---

title: "Named Map Builder"

author: "John Mount"

date: "`r Sys.Date()`"

output: rmarkdown::html_vignette

vignette: >

%\VignetteIndexEntry{Named Map Builder}

%\VignetteEngine{knitr::rmarkdown}

%\VignetteEncoding{UTF-8}

---

[`wrapr`](https://github.com/WinVector/wrapr) introduces an operator called "named map builder" that is written as "`:=`". Named map builder is a *very* simple bit of code that performs a very simple task: it adds names to vectors or lists (making them work more like maps).

Here are some examples:

```{r ex1}

library("wrapr")

'a' := 5

c('a' := 5, 'b' := 6)

c('a', 'b') := c(5, 6)

```

The left-side argument of the `:=` operator is called "the names", and the right-side argument is called "the values". The `:=` operators returns the values with the names set to names.

A key use of the named map builder is the following:

```{r key1}

key = 'keycode'

key := 'value'

```

Notice the value inside the variable `key` was used as the array name, this differs from

what is easily done with `R`'s native `c(key = 'value')` style notation.

A great use of the `:=` operator is using it to conveniently build arguments lists for functions such as `seplyr::mutate_se()`. This works for simple explicit code such as the following.

```{r ex2}

library("seplyr")

datasets::iris %.>%

summarize_se(., "Mean_Sepal_Length" := "mean(Sepal.Length)")

```

Slightly more complicated code such as:

```{r ex3}

datasets::iris %.>%

group_by_se(., "Species") %.>%

summarize_se(., c("Mean_Sepal_Length" := "mean(Sepal.Length)",

"Mean_Sepal_Width" := "mean(Sepal.Width)"))

```

Or even parametric code such as:

```{r ex4}

resultColumn <- "summary_value"

datasets::iris %.>%

group_by_se(., "Species") %.>%

summarize_se(., resultColumn := "mean(Sepal.Length)")

```

For more details please see: [`help(`:=`, package = 'wrapr')`](https://winvector.github.io/wrapr/reference/named_map_builder.html) and

[`help("%.>%", package="wrapr")`](https://winvector.github.io/wrapr/reference/dot_arrow.html).