[R] sapply Call Returning " the condition has length > 1" Error

[John Fox]

Dear Alex,

> -----Original Message-----
> From: Alex Zhang [mailto:alex.zhang at ymail.com]
> Sent: December-27-11 3:34 PM
> To: John Fox
> Cc: r-help at r-project.org
> Subject: Re: [R] sapply Call Returning " the condition has length > 1"
> Error
> 
> John,
> 
> Thanks for the pointers.
> 
> The DummyFunc is just a made-up example. The true function I need to
> use is more complicated and would be distractive to include.

You'll probably get a better answer if you don't keep what you want to do a
secret.

> 
> Do you mean that sapply would take columns in the input data.frame and
> feed them into "FUN" as "whole" vectors? That explains the behavior.

Yes. As I said, a data frame is a list of columns, so FUN is called with
each column as its argument.

> Is there an "*apply" function that will fee elements of the input
> data.frame into "FUN" instead of whole columns? Thanks.

I'm afraid that I don't know what you mean. Do you want to deal with the
columns of the data frame separately (in general, they need not all be of
the same class), and within each column, apply a function separately to each
element? You could nest calls to lapply() or sapply(), as in

sapply(D, function(DD) sapply(DD, abs))

assuming, of course, that D is an entirely numeric data frame. But in this
case,

abs(as.matrix(D))

would be more sensible, and using sapply() like this isn't necessarily
better than a loop. Again, not knowing what you want to do makes it hard to
suggest a solution.

Best,
 John

> 
> ________________________________
> 
> From: John Fox <jfox at mcmaster.ca>
> To: 'Alex Zhang' <alex.zhang at ymail.com>
> Cc: r-help at r-project.org
> Sent: Tuesday, December 27, 2011 3:10 PM
> Subject: RE: [R] sapply Call Returning " the condition has length > 1"
> Error
> 
> Dear Alex,
> 
> > -----Original Message-----
> > From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> > project.org] On Behalf Of Alex Zhang
> > Sent: December-27-11 2:14 PM
> > To: r-help at r-project.org
> > Subject: [R] sapply Call Returning " the condition has length > 1"
> > Error
> >
> > Dear all,
> >
> > Happy new year!
> >
> > I have a question re using sapply. Below is a dummy example that
> would
> > replicate the error I saw.
> >
> > ##Code Starts here
> > DummyFunc <- function(x) {
> >
> > if (x > 0) {
> > return (x)
> > } else
> > {
> > return (-x)
> > }
> >
> > }
> >
> > Y = data.frame(val = c(-3:7))
> > sapply(Y, FUN = DummyFunc)
> > ##Code ends here
> >
> > When I run it, I got:
> >      val
> >  [1,]  3
> >  [2,]  2
> >  [3,]  1
> >  [4,]  0
> >  [5,]  -1
> >  [6,]  -2
> >  [7,]  -3
> >  [8,]  -4
> >  [9,]  -5
> > [10,]  -6
> > [11,]  -7
> > Warning message:
> > In if (x > 0) { :
> >  the condition has length > 1 and only the first element will be
> used
> >
> > The result is different from what I would expect plus there is such
> an
> > error message.
> 
> This is a warning, not really an error message. A data frame is
> essentially a list of variables (columns), and sapply() applies its
> FUN argument to each list element, that is, each variable -- the one
> variable val in your case.
> That produces a warning because val > 0 is a vector of 11 elements,
> and the first comparison, 3 > 0, which is TRUE, controls the result.
> 
> >
> > I guess if the DummyFunc I provided is compatible with vectors, the
> > problem would go away. But let's suppose I cannot change DummyFunc.
> Is
> > there still a way to use sapply or alike without actually writing a
> > loop? Thanks.
> 
> Well, you could just use
> 
> > abs(Y$val)
> [1] 3 2 1 0 1 2 3 4 5 6 7
> 
> but I suppose that you didn't really want to write your own version of
> the absolute-value function as something more than an exercise.
> 
> An alternative is
> 
> > with(Y, ifelse(val > 0, val, -val))
> [1] 3 2 1 0 1 2 3 4 5 6 7
> 
> I hope this helps,
> John
> 
> --------------------------------
> John Fox
> Senator William McMaster
>   Professor of Social Statistics
> Department of Sociology
> McMaster University
> Hamilton, Ontario, Canada
> http://socserv.mcmaster.ca/jfox
> 
> 
> 
> >
> > - Alex
> >     [[alternative HTML version deleted]]
> 
> 
> 
>