---
title: "A Quick Introduction to the `lookup` Package"
author: "Kevin Wright"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{lookup}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

## A simple example

The following example shows one use-case.  Using Anderson's `iris` data, we calculate the regression of `Petal.Length` on `Sepal.Length` for each `Species` and then merge this slope coefficient back into the original data.

```{r}
coefs  <- lapply(split(iris, iris$Species),
                 function(dat) lm(Petal.Length~Sepal.Length, dat)$coef)
coefs <- do.call("rbind",coefs)
coefs <- as.data.frame(coefs)
coefs$Species <- rownames(coefs)
coefs

library(lookup)
iris = transform(iris,
                 slope1 = lookup(iris$Species, coefs$Species, coefs[,"Sepal.Length"]),
                 slope2 = vlookup(iris$Species, coefs, "Species", "Sepal.Length"))
head(iris)
```

Admittedly, a better way to approach this problem would be with the `dplyr` package and the `group_by` and `summarize` functions. But, this example does not depend on external packages.

## History

I wrote the `lookup()` function for my own personal in 2004 for the simple reason that I could never remember the syntax of the `merge()` function. When Jenny Bryan posted `vlookup()` on [Twitter](https://twitter.com/JennyBryan/status/980978609794895872), I modified her version and decided there would be value in making both of these functions widely available in a package.