sapply Call Returning " the condition has length > 1" Error

John Fox

Tue, Dec 27, 2011 1:06 PM

Dear Alex,

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

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

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
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sapply Call Returning " the condition has length > 1" Error

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