R Loop to Select Specific Columns from Multiple Dataframes

Question

I am trying to write a loop in R that will select the same specific column names in multiple data frames. The below code seems to achieve the desired output but the dataframes are not re-assigned. How can I re-assign the dataframes df1 and df2 to the lapply output?

 col1 <- c(1,2,3,4)
 col2 <- c("A","B","C","D")
 col3 <- c(4,15,"BLANK","ZZ")

 df1 <- data.frame(col1,col2, col3)

 col1 <- c(500,546,47,87)
 col2 <- c("E","L","J","U")
 col3 <- c(6,10,"F","R")

 df2 <- data.frame(col1,col2, col3)

 df_list <- list(df1,df2)

 lapply(df_list,function(x) {x<- x %>% select("col1","col2")} )

akrun · Accepted Answer · 2020-03-11 17:24:35Z

1

We can use a named list and then with list2env to update the objects in the global env (but, it is better to keep it in a list)

list2env(setNames(lapply(df_list, `[`, c("col1", "col2")),
         c("df1", "df2")), .GlobalEnv)
df1
#  col1 col2
#1    1    A
#2    2    B
#3    3    C
#4    4    D
df2
#  col1 col2
#1  500    E
#2  546    L
#3   47    J
#4   87    U

Instead of creating the 'df_list' by specifying list(df1, df2), we can have a named list by making use of mget

df_list <- mget(ls(pattern= "^df\\d+$"))

then, it is more easier

list2env(lapply(df_list, `[`, c("col1", "col2")), .GlobalEnv)

edited Mar 11, 2020 at 17:24

answered Mar 11, 2020 at 17:19

akrun

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5 Comments

Matt Gossett Over a year ago

Wow that is fantastic! Very efficient with using mget.

Matt Gossett Over a year ago

when I apply this solution in AWS I get this error. any thoughts as to why?: Error in [.data.table(X[[i]], ...) : When i is a data.table (or character vector), the columns to join by must be specified using 'on=' argument (see ?data.table), by keying x (i.e. sorted, and, marked as sorted, see ?setkey), or by sharing column names between x and i (i.e., a natural join). Keyed joins might have further speed benefits on very large data due to x being sorted in RAM.

akrun Over a year ago

@MattGossett In the code, we are not doing any join though. It could be because you have a data.table object while the input showed here is data.frame?

akrun Over a year ago

You may need lapply(df_list, function(x) x[, .(col1, col2)])

Matt Gossett Over a year ago

thank you! ended up using this to remove a column and it worked lapply(df_list, function(x) subset(x, select=-c(col1)))

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