### R length Function

`length()` function gets or sets the length of a vector (list) or other objects.

Get vector length:

```>x <- c(1,2,5,4,6,1,22,1)
>length(x)
```
```[1] 8
```

Set vector length:
```>length(x) <- 4
>x
```
```[1] 1 2 5 4
```

Get the length of a list:
```>y <- list(batch=3,label="Lung Cancer Patients", subtype=c("A","B","C"))
>y
```
```\$batch
[1] 3

\$label
[1] "Lung Cancer Patients"

\$subtype
[1] "A" "B" "C"
```
```>is.list(y)
```
```[1] TURE
```

```>length(y)
```
```[1] 3
```

If the parameter is a matrix or dataframe, it returns the number of variables:
```>length(BOD)
```
```[1] 2
```

```>BOD
```
```  Time demand
1    1    8.3
2    2   10.3
3    3   19.0
4    4   16.0
5    5   15.6
6    7   19.8
```

When resets a list or matrix, if the list is shortened, extra values will be discarded, if the list is lengthened, `NAs` (or `nul`) is added to the list.
```> length(BOD) <- 1
> BOD
```
```\$Time
[1] 1 2 3 4 5 7
```

```> length(BOD) <- 3
> BOD
```
```\$Time
[1] 1 2 3 4 5 7

\$demand
[1]  8.3 10.3 19.0 16.0 15.6 19.8

[[3]]
NULL
```

`length()` function can be used for all R objects. For an environment it returns the object number in it. NULL returns 0. Most other objects return length 1.