# 02. Sequences and Repetitions in R seq(), rep()

To avoid repetitiveness and tediously filling the elements of a vector, we may use the `seq()` and `rep()` functions.

## Sequences

We can create a vector with a known pattern or sequence with either the `seq()` command or `:` notation.

### 1) `:` (colon) notation

The `:` is used to indicate a sequence of integer values.

``````> x <- c(1:9)
> x
[1] 1 2 3 4 5 6 7 8 9``````

### 2) `seq()` function

To get more specific, we can use the `seq()` function.

``````> x <- seq(from=15, to=45, by=3)
> x
[1] 15 18 21 24 27 30 33 36 39 42 45``````

Notice how the values `from` and `to` are inclusive.

We can also use `length` parameter instead, which will equally split our sequence.

``````> x <- seq(from=1.1, to=3, length=20)
[1] 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1
[12] 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0
# Every tenth number up to 100
> 1:10*10
[1] 10 20 30 40 50 60 70 80 90 100``````

Remember that you can open the manual page for any function by typing `?seq`

### Indexing with Sequences

Indexing can be performed not just to return one value, but multiple. We can do this using the `seq()` function above.

``````> x <-seq(1:20)
# Pull out just the first five elements
> x[1:5]
[1] 1 2 3 4 5
# Pull out every third element
> x[seq(1,20,3)]
[1] 1 4 7 10 13 16 19``````

## Repetitions

R also allows you to easily create vectors containing repetitions with the `rep()` function.

``````> x <- rep(c("hello there"), 4)
> x
[1] "hello there" "hello there" "hello there"
[4] "hello there"``````

The first parameter is the constant to be repeated, while the second parameter is the number of times.

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