for loop in R using a list of strings as variables - with and without quotes-CodePudding

I would like to create a loop that would run over a list of strings. This list of strings contains name of the variables that need to be plugged-in the syntax. My syntax sometimes need them with quotes (and then it works) and sometimes without quotes (and then it fails). I have tried solutions from here Remove quotes from a character vector in R but it didn't work.

here is an example of what I try to do:

library(dplyr)

data(mtcars)
list=c("mpg", "cyl")
for (i in list) {
  print(i)
  df=mtcars[,c(i, "gear")] #this works
  df1 = group_by(df, gear) %>% summarize(Y=sum(i)) # this doesn't work - it doesn't recognize i as variable name
}

I'd appreciate your help

Regards, Polina

CodePudding user response：

Try wrapping i in a get.

for (i in list) {
  print(i)
  df=mtcars[,c(i, "gear")] #this works
  df1 = group_by(df, gear) %>% summarize(Y=sum(get(i))) # this doesn't work - it doesn't recognize i as variable name
}

It's also worth reading about sym and !! etc

CodePudding user response：

To correct the error of accessing the variable given by a character variable, use !!rlang::sym(i).

for (i in list) {
  print(i)
  df=mtcars[,c(i, "gear")] #this works
  df1 = group_by(df, gear) %>% summarize(Y = sum(!!rlang::sym(i)))
}

But there is another error in the question's code, the output df1 is rewritten every time through the loop.
Here is a corrected loop.

df_list <- lapply(list, function(i){
  print(i)
  df <- mtcars[,c(i, "gear")] #this works
  group_by(df, gear) %>% summarize(Y = sum(!!rlang::sym(i)))
})

df_list[[1]]
## A tibble: 3 x 2
#   gear     Y
#  <dbl> <dbl>
#1     3  242.
#2     4  294.
#3     5  107.