looping through dataframes using 'for' [duplicate] - r

Question

This question already has answers here:
How can R loop over data frames?
(2 answers)
Closed 6 years ago.
Here is a simple made up data set:
df1 <- data.frame(x = c(1,2,3),
y = c(4,6,8),
z= c(1, 6, 7))
df2 <- data.frame(x = c(3,5,6),
y = c(3,4,9),
z= c(6, 7, 7))
What I want to do is to create a new variable "a" which is just the sum of all three variables (x,y,z)
Instead of doing this separately for each dataframe I thought it would be more efficient to just create a loop. So here is the code I wrote:
my.list<- list(df1, df2)
for (i in 1:2) {
my.list[i]$a<- my.list[i]$x +my.list[i]$y + my.list[i]$z
}
or alternatively
for (i in 1:2) {
my.list[i]<- transform(my.list[i], a= x+ y+ z)
}
In both cases it does not work and the error "number of items to replace is not a multiple of replacement length" is returned.
What would be the best solution to writing a loop code where I can loop through dataframes?

Answer 1

See ?Extract:
Recursive (list-like) objects
Indexing by [ is similar to atomic vectors and selects a list of the
specified element(s).
Both [[ and $ select a single element of the list.
In short, my.list[i] returns a list of length 1, and you are trying to assign it a data.frame, so that doesn't work; whereas my.list[[i]] returns the data.frame #i in your list, which you can replace with a data.frame.
So you can use either:
for (i in 1:2) {
my.list[[i]]$a<- my.list[[i]]$x +my.list[[i]]$y + my.list[[i]]$z
}
or
for (i in 1:2) {
my.list[[i]]<- transform(my.list[[i]], a= x+ y+ z)
}
But it would be even simpler to use lapply, where you don't need [[:
my.list <- lapply(my.list, function(df) df$a <- df$x + df$y + df$z)

Answer 2

Rather than using an explicit loop to extract the data.frames from the list, just use lapply. It takes a list of data.frames (or any object) and a function, applies the function to every element of the list, and returns a list with the results.
# Sample data
df1 <- data.frame(x = c(1,2,3), y = c(4,6,8), z = c(1, 6, 7))
df2 <- data.frame(x = c(3,5,6), y = c(3,4,9), z = c(6, 7, 7))
# Put them in a list
df_list <- list(df1, df2)
# Use lapply to iterate. FUN takes the function you want, and
# then its arguments (a = x + y + z) are just listed after it.
result_list <- lapply(df_list, FUN = transform, a = x + y + z)