Replacing subsets in R with Tidyverse

Question

How do one replace the values of a subset in R with Tidyverse?

Using the cars data as an example, if I would like to change all the speed lower than 30 into 0, I can use the command below:

 cars[cars["speed"] < 30,] <- 0

With Tidyverse, one can generate the same subset with more readable commands:

 cars %>% filter(speed < 30) %>% mutate(speed =0)

However, this is changing the subset of data we have taken out from cars, not the values of observations within cars.

I might have missed something obvious but is there an intuitive way to do the same thing with Tidyverse as well? While cars[cars["speed"] < 30,] <- 0 works fine in most cases, it becomes really unwieldy when one has more than 5 conditions to meet.

Answer 1

You could use the replace function:

cars %>% mutate(speed = replace(speed, speed < 30, 0))

An ifelse condition would also work:

cars %>% mutate(speed = ifelse(speed < 30, 0, speed))

I tested this on a one-million-row data frame and replace ran in about one-eighth the time of ifelse.

library(microbenchmark)

set.seed(2)
dat = data.frame(x=runif(1e6, 0, 1000), y=runif(1e6, 0, 1000))

microbenchmark(
  replace=dat %>% mutate(x=replace(x, x<200, 0)),
  ifelse=dat %>% mutate(x=ifelse(x<200, 0, x)),
  if_else=dat %>% mutate(x=if_else(x<200, 0, x)),
  times=100
)

Unit: milliseconds
    expr       min       lq      mean   median        uq      max neval cld
 replace  8.352943  9.55682  18.16755 11.45507  15.33215 224.8759   100 a  
  ifelse 71.782371 87.37754 165.95928 95.12722 262.73016 287.3633   100   c
 if_else 39.947845 47.83934  88.72291 51.99449  59.76760 251.0381   100  b