matrix.Rmd

## Matrices

* A matrix is a **2 dimensional** vector.

* All columns in a matrix must have:
	+ the same **type** (numeric, character or logical)
	+ the same **length**

### Creating a matrix

* From vectors with the **rbind** function:

```{r}
x <- c(1, 44)
y <- c(0, 12)
z <- c(34, 4)
# rbind: bind rows
b <- rbind(x, y, z)
```

* From vectors with the **cbind** function:

```{r}
i <- c(1, 0, 34)
j <- c(44, 12, 4)
# cbind: bind columns
k <- cbind(i, j)
```

* From scratch with the *matrix* function:

```{r}
# nrow: number of rows
# ncol: number of columns
pp <- matrix(c(1, 0, 34, 44, 12, 4), 
	nrow=3,
	ncol=2)
```

### Two-dimensional object

Vectors have one index per element (1-dimension).<br>
Matrices have **two indices (2-dimensions)** per element, corresponding to its corresponding location: row and column number:

<img src="images/matrix_indices.png" width="300"/>

* Fetching elements of a matrix:

The "coordinates" of an element in a 2-dimensional object will be first the row (on the left of the comma), then the column (on the right of the comma):

<img src="images/matrix_rc.png" alt="rstudio logo" width="250"/>

### Matrix manipulation

* Add 1 to all elements of a matrix

```{r}
b <- b + 1
```

* Multiply by 3 all elements of a matrix

```{r}
b <- b * 3
```

* Subtract 2 to each element of **the first row** of a matrix

```{r}
b[1, ] <- b[1, ] - 2
```

* Replace elements that comply a condition:

```{r}
# Replace all elements that are greater than 3 with NA
b[ b > 3 ] <- NA
```

**HANDS-ON**

1. Create a matrix, using the 2 following vectors **vv1** and **vv2** as columns:
```{r}
vv1 <- c(43, 21, 54, 65, 25, 44)
vv2 <- c(0, 1, 1, 0, 1, 0)
```
Save the matrix in the object **mm1**.

2. Add **2** to each element of **mm1**.