Guards! Guards! purrr::map_if

I found a cool use for purrr’s map_if function recently that both felt wizardly and helped me better understand when to use map_if. Problem: I had a list called params with 3 items of different types

• start_date - a Date
• end_date - a Date
• confidence_interval - a Numeric

params <-  list(
                start_date = as.Date("2015-01-01"),
                end_date = as.Date("2015-12-31"),
                confidence_interval = 0.8
                )
params

## $start_date
## [1] "2015-01-01"
## 
## $end_date
## [1] "2015-12-31"
## 
## $confidence_interval
## [1] 0.8

I wanted to convert this to a list where the two dates were converted to characters in the format 01 January 2015 and the confidence_interval left as is. So, if the item was a date I wanted to apply the format(x, format = "%d %B %Y") function to it and if not a date do nothing to the item.

Iterating over a list and applying a function? I knew I would want to use the purrr package.

I just needed to find a function to only do something if the item passed some test.

Enter purrr::map_if().

map_if takes the list or vector you want to iterate over (a list in my case), a predicate function(?) and the function you want to map to items of the list.

Back to the predicate function. Predicate is a word that hurts my head. I like to think of it as a guard function.

Each item in the list gets passed to the guard function which should return either TRUE or FALSE for each item. If an item passes the guard (TRUE) it gets passed to your main function. If an item doesn’t pass the guard then nothing happens to it, it goes to the output as is.

In the example below I use map_if to test each item in the params list. My guard (predicate) function is lubridate::is.Date. If the guard returns TRUE for an item, that item gets passed to the format function with the additional format = "%d %B %Y" argument. Sweet! 🎉

library(purrr)
library(lubridate)
params %>%
  map_if(is.Date, format , format = "%d %B %Y")

## $start_date
## [1] "01 January 2015"
## 
## $end_date
## [1] "31 December 2015"
## 
## $confidence_interval
## [1] 0.8

I then changed my mind and also wanted to format the numeric confidence_interval in the format 80%. I decided to pipe (%>%) to another map_if and use is.numeric as my guard ans scales::percent as my map function. Here is how it looks

params %>%
  map_if(is.Date, format , format = "%d %B %Y") %>%
  map_if(is.numeric, scales::percent)

## $start_date
## [1] "01 January 2015"
## 
## $end_date
## [1] "31 December 2015"
## 
## $confidence_interval
## [1] "80.0%"

While reading the map_if help I also noted that there is a map_at function which doesn’t take a guard function to decide which items to map but instead takes a guard vector of names of items or item positions. Useful if you know ahead of time the names or positions of items you want to act on. Here is how it would work in my example.

params %>%
  map_at(c("start_date","end_date"), format , format = "%d %B %Y")

## $start_date
## [1] "01 January 2015"
## 
## $end_date
## [1] "31 December 2015"
## 
## $confidence_interval
## [1] 0.8

params %>%
  map_at(c(1,2), format , format = "%d %B %Y")

## $start_date
## [1] "01 January 2015"
## 
## $end_date
## [1] "31 December 2015"
## 
## $confidence_interval
## [1] 0.8