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I am trying to compute means of data frames inside a list using lapply function. 

df_list <- list(cars, mtcars)
sapply(df_list, mean)

The above code doesn't seem to work. However when i changed it to:

 df_list <- c(cars, mtcars)
 sapply(df_list, mean)

The output had the means of all the variables of both data frames.

is there a way to compute the means using the first approach.

Karthik Shanmukha
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  • `c(cars, mtcars)` creates a list with each element from each column, while `list(cars, mtcars)` creates a list with two data frames. Since `mean(mtcars)` or `mean(cars)` result in `NA`, of course the first apporach will not work. What exactly do you want to achieve? – www Jul 29 '17 at 04:46

2 Answers2

3

use the purrr library to achieve this...much simpler:

library(purrr) 
map(df_list, ~map_if(., is.numeric, mean))

If you want a df to be returned then:

map_df(df_list, ~map_if(., is.numeric, mean))

answer is from here: why does map_if() not work within a list credit should go to @Axeman

Uwe
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cephalopod
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2

In base R, you can use rapply in order to calculate the means of variables contained in a list of data.frames.

# data
df_list <- list(cars, mtcars)

The simplest output is to run rapply with two arguments, the function and the list of data.frames. The function if(is.numeric(x)) mean(x) checks if the variable is numeric, and if so, returns the mean.

# returns a vector of means
rapply(df_list, function(x) if(is.numeric(x)) mean(x))

This output destroys the relationship between the variables and their data.frames. If desired, we can return the values in a structure that preserves that of the original object, a nested list of length 2 and inner lists of length 2 and 11.

rapply(df_list, function(x) if(is.numeric(x)) mean(x), how="list")

The resulting structure is probably more complicated than desired. For my taste,

lapply(rapply(df_list, function(x) if(is.numeric(x)) mean(x), how="list"), unlist)
[[1]]
speed  dist 
15.40 42.98 

[[2]]
       mpg        cyl       disp         hp       drat         wt       qsec 
 20.090625   6.187500 230.721875 146.687500   3.596563   3.217250  17.848750 
        vs         am       gear       carb 
  0.437500   0.406250   3.687500   2.812500

results in a nice balance, a list of length 2 each containing named vectors of the means.

lmo
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