how to pass an expression through a function for the subset function to evaluate in R

Question

i'm trying to write a subset method for a different object class that i'd like users to be able to execute the same way they use the subset.data.frame function. i've read a few related articles like this and this, but i don't think they're the solution here. i believe i'm using the wrong environment, but i don't understand enough about environments and also the substitute function to figure out why the first half of this code works but the second half doesn't. could anyone explain what i'm doing wrong below and then how to make a working lsubset function that could take gear == 4 as its second parameter? sorry if my searches missed a similar question.. thanks!!

# create a list of mtcars data frames
mtlist <- list( mtcars , mtcars , mtcars )
# subset them all - this works
lapply( mtlist , subset , gear == 4 )


# create a function that simply replicates the task above
lsubset <- 
    function( x , sset ){
        lapply( x , subset , sset )
    }

# this does not work for some reason
lsubset( mtlist , gear == 4 )

I'd take a look at [this](http://stackoverflow.com/questions/9860090/in-r-why-is-better-than-subset) question thread and [this one](http://stackoverflow.com/questions/12850141/programming-safe-version-of-subset-to-evaluate-its-condition-while-called-from/12850252#12850252). It is commonly said that using `subset` programatically is a bad idea, and I think you're about to discover why! — Justin, Jul 01 '13 at 15:17

score 5 · Accepted Answer · answered Jul 01 '13 at 15:18

5

What about:

lsubset <- 
    function( x , ... ){
        lapply( x , subset , ... )
    }

lsubset( mtlist , gear == 4 )

answered Jul 01 '13 at 15:18

Tyler Rinker

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Kudos to me for resisting the urge to end my answer with "This has been a work of R" – Tyler Rinker Jul 01 '13 at 15:22
The trick here is that `gear==4` is passed down to `subset` unevaluated, am I right? – Ferdinand.kraft Jul 01 '13 at 16:43
That was my understanding but I'm no programmer. – Tyler Rinker Jul 01 '13 at 16:52

how to pass an expression through a function for the subset function to evaluate in R

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