R: Loop through multiple data frames, save to list

Question

I'm trying to do a simple for loop on multiple data frames and save the output (from all data frames) to one list.

Here is example data:

# Two example data frames (variables x and y)
a <- data.frame(x=rnorm(1:100, 10, 1), y=1:100)
b <- data.frame(x=rnorm(1:100, 500, 50), y=1:100)

# Merging data frames to a list, which we aim to loop through
data_frame_list <- list(a, b)

# Desired list
new_list <- list()

# The loop
# I want two simple values from each data frame (value1 and value2)
for(i in data_frame_list) {
     for(j in 1:5) {
          value1 = i[j,1]
          value2 = i[j,2]
          new_list[i] <- cbind(value1, value2)
     }
}

So from each data frame i want value1 and value 2 for all the j values (1 to 5). I'd like to have all the extracted data in a list (new_list).

Thanks for help

Andy McKenzie · Accepted Answer · 2016-02-15 01:16:47Z

The other answers are helpful, but I wanted to try to help you fix your for loop.

for(i in 1:length(data_frame_list)) {
   for(j in 1:5) {
      value1 = data_frame_list[[i]][j,1]
      value2 = data_frame_list[[i]][j,2]
      new_list[[5*(i-1)+j]] = c(value1, value2)
   }
}

A few differences from your code:

You need to assign to the next element of the list by subsetting using [[, i.e. double brackets, instead of single brackets, [. In your code, I was getting a " invalid subscript type 'list'" error, which I'm pretty sure was due to this.
cbind creates a data structure that you might not want -- if you just want the values, might be easier to create a vector using c()
in your code, the new_list values are replaced on the second iteration of the outer loop. In order to fix this, I needed to access the data frames within the loop using subsetting. This allowed me to extend the length of the list so that it appends the elements of the second data frame.

Hope this helps.

windrunn3r.1990 · Accepted Answer · 2016-02-15 00:04:27Z

1

Are you trying to select the first 5 columns of data frames a and b?

If so, try this:

new_list <- cbind(a[, 1:5], b[, 1:5])

answered Feb 15, 2016 at 0:04

windrunn3r.1990

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Comments

Frank Pinto · Accepted Answer · 2016-02-15 00:59:28Z

1

If I read correctly your problem, you are trying to get the first 5 rows of data (including the columns) from each element of the list data_frame_list. If so, then this might help:

for(i in data_frame_list) { new_list <- i[1:5, ]} It provides a new list with ONLY the first 5 rows (columns included) of each element in the first list. It that list were to get bigger it should incorporate the first five elements of each new list.

Hope it helps!

answered Feb 15, 2016 at 0:59

Frank Pinto

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