Evaluate dataframe$column expression stored as a string value

Question

Can a string of the form below be evaluated so that it is equivalent to the same \"literal\" expression?

Example data and code:

df.name = data.frame(col1         


        

Answer 1

You are almost there:

> eval(parse(text = var1))
[1] "D"

Because parse expecting file by default, you need to specify the text parameter.

Answer 2

I'm trying to create a function that will search (subset) df.name using the col.name passed to the function.

Set up data:

df.name = data.frame(col1 = 1:5, col2 = LETTERS[1:5], ## seq() is unnecessary
                     col3 = letters[1:5], 
                     stringsAsFactors = FALSE)
col.name = "col2"
row.num = "4"

Solving your ultimate (index the data frame by column name) rather than your proximal (figure out how to use get()/eval() etc.) question: as @RichardScriven points out,

f <- function(col.name,row.num,data=df.name)
   return(data[[col.name]][as.numeric(row.num)])
}

should work. It would probably be more idiomatic if you specified the row number as numeric rather than character, if possible ...