Unknown result of select command - r

Question

I have multiple .csv files (mydata_1, mydata_2,...) with the same amount of columns and column names(, different row lengths if that helps finding an answer). After reading them into my environment they have the class data.frame . I was putting them all in a list and now want to select specific columns by name from all of them, resulting in in the same variable name with just the chosen columns.
mydata_1 = matrix(c(1:21), nrow=3, ncol=7,byrow = TRUE)
mydata_2 = matrix(c(1:21), nrow=3, ncol=7,byrow = TRUE)
colnames(mydata_1) = c(paste0("X","1":"7"))
colnames(mydata_2) = c(paste0("X","1":"7"))
df1 = as.data.frame(mydata_1)
df2 = as.data.frame(mydata_2)
all_data = c(df1, df2)
class(all_data)
class(df1)
for (i in all_data){
i = select(i,"X3":"X5")
}
My for command shall output the data.frames df1 and df2 with just three columns (instead of the prior seven), but when running the code an error message regarding the select command appears.
Error in UseMethod("select_") :
no applicable method for 'select_' applied to an object of class "c('integer', 'numeric')"
How can I get an working output of my new dfs?

Answer 1

The first issue here is that your are trying to create a list using c(df1, df2), while you have to use list(df1, df2)
Data
library(dplyr)
library(purrr)
mydata_1 = matrix(c(1:21), nrow=3, ncol=7,byrow = TRUE)
mydata_2 = matrix(c(1:21), nrow=3, ncol=7,byrow = TRUE)
colnames(mydata_1) = c(paste0("X","1":"7"))
colnames(mydata_2) = c(paste0("X","1":"7"))
df1 = as.data.frame(mydata_1)
df2 = as.data.frame(mydata_2)
all_data = list(df1 = df1, df2 = df2)
The second problem is within your loop. look, in this approach you have to create an empty list before running the loop, and then aggregate elements in each iteration.
all_data2 <- list()
for(i in 1:length(all_data)) {
all_data2[[i]] <- all_data[[i]] %>% select(X3, X4, X5)
}
try using map from purrr which is part of the tidyverse package and lead to a cleaner code with the same result.
# Down here the `.x` is replaced by each element of the list all_data
# in each iteration, ending wiht a list of two data frames
all_data2 = map(all_data, ~.x %>%
select(X3, X4, X5))

Answer 2

Consider base R's subset with select argument for contiguous column selection, wrapped in an lapply call. Unlike for loop, lapply does not require the bookkeeping to reassign each element back into a list:
all_data <- list(df1 = df1, df2 = df2)
all_data_sub <- lapply(all_data, function(df) subset(df, select=X3:X5))