# Rebuilding Map Example With Apply Functions

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Yesterday Hadley’s functional programming package purrr was published to CRAN. It is designed to bring convenient functional programming paradigma and add another data manipulation framework for R.

“Where dplyr focusses on data frames, purrr focusses on vectors” – Hadley Wickham in a blogpost

The core of the package consists of map functions, which operate similar to base apply functions. So I tried to rebuild the first example used to present the map function in the blog and Github repo with apply. The example looks like this:

library(purrr) mtcars %>% split(.$cyl) %>% map(~ lm(mpg ~ wt, data = .)) %>% map(summary) %>% map_dbl("r.squared") ## 4 6 8 ## 0.5086326 0.4645102 0.4229655

Let’s do it with apply now:

mtcars %>% split(.$cyl) %>% lapply(function(x) lm(mpg~wt, data = x)) %>% lapply(summary) %>% sapply(function(x) x$r.squared) ## 4 6 8 ## 0.5086326 0.4645102 0.4229655

As you can see, map works with 3 different inputs – function names (exactly like apply), anonymous function as formula and character to select elements. Apply on the other hand only accepts functions, but with a little piping voodoo we can also shortcut the anonymous functions like this:

mtcars %>% split(.$cyl) %>% lapply(. %>% lm(mpg~wt, data = .)) %>% lapply(summary) %>% sapply(. %>% .$r.squared) ## 4 6 8 ## 0.5086326 0.4645102 0.4229655

Which is almost as appealing as the map alternative. Checking the speed of both approaches also reveals no significant time differences:

library(microbenchmark) map <- function() {mtcars %>% split(.$cyl) %>% map(~ lm(mpg ~ wt, data = .)) %>% map(summary) %>% map_dbl("r.squared")} apply <- function() {mtcars %>% split(.$cyl) %>% lapply(. %>% lm(mpg~wt, data = .)) %>% lapply(summary) %>% sapply(. %>% .$r.squared)} microbenchmark(map, apply) ## Unit: nanoseconds ## expr min lq mean median uq max neval ## map 14 27 31.69 27 27 513 100 ## apply 14 27 281.34 27 27 24556 100

Now don’t get me wrong, I don’t want to say, that purrr is worthless or reduntant. I just picked the most basic function of the package and explained what it does by rewriting with apply functions. The map function and their derivatives are convenient alternatives to apply in my eyes without computational overhead. Furthermore the package offers very interesting functions like `zip_n`

and `lift`

. If you love apply and word with lists, you definitely should check out `purrr`

.

Rebuilding Map Example With Apply Functions was originally published by Kirill Pomogajko at Opiate for the masses on September 30, 2015.

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