---
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}
---

"named map builder" is an operator 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.

`:=` is a left-over assignment operator in `R`. It is part of the syntax, but by default not defined.

`data.table` has long used `:=` to denote "in-place assignment" as in the following.

```
library("data.table")

data.table(x = 1)[, y := x + 1][]
#    x y
# 1: 1 2
```

`dplyr` later adopted the `:=` notation as this allows for substitution on the left-hand sides of assignments.  [`wrapr::qc()`](https://winvector.github.io/wrapr/articles/QuotingConcatinate.html) uses the `:=` for the same purpose.

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

```{r key1}
`:=` <- wrapr::`:=` # in case data.tables "catch calls" definition is active

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.


```{r print, eval=FALSE}
help(`:=`, package = 'wrapr')
```