Using map function instead of for loop in R

Question

Apologies if this is a dumb question- I am quite new to R and have been trying to teach myself. I've been trying to rewrite the following code using a map function instead of a for loop.

  columnmean <- function(y){
  nc <- ncol(y)
  means <- numeric(nc)
  for(i in 1:nc){
    means[i] <- mean(y[, i])
  }
  means
}

columnmean(mtcars)

My code which uses map prints out means but it also adds the column names as well. How do I utilize map properly to avoid this? Should I be using imap or map2? Thanks!

columnmean1 <- function(y){
  nc <- ncol(y)
  means <- numeric(nc)
  map(y, function (y) means <- mean(y) )
}

columnmean1(mtcars)

Welcome to Stack Overflow! Can you please read and incorporate elements from How to make a great R reproducible example? Especially the aspects of using dput() for the input and then an explicit example of your expected dataset? — wibeasley
– wibeasley, Commented Mar 11, 2020 at 1:46
Thanks for the suggestion! I don't quite understand how to use dput() but I hope that by including the mtcars dataset it makes the issues I'm seeing more readily accessible to everyone. — nzhanggh
– nzhanggh, Commented Mar 11, 2020 at 1:59
A couple things: (1) Careful about variable reuse in nested functions. You use y for two different variables in columnmean1. (2) Make your anonymous function in map return something, and assign the results of the whole map() call, not within the function, e.g. means <- map(mtcars, function(col) mean(col)) (which is the same as means <- map(mtcars, mean)). — alistaire
– alistaire, Commented Mar 11, 2020 at 2:04
And don't worry about map2 or imap until you understand map (and map_dbl, map_chr, etc.) — alistaire
– alistaire, Commented Mar 11, 2020 at 2:06

Ronak Shah · Accepted Answer · 2020-03-11 02:05:09Z

1

You can use map_dbl :

columnmean_map <- function(y){
   purrr::map_dbl(y, mean)
}

You can also use summarise_all

columnmean_summarise <- function(y){
   dplyr::summarise_all(y, mean)
}

answered Mar 11, 2020 at 2:05

Ronak Shah

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