[R] Applying a function to a column of a data frame

[R. Michael Weylandt]

No worries -- it's an important question and it introduces you to one
of the most important R idioms. [And you get bonus points for having a
well formed question!]

You're probably looking for something like this:

apply(testframe, 1, function(x) testfun(x[2]))

Which goes row-by-row and substitutes the second element (your col2)
into testfun leaving the y= unsupplied and hence defaulting to 2. This
seems a little kludgy, however, in light of the below. (Also, if you
have a data frame of multiple types, it seems to get in trouble
quicker than I would have imagined)

Note that this is not particularly efficient and you could just use

testfun(testframe[["col2"]])

instead using the magic of vectorization. Very rarely do you need to
do things "one-by-one" except for recursive/state-dependent
simulations.

Best,
Michael

On Sat, Jun 9, 2012 at 9:12 AM, Onur Uncu <onuruncu at gmail.com> wrote:
> Apologees the novice question. Currently climbing up the learning curve of R.
>
> Suppose I have the following function and the data.frame:
>
> testfun<-function(x=1,y=2) x+y
>
> testframe=data.frame(col1=c(1,2),col2=c(3,4))
>
> When evaluating testfun, I want to use the default value for y (which
> is 2) and for x, I want to feed (one by one) the values in col2 of
> testframe. How can I achieve this please? Not having much success with
> apply()
>
> Thanks for any help.
>
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