Performing dplyr mutate on subset of columns

dplyr, r

Solution

Am I missing something or would this work as expected:

cols <- paste0("X", c(2,4))
dd %>% mutate(evensum = rowSums(.[cols]), evenmean = rowMeans(.[cols]))
#  id        X1        X2        X3        X4        X5   evensum  evenmean
#1  a 0.6021140 0.3670719 0.6872308 0.5090904 0.4474437 0.8761623 0.4380811
#2  b 0.1950439 0.9888592 0.8314290 0.7066286 0.9646670 1.6954878 0.8477439
#3  c 0.9664587 0.8151934 0.1046694 0.8623137 0.1411871 1.6775071 0.8387535
#4  d 0.6509055 0.2539684 0.6461509 0.8417851 0.7767125 1.0957535 0.5478768

Or are you specifically looking for a custom function to do this?

Not exactly what you are looking for but if you want to do it inside a pipe you could use `select` explicitly inside `mutate` like this:

dd %>% mutate(xy = select(., num_range("X", c(2,4))) %>% rowSums)
#  id        X1        X2        X3        X4        X5        xy
#1  a 0.6021140 0.3670719 0.6872308 0.5090904 0.4474437 0.8761623
#2  b 0.1950439 0.9888592 0.8314290 0.7066286 0.9646670 1.6954878
#3  c 0.9664587 0.8151934 0.1046694 0.8623137 0.1411871 1.6775071
#4  d 0.6509055 0.2539684 0.6461509 0.8417851 0.7767125 1.0957535

However, it is a bit more complicated if you want to apply several functions. You could use a helper function along the lines of (..not thoroughly tested.. ):

f <- function(x, ...) {
  n <- nrow(x)
  x <- lapply(list(...), function(y) if (length(y) == 1L) rep(y, n) else y)
  matrix(unlist(x), nrow = n, byrow = FALSE)
}

And then apply it like this:

dd %>% mutate(xy = select(., num_range("X", c(2,4))) %>% f(., rowSums(.), max(.)))
#  id        X1        X2        X3        X4        X5      xy.1      xy.2
#1  a 0.6021140 0.3670719 0.6872308 0.5090904 0.4474437 0.8761623 0.9888592
#2  b 0.1950439 0.9888592 0.8314290 0.7066286 0.9646670 1.6954878 0.9888592
#3  c 0.9664587 0.8151934 0.1046694 0.8623137 0.1411871 1.6775071 0.9888592
#4  d 0.6509055 0.2539684 0.6461509 0.8417851 0.7767125 1.0957535 0.9888592

Problem

I have a data.frame such as this (the real data set has many more rows and columns) ``` set.seed(15) dd <- data.frame(id=letters[1:4], matrix(runif(5*4), nrow=4)) # id X1 X2 X3 X4 X5 # 1 a 0.6021140 0.3670719 0.6872308 0.5090904 0.4474437 # 2 b 0.1950439 0.9888592 0.8314290 0.7066286 0.9646670 # 3 c 0.9664587 0.8151934 0.1046694 0.8623137 0.1411871 # 4 d 0.6509055 0.2539684 0.6461509 0.8417851 0.7767125 ``` I would like to be able to write a dplyr statement where I can select a subset of columns and mutate them. (I'm trying to do something similar to using .SDcols in data.table). For a simplified example, here's the function I would like to be able to write to add columns for the sums and means of the even "X" columns while preserving all other columns. The desired output using base R is ``` (cols<-paste0("X", c(2,4))) # [1] "X2" "X4" cbind(dd,evensum=rowSums(dd[,cols]),evenmean=rowMeans(dd[,cols])) # id X1 X2 X3 X4 X5 evensum evenmean # 1 a 0.6021140 0.3670719 0.6872308 0.5090904 0.4474437 0.8761623 0.4380811 # 2 b 0.1950439 0.9888592 0.8314290 0.7066286 0.9646670 1.6954878 0.8477439 # 3 c 0.9664587 0.8151934 0.1046694 0.8623137 0.1411871 1.6775071 0.8387535 # 4 d 0.6509055 0.2539684 0.6461509 0.8417851 0.7767125 1.0957535 0.5478768 ``` but I wanted to use a dplyr-like chain to do the same thing. In the general case, I'd like to be able to use any of `select()`'s helper functions such as `starts_with`, `ends_with`, `matches`, etc and any function. Here's what I tried ``` library(dplyr) partial_mutate1 <- function(x, colspec, ...) { select_(x, .dots=list(lazyeval::lazy(colspec))) %>% transmute_(.dots=lazyeval::lazy_dots(...)) %>% cbind(x,.) } dd %>% partial_mutate1(num_range("X", c(2,4)), evensum=rowSums(.), evenmean=rowMeans(.)) ``` However, This throws an error that says ``` Error in rowSums(.) : 'x' must be numeric ``` Which appears to be because `.` seems to be referring to the entire date.frame rather than the selected subset. (same error as `rowSums(dd)`). However, note that this produces the desired output ``` partial_mutate2 <- function(x, colspec) { select_(x, .dots=list(lazyeval::lazy(colspec))) %>% transmute(evensum=rowSums(.), evenmean=rowMeans(.)) %>% cbind(x,.) } dd %>% partial_mutate2(seq(2,ncol(dd),2)) ``` I'm guessing this is some sort of environment problem? Any suggestions on how to pass the arguments to `partial_mutate1` so that the `.` will correctly take values from the "select()-ed" dataset?

Original source

Related problems