c / arrays / matrices

One-dimensional and multidimensional arrays in C

This presentation is designed for someone just starting out. We go from the problem of having many separate variables to understanding why an array organizes data better and how a matrix adds rows and columns.

1 namemany data of the same type

index 0always starts from zero

2 indicesrow and column in matrices

Memory concept of an array: int notas[5] = {8, 9, 6, 10, 7};

notas[0]

notas[1]

notas[2]

notas[3]

notas[4]

1. Why do arrays exist?

Because having many separate variables is chaos. An array lets you store several values of the same type under a single name and access each one using its position.

Bad approach with lots of data

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

This becomes repetitive, hard to maintain, and later it is very difficult to traverse or process the information.

Better approach with array

int notas[5];

Single name: notas.
All elements are the same type.
Each value lives at a position: notas[0], notas[1], etc.

2. One-dimensional array: a row of lockers

A one-dimensional array can be thought of as a single row. It has several positions but only one dimension. The important thing is to understand that the index starts at 0.

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

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

numeros[0] is 10.
numeros[2] is 30.
If the array has 5 elements, the last valid index is 4.

Correct mental model

Don't think "5 values → last index 5". That's a classic mistake. Think like this: count = 5, last index = 4.

3. Step by step: load and read an array

Here you can see how an array is filled with a loop. Notice that we don't write one line per position: we let the index do the repetitive work.

Guided code Active line: 1

int numeros[3];

int i;

for (i = 0; i < 3; i = i + 1) {

numeros[i] = i + 1;

}

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

Step 1: Space is reserved for 3 integers.

Step 1 / 6

Array state

numeros[0]

numeros[1]

numeros[2]

./arreglo-demostdout

4. Two-dimensional array: a table with rows and columns

When one row is not enough, the matrix appears. A matrix is an array with two dimensions. To access its data you need two indices: one for the row and one for the column.

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

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

That access returns 6, because row 1 is the second row and column 2 is the third column.

Matrix visualization

[0][0]1

[0][1]2

[0][2]3

[1][0]4

[1][1]5

[1][2]6

Think of it as a table: first you choose the row, then the column. If you mix up that order, you get lost.

5. When to use an array and when a matrix?

Simple array

List of ages.
Student grades.
Week temperatures.

Matrix

Grades of several students in several subjects.
Game board.
Seats in rows and columns.

Simple rule

If the data can be imagined as a single list, use a one-dimensional array. If the data is organized like a table, use a matrix.

6. Typical beginner mistakes

Error 1: thinking that if an array has 5 elements, the last index is 5. NO. The last index is 4.

Error 2: using a position that does not exist, for example numeros[5] in an array of 5 elements.

Error 3: believing that a matrix is accessed with a single index. You need two: row and column.

Error 4: not understanding that all array elements must be of the same type.

7. Final idea

An array in C is not just "many data together". It is an orderly way of representing related information. When you understand that each data has a position and that position is accessed by index, you start thinking about problems better.

And when a second dimension appears, the idea does not change: you simply go from a list to a table. No magic. Just data organization.

What you should take away

An array stores several data of the same type.
The index starts at 0.
A matrix uses row and column.
Without understanding indexes, everything later becomes complicated.