Array indexing starts at 0, not 1.

This is extremely important.

If the array has 5 elements:

int scores[5];

Valid indices are:

0, 1, 2, 3, 4

The last index is always:

size - 1

So for scores[5], the last index is 4.

5. Accessing Array Elements

You access elements using square brackets:

scores[0]
scores[1]

Example:

#include <stdio.h>

int main() {
    int scores[3] = {10, 20, 30};

    printf("%d\n", scores[0]);
    printf("%d\n", scores[1]);
    printf("%d\n", scores[2]);

    return 0;
}

Output:

10
20
30

6. Modifying Array Elements

You can change values using indexing:

scores[1] = 50;

Now the array becomes:

10, 50, 30

Arrays are mutable (meaning their elements can change.)

7. Memory Layout (Why Arrays Matter)

Arrays are stored in contiguous memory.

If this is memory:

Address:   100   104   108   112   116
Value:      85    90    78    92    88

Each integer takes 4 bytes (on most systems).

So the addresses increase by 4.

This layout is important because:

• It makes arrays efficient.

• It allows pointer arithmetic later (we’ll get there).

8. Looping Through an Array

Arrays become powerful when combined with loops.

Instead of printing each element manually:

printf("%d\n", scores[0]);
printf("%d\n", scores[1]);
printf("%d\n", scores[2]);

We use a loop:

#include <stdio.h>

int main() {
    int scores[3] = {10, 20, 30};

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

    return 0;
}

This prints all elements automatically.

9. Common Beginner Mistakes

❌ 1. Going Out of Bounds

scores[3] = 100;

If array size is 3, valid indices are 0–2.

Accessing scores[3] is undefined behavior.

C will not protect you.

It may:

• Crash

• Corrupt memory

• Appear to work (but cause problems later)

Always stay within bounds.

❌ 2. Confusing Size With Last Index

If:

int a[5];

The last valid index is 4, not 5.

❌ 3. Forgetting Array Size in Loops

Always match your loop condition to the array size:

for (int i = 0; i < 5; i++)

Not:

i <= 5

10. Why Arrays Are Important

Arrays allow us to:

• Store collections of data

• Process bulk information

• Prepare for strings

• Understand memory structure

• Work with pointers (next lesson)

Arrays are the foundation of more advanced data structures.

11. Practice Exercises

1. Create an array of 5 integers and print them using a loop.

2. Create an array of 4 numbers and calculate their sum.

3. Ask the user to input 3 numbers and store them in an array.

4. Find the largest number in an array.

Final Thoughts

Arrays teach you something important about C:

C gives you power.

But it does not protect you from mistakes.

You must:

• Manage memory carefully

• Respect array boundaries

• Think about how data is stored

In the next lesson, we’ll go deeper into memory by learning about pointers.

And that’s where C starts becoming truly powerful.