Using a string from a list to select a column in R

Try this:

list<-list("Var1", "Var2", "Var3")
df1 <- data.frame("Var1" = 1:2, "Var2" = c(21,15), "Var3" = c(10,9))
df2<- data.frame("Var1" = 1, "Var2" = 16, "Var3" = 8)
#Sum
df1$Var4<- df1[,list[[1]]]+df2[,list[[1]]]

  Var1 Var2 Var3 Var4
1    1   21   10    2
2    2   15    9    3

To follow up with the reasoning to Duck's obviously correct answer, your approach isn't working for two reasons:

1. The $ operator does "not allow computed indices", so you can't pass a character vector to it. See help(`$`).

`$`(df1,"Var1")
#[1] 1 2

`$`(df1,list[[1]])
#Error in df1$list[[1]] : invalid subscript type 'language'

2. You were trying to subset df1 with a list. You need a character vector.

list[1]
#[[1]]
#[1] "Var1"

class(list[1])
#[1] "list"

The [[ operator selects an individual element from a list. In this case, the character vector you needed.

list[[1]]
#[1] "Var1"

class(list[[1]])
#[1] "character"

You can try the code below

df1$Var4 <- Map(`+`,df1,df2)[[lst[[1]]]]

such that

> df1
  Var1 Var2 Var3 Var4
1    1   21   10    2
2    2   15    9    3