05-write-some-code.Rmd

```{r echo = FALSE}
knitr::opts_chunk$set(
  comment = "#>",
  warning = FALSE,
  message = FALSE
)
```


# Coding in a package {#write-some-code}

## Edit code, run, modify, repeat

When developing a package, find whatever method is most efficient for you 
to develop the package quickly. 

You want to be able to edit code, then run/test it, modify, rerun, modify, etc. 
until you are happy with it. 

For some this may mean working in an IDE (RStudio) - for others this means
a text editor and console - for others maybe Vim or Emacs. 

Get used to the keyboard shortcuts or buttons to make this flow easy and 
quick.

`devtools::load_all()` (or equivalent buttons/shortcuts in IDE) is your friend - though beware you still want to check your package outside of a load all context (See \@ref(package-checks))

## Code

### Exercise

__Write your own function__

* Write a function
* Put it in a file in your R package

## Docs

Time to document your function! 

Above your function start each line with `#'`. There's many, many 
options for documenting functions. 

### Exercise

__Let's do a minimal set:__

* Title
* Export tag
* Parameter definitions
* Return tag
* Examples

```r
#' Multiply a number by 2
#' 
#' @export
#' @param x a numeric value. required
#' @return a number
#' @examples
#' multiply2(x = 5)
multiply2 <- function(x) {
  stopifnot(is.numeric(x))
  x * 2
}
```

## Generate documentation

In RStudio push the document button or keyboard shortcut, or on the command line 
run `devtools::document()` or similar. 

Notice what state your `NAMESPACE` file is in before and after.

## Install the package

Now install the package. 

* in RStudio: Install and Restart
* command line: `devtools::install()`
* restart your R session
* load your package `library(packagename)`
* Can you run your function?
  * if not, let's dig in and sort it out

## Yay!