Arrays – Storing Data in Blocks of Memory

UpdatedMarch 2, 2026

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C Programming

Part 19 of 40

From today, I will be starting lessons on C programming in my ALX Software Engineering class and I look forward to sharing with you everything I learn through this series.

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Pointers – Understanding Memory Addresses

If arrays taught you that data is stored next to each other in memory,pointers will teach you how to access memory directly. This is where C becomes powerful and a little scary. But don’t worry. We’ll

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Dr. Ehoneah Obed

86 posts

Software engineer writing about systems: in code, in learning, in life. I reverse-engineer complex problems into frameworks. Pharmacist → SWE → Founder.

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

Create an array of 5 integers and print them using a loop.
Create an array of 4 numbers and calculate their sum.
Ask the user to input 3 numbers and store them in an array.
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.

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