在if语句中使用变量名的向量

Question

I'm fairly new to using R, but I've been encountering this issue a lot in what I'm trying to do. I'd like to apply a function to a specific column in a data.frame depending on the value passed to the containing function. It would look something like this:

myfunction <- function(var) {
        < do something to mydata$var >
}

Where var could be the name of any of the columns in mydata (a data.frame).

The issue is with the dataframe$var part of the code. I can't figure out how to dynamically call a variable name like that. dataframe[,var] doesn't seem to work, nor does dataframe[,get(var)] or dataframe[,eval(var)] .

I'd also like to use this same theory to loop through a vector of specific columns in mydata :

varlist <- c("var1","var2","var3")
for (var in varlist) {
    < do something to mydata$var >
}

But again, mydata$var is the problem.

Any advice or help would be appreciated!

Answer 1

Try this:

test_frame = data.frame("a"=1:4,"b"=2:5,"c"=3:6)

varlist = c("a","c")

myfunction = function(var_name){
  return(mean(test_frame[,var_name]))
}

for (var in varlist){
  myfunction(var)
}

Or in a cleaner way...

sapply(varlist,myfunction)
  a   c 
2.5 4.5