# 5. Algebra¶

Vectors and matrices can be used to compute new vectors (matrices) with simple and intuitive algebraic expressions.

## Vector algebra¶

We have two vectors, `a`

and `b`

```
a <- 1:5
b <- 6:10
```

Multiplication works element by element. That is `a[1] * b[1]`

,
`a[2] * b[2]`

, etc

```
d <- a * b
a
## [1] 1 2 3 4 5
b
## [1] 6 7 8 9 10
d
## [1] 6 14 24 36 50
```

The examples above illustrate a special feature of *R* not found in most
other programming languages. This is that you do not need to ‘loop’ over
elements in an array (vector in this case) to compute new values. It is
important to use this feature as much as possible. In other programming
languages you would need to do something like the ‘for-loop’ below to
achieve the above (for-loops do exist in *R* and are discussed in a
later chapter).

You can also multiply with a single number.

```
a * 3
## [1] 3 6 9 12 15
```

In the examples above the computations used either vectors of the same
length, or one of the vectors had length 1. But be careful, you can use
algebraic computations with vectors of different lengths, as the shorter
ones will be “recycled”. *R* only issues a warning if the length of the
longer vector is not a multiple of the length of the shorter object.
This is a great feature when you need it, but it may also make you
overlook errors when your data are not what you think they are.

```
a + c(1,10)
## Warning in a + c(1, 10): longer object length is not a multiple of shorter
## object length
## [1] 2 12 4 14 6
```

No warning here:

```
1:6 + c(0,10)
## [1] 1 12 3 14 5 16
```

### Logical comparisons¶

Recall that `==`

is used to test for equality

```
a == 2
## [1] FALSE TRUE FALSE FALSE FALSE
f <- a > 2
f
## [1] FALSE FALSE TRUE TRUE TRUE
```

`&`

is Boolean “AND”, and `|`

is Boolean “OR”.

```
a
## [1] 1 2 3 4 5
b
## [1] 6 7 8 9 10
b > 6 & b < 8
## [1] FALSE TRUE FALSE FALSE FALSE
# combining a and b
b > 9 | a < 2
## [1] TRUE FALSE FALSE FALSE TRUE
```

“Less than or equal” is `<=`

, and “more than or equal” is `>=`

.

```
b >= 9
## [1] FALSE FALSE FALSE TRUE TRUE
a <= 2
## [1] TRUE TRUE FALSE FALSE FALSE
b >= 9 | a <= 2
## [1] TRUE TRUE FALSE TRUE TRUE
b >= 9 & a <= 2
## [1] FALSE FALSE FALSE FALSE FALSE
```

### Functions¶

There are many functions that allow us to do vectorized algebra. For example:

```
sqrt(a)
## [1] 1.000000 1.414214 1.732051 2.000000 2.236068
exp(a)
## [1] 2.718282 7.389056 20.085537 54.598150 148.413159
```

Not all functions return a vector of the same length. The following functions return just one or two numbers:

```
min(a)
## [1] 1
max(a)
## [1] 5
range(a)
## [1] 1 5
sum(a)
## [1] 15
mean(a)
## [1] 3
median(a)
## [1] 3
prod(a)
## [1] 120
sd(a)
## [1] 1.581139
```

If you cannot guess what `prod`

and `sd`

do, look it up in the help
files (e.g. `?sd`

)

### Random numbers¶

It is common to create a vector of random numbers in data analysis, and also to create example data to demonstrate how a procedure works. To get 10 numbers sampled from the uniform distribution between 0 and 1 you can do

```
r <- runif(10)
r
## [1] 0.07003268 0.96431704 0.44251011 0.37027238 0.14118509 0.05419043
## [7] 0.65782807 0.57816192 0.98710176 0.60379240
```

For Normally distributed numbers, use `rnorm`

```
r <- rnorm(10, mean=10, sd=2)
r
## [1] 6.971006 9.876585 9.705458 13.083186 8.036289 10.993156 13.393896
## [8] 9.478527 8.588143 9.677643
```

If you run the functions above, you will get different numbers. After
all, they are random numbers! Well, computer generated numbers are not
truly random, but ‘pseudo-random’. To be able to exactly reproduce
examples or data analysis we often want to assure that we take exactly
the same “random” sample each time we run our code. To do that we use
`set.seed`

. This function initializes the random number generator to a
specific point. This is illustrated below.

```
set.seed(12)
runif(3)
## [1] 0.06936092 0.81777520 0.94262173
runif(4)
## [1] 0.26938188 0.16934812 0.03389562 0.17878500
runif(5)
## [1] 0.641665366 0.022877743 0.008324827 0.392697197 0.813880559
set.seed(12)
runif(3)
## [1] 0.06936092 0.81777520 0.94262173
runif(5)
## [1] 0.26938188 0.16934812 0.03389562 0.17878500 0.64166537
set.seed(12)
runif(3)
## [1] 0.06936092 0.81777520 0.94262173
runif(5)
## [1] 0.26938188 0.16934812 0.03389562 0.17878500 0.64166537
```

Note that each time `set.seed`

is called, the same sequence of
(pseudo) random numbers will be generated. This is a very important
feature, as it allows us to exactly reproduce results that involve
random sampling. The seed number is arbitrary; a different seed number
will give a different sequence.

```
set.seed(12)
runif(3)
## [1] 0.06936092 0.81777520 0.94262173
runif(5)
## [1] 0.26938188 0.16934812 0.03389562 0.17878500 0.64166537
```

## Matrices¶

Computation with matrices is also ‘vectorized’. For example, with matrix
`m`

you can do `m * 5`

to multiply all values of m3 with 5, or do
`m^2`

or `m * m`

to square the values of m.

```
# set up an example matrix
m <- matrix(1:6, ncol=3, nrow=2, byrow=TRUE)
m
## [,1] [,2] [,3]
## [1,] 1 2 3
## [2,] 4 5 6
m * 2
## [,1] [,2] [,3]
## [1,] 2 4 6
## [2,] 8 10 12
m^2
## [,1] [,2] [,3]
## [1,] 1 4 9
## [2,] 16 25 36
```

We can also do math with a matrix and a vector. Note, again, that
computation with matrices in *R* is column-wise, and that shorter
vectors are recycled.

```
m * 1:2
## [,1] [,2] [,3]
## [1,] 1 2 3
## [2,] 8 10 12
```

You can multiply two matrices.

```
m * m
## [,1] [,2] [,3]
## [1,] 1 4 9
## [2,] 16 25 36
```

Note that this is “cell by cell” multiplication. For ‘matrix
multiplication’
in the mathematical sense, you need to use the `%*%`

operator.

```
m %*% t(m)
## [,1] [,2]
## [1,] 14 32
## [2,] 32 77
```