Hi:

On Fri, Jun 25, 2010 at 6:12 PM, Vadim Ogranovich <
vogranovich@jumptrading.com> wrote:

> Dear R users,
>
> Please disregard my previous post "converting result of substitute to
> 'ordidnary' expression". The problem I have has nothing to do with
> substitute.
>
> Consider:
>
> > dat <- data.frame(x=1:10, y=1:10)
>
> > subsetexp <- expression(5<x)
>

> class(subsetexp)
[1] "expression"


> > ## this does work
> > subset(dat, eval(subsetexp))
>    x  y
> 6   6  6
> 7   7  7
> 8   8  8
> 9   9  9
> 10 10 10
>
> eval(subsetexp)
 [1] FALSE FALSE FALSE FALSE FALSE  TRUE  TRUE  TRUE  TRUE  TRUE
>
>
> > ## and so does this
> > subset(dat, 5<x)
>    x  y
> 6   6  6
> 7   7  7
> 8   8  8
> 9   9  9
> 10 10 10
>

> 5 < x
 [1] FALSE FALSE FALSE FALSE FALSE  TRUE  TRUE  TRUE  TRUE  TRUE

>
> > ## but this doesn't work
> > subset(dat, subsetexp)
> Error in subset.data.frame(dat, subsetexp) :
>  'subset' must evaluate to logical
>
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

>
> Why did the last expression fail and why it worked with eval()?
>

subsetexp is an object of class expression, whereas the second argument of
subset()
expects a logical [vector]. As shown above, both 5 < x and eval(subsetexp)
evaluate to
logical vectors.

HTH,
Dennis

>
> Thank you very much for your help,
> Vadim
>
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