One-Dimensional and Multi-Dimensional Arrays — C Course

So far you’ve worked many times with one variable at a time.

For example:

int edad;
float precio;
char inicial;

That’s fine when you need to store a single piece of data.

But what if you need to store:

5 grades
10 ages
30 temperatures
7 numbers entered by the user

Are you going to create a variable for each one?

int nota1;
int nota2;
int nota3;
int nota4;
int nota5;

You could, yes. But it would be awkward, messy, and impractical.

That’s where arrays come in.

What is an array?

An array is a structure that lets you store several data of the same type under a single name.

Each piece of data is stored in a position and that position is accessed through an index.

Simple example

int notas[5];

This means:

int is the data type of each element
notas is the name of the array
5 is the number of reserved positions

In other words: notas can store 5 integer values.

Why is an array useful?

Because it lets you group many related data.

For example, if you want to store 5 grades, instead of having:

int nota1, nota2, nota3, nota4, nota5;

you can have:

int notas[5];

That organizes the program better and makes subsequent work much easier.

All elements must be the same type

In an array, all elements have the same type.

For example:

int numeros[4];

here all elements are int.

And in:

char letras[6];

here all elements are char.

You can’t mix an int, a float, and a char in the same array.

Indices: array positions

In C, arrays start at index 0.

This is FUNDAMENTAL. If you don’t understand this well, everything starts to break later.

If we have:

int notas[5];

the valid positions are:

notas[0]
notas[1]
notas[2]
notas[3]
notas[4]

Watch out for this

If the array has 5 elements:

the number of elements is 5
the last index is 4

They are not the same.

Mental picture of an array

You can think of an array as a row of lockers.

Index:   0    1    2    3    4
Value:   [ ]  [ ]  [ ]  [ ]  [ ]

Each locker has:

a position
a possible value

Declaring an array

Declaring an array means reserving space for its elements.

int numeros[5];

Here we reserve space for 5 integers.

Assigning values to positions

After declaring the array, we can load values into its positions.

#include <stdio.h>

int main() {
    int numeros[5];

    numeros[0] = 10;
    numeros[1] = 20;
    numeros[2] = 30;
    numeros[3] = 40;
    numeros[4] = 50;

    printf("First value: %d\n", numeros[0]);
    printf("Last value: %d\n", numeros[4]);

    return 0;
}

Initializing an array

We can also declare and load values at the same time.

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

That means exactly this:

numeros[0] = 10
numeros[1] = 20
numeros[2] = 30
numeros[3] = 40
numeros[4] = 50

Accessing elements

To read or use a value from the array, we write the array name and the index in brackets.

int primero = numeros[0];
int tercero = numeros[2];

Complete example with position access

#include <stdio.h>

int main() {
    int edades[4] = {15, 18, 20, 22};

    printf("Age at position 0: %d\n", edades[0]);
    printf("Age at position 1: %d\n", edades[1]);
    printf("Age at position 2: %d\n", edades[2]);
    printf("Age at position 3: %d\n", edades[3]);

    return 0;
}

Modifying an array element

Array values can also change.

#include <stdio.h>

int main() {
    int notas[3] = {6, 7, 8};

    printf("Before: %d\n", notas[1]);

    notas[1] = 10;

    printf("After: %d\n", notas[1]);

    return 0;
}

One-dimensional array

A one-dimensional array is the simplest array.

It has one dimension and can be imagined as a row of elements.

[10, 20, 30, 40, 50]

Or more visually:

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

Example with `float`

#include <stdio.h>

int main() {
    float precios[3] = {1250.5, 980.0, 1500.75};

    printf("Price 1: %.2f\n", precios[0]);
    printf("Price 2: %.2f\n", precios[1]);
    printf("Price 3: %.2f\n", precios[2]);

    return 0;
}

This shows something important:

an array doesn’t always have to be of integers. It can be of any type, as long as all its elements are the same type.

Loading data into an array with a loop

One of the great advantages of arrays is that they combine very well with repetitive structures.

#include <stdio.h>

int main() {
    int numeros[5];
    int i;

    for (i = 0; i < 5; i = i + 1) {
        printf("Enter a number: ");
        scanf("%d", &numeros[i]);
    }

    printf("\nStored values:\n");

    for (i = 0; i < 5; i = i + 1) {
        printf("numeros[%d] = %d\n", i, numeros[i]);
    }

    return 0;
}

What’s good about this example?

So much.

Because it shows you don’t need to write:

scanf("%d", &numeros[0]);
scanf("%d", &numeros[1]);
scanf("%d", &numeros[2]);

That would be repetitive and awkward. With a loop, we work through all positions in an orderly way.

Multidimensional arrays

A multidimensional array is an array with more than one dimension.

The most common case when starting to program is the two-dimensional array, also called a matrix.

What is a matrix?

A matrix organizes data into:

rows
columns

So instead of imagining a single row of lockers, we imagine a table.

Declaring a matrix

int matriz[2][3];

This means:

2 rows
3 columns

In other words, there’s room for 6 integers total.

Mental picture of a matrix

       Column 0  Column 1  Column 2
Row 0     [ ]        [ ]        [ ]
Row 1     [ ]        [ ]        [ ]

Initializing a matrix

int matriz[2][3] = {
    {1, 2, 3},
    {4, 5, 6}
};

This means:

matriz[0][0] = 1
matriz[0][1] = 2
matriz[0][2] = 3
matriz[1][0] = 4
matriz[1][1] = 5
matriz[1][2] = 6

Accessing a matrix element

To access an element in a matrix we need two indices:

one for the row
another for the column

int valor = matriz[1][2];

That value is 6.

Why?

Because:

row 1 is the second row
column 2 is the third column

Complete matrix example

#include <stdio.h>

int main() {
    int matriz[2][3] = {
        {1, 2, 3},
        {4, 5, 6}
    };

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

    return 0;
}

Modifying a position in a matrix

We can also change a value in the matrix.

#include <stdio.h>

int main() {
    int tablero[2][2] = {
        {1, 0},
        {0, 1}
    };

    tablero[0][1] = 9;

    printf("New value: %d\n", tablero[0][1]);

    return 0;
}

What are multidimensional arrays used for?

They’re useful when data has a table-like organization.

For example:

grades of several students in several subjects
seats in a room
game board
mathematical matrices
temperatures by day and shift

Conceptual example: grades by student and subject

int notas[3][2];

Could mean:

3 students
2 subjects

So:

notas[0][0] = grade of student 1 in subject 1
notas[0][1] = grade of student 1 in subject 2
notas[1][0] = grade of student 2 in subject 1
and so on

Difference between one-dimensional and multidimensional arrays

One-dimensional

It has a single line of positions.

int numeros[5];

It’s accessed with a single index.

numeros[2]

Multidimensional

It has more than one dimension.

int matriz[2][3];

It’s accessed with more than one index.

matriz[1][2]

Common mistakes

Forgetting that the index starts at `0`

This is the classic mistake.

If the array has 5 elements, the last index is 4, not 5.

Accessing a position that doesn’t exist

This is wrong:

int numeros[5];
int x = numeros[5];

Because numeros[5] is outside the array.

Confusing count with last index

If there are 3 rows and 2 columns:

int notas[3][2];

the valid rows are:

And the valid columns are:

Thinking a matrix works the same as a simple array

No. In a matrix you need two indices.

Summary

an array lets you store several data of the same type
each element is identified by its index
in C, the first index is always 0
a one-dimensional array looks like a row of data
a two-dimensional array looks like a table with rows and columns
to access a matrix you need two indices

Final thought

Arrays are one of the first truly powerful tools for working with several related data.

If you understand these three ideas well:

that data shares a single name
that each position is identified by an index
that a matrix adds rows and columns

then you already have a very solid foundation for everything that comes next.

What is an array?

Simple example

Why is an array useful?

All elements must be the same type

Indices: array positions

Watch out for this

Mental picture of an array

Declaring an array

Assigning values to positions

Initializing an array

Accessing elements

Complete example with position access

Modifying an array element

One-dimensional array

Example with float

Loading data into an array with a loop

What’s good about this example?

Multidimensional arrays

What is a matrix?

Declaring a matrix

Mental picture of a matrix

Initializing a matrix

Accessing a matrix element

Why?

Complete matrix example

Modifying a position in a matrix

What are multidimensional arrays used for?

Conceptual example: grades by student and subject

Difference between one-dimensional and multidimensional arrays

One-dimensional

Multidimensional

Common mistakes

Forgetting that the index starts at 0

Accessing a position that doesn’t exist

Confusing count with last index

Thinking a matrix works the same as a simple array

Summary

Final thought

Example with `float`

Forgetting that the index starts at `0`