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 = 1:5, col2 = LETTERS[seq(1:5)], col3 = letters[seq(1:5)], stringsAsFactors = FALSE)
col.name = "col2"
row.num = "4"

var1 = str_c("df.name$", col.name,"[",row.num,"]")

> var1
[1] "df.name$col2[4]"

The literal works as expected

> df.name$col2[4]
[1] D

get() is not equivalent:

get(var1)
## Error in get(var1) : object 'df.name$col2[4]' not found

This form of get() "works" but does not solve the problem

get("df.name")$col2[4]
[1] D

Per other posts I've tried eval(parse()) and eval(parse(text())) without success.

I'm trying to create a function that will search (subset) df.name using the col.name passed to the function. I want to avoid writing a separate function for each column name, though that will work since I can code df.name$col2[row.num] as a "literal".

EDIT

The example code should have shown the row.num as type numeric / integer, i.e., row.num = 4

Parsing code as strings is never the answer. df.name[[col.name]][as.numeric(row.num)] is a possibility for you. But I think you should ask yourself how you ended up with a character representation of a row number and start there. — Rich Scriven
– Rich Scriven, Commented Sep 20, 2016 at 18:39
if you really need to do this, eval(parse(text=var1)) should work (it works for me). But @RichScriven's solution (but with get(df.name) instead of df.name) should work. — Ben Bolker
– Ben Bolker, Commented Sep 20, 2016 at 18:48
@Rich Scriven - the row number comes from a "for loop". The sample code is just to show the construct. The task at hand is not vectorisable since future rows depend on results of earlier rows. Again, I can avoid the paste/str_c via a separate function for each of the columns from which I need to retrieve a value. However, if I can pass the column name as a parameter to the function, well then I need only one function. — LWRMS
– LWRMS, Commented Sep 20, 2016 at 19:01

dracodoc · Accepted Answer · 2016-09-20 19:38:13Z

3

You are almost there:

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

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

answered Sep 20, 2016 at 19:38

dracodoc

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Ben Bolker · Accepted Answer · 2016-09-20 19:53:56Z

1

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 ...

answered Sep 20, 2016 at 19:53

Ben Bolker

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1 Comment

LWRMS Over a year ago

I will edit my question to note that the example row number should, more appropriately, have been numeric().

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