functionbasics.Rmd

# Functions

In programming, a function is a section of a program that **performs a specific task**.

For example, the function **getwd**, that we have seen before, is used as:
```{r, eval=FALSE}
getwd()
```
and has the task of outputting the **current working directory**.

You can recognize a function by its **round brackets**: functionname**()**
 
A function can also take *arguments/parameters*:
```{r, eval=FALSE}
setwd(dir="Rcourse")
```
**setwd** changes the current working directory to the directory specified with argument **dir**. 

<img src="images/func_arg1.png"  width="250"/>

* Assign the output of a function to an object:

<img src="images/func_arg2.png"  width="250"/>

* Getting help / know how a function works: <br>

From the console:

```{r}
help(getwd)
```

or (shortcut):

```{r}
?getwd
```

From the RStudio bottom-right panel:<br>

<img src="images/func_help.png"  width="500"/>

* The help pages show:
	+ required/optional argument(s), if any.
	+ default values for each argument(s), if any.
	+ examples.
	+ detailed description.

* Get the example of a function:
```{r, eval=FALSE}
example(mean)
```

* Need more help? Ask your favourite **Web search engine !**

* **Note on arguments**

The help page shows the compulsory arguments in the **Usage** section: in the help page of getwd and setwd (above), you can see that getwd *doesn't take any compulsory argument*, and setwd takes one compulsory argument that is called dir.
<br>

Compulsory arguments can be given **with their names**: in such case you don't need to respect a specific order, or **without their names**, in which case you have to respect the order specified in the help page!<br>

For example, the **rep.int** function (a variant of the rep function) takes 2 arguments (see in help page): **x** and **times**, in that order:

```{r}
# use arguments with their names:
rep.int(x=1, times=3)
# use arguments with their names without respecting the order:
rep.int(times=3, x=1)
# use arguments without their names but respecting the order:
rep.int(1, 3)
# use arguments without their names without respecting the order:
rep.int(3, 1)
# It works, but is not giving the expected output!
```