sapply Call Returning " the condition has length > 1" Error
Jeff, Could you please tell me which part of the guide that I didn't follow? I provide a piece of code that can run in R, producing the problem and I also provided my results. Other people wanted to help me to learn more so asked me about more details. I replied very carefully explaining that i had provided all relevant information. Jeff, before you response, did you read any of previous posts? Sent from my iPhone
On Dec 27, 2011, at 6:19 PM, Jeff Newmiller <jdnewmil at dcn.davis.ca.us> wrote:
I suggest you (re-?)read the posting guide. Proper etiquette on this list is to provide a fully self-contained ("reproducible") example that demonstrates your problem. You are free to supply an alternate to your actual function as long as it illustrates your problem and you can deal with the re-substitution on your own.
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Alex Zhang <alex.zhang at ymail.com> wrote:
John, Thank you for your comment.? There is no secret. But the actual function I need to call is rather irrelevant.?However don't take it as the "abs" function. If you like to know, it is a function that converts 4 kinds of old ids from several old database tables into a new id in a new database. Again, I don't think providing?such detail is better than saying MyDummyFunc maps a number into a number but doesn't work?with vectors. All I need to do, is to call DummyFunc for every element in a column of a data.frame and returns?the resulted vector. But, I cannot change DummyFunc. Correct me if I am wrong: this is rather?common in a group?collaboration enviroment. Person A may be responsible for writing a function and person B who needs to use that function cannot or better not change it. Obviously, I could write a loop. Michael in a previous post suggested using vectorize which works perfectly. As a newbie of R, I would wish to learn more ways to achieve my goal (sorry, it automatically involves "how" not just "what" ;). Is there a way using?a "*apply" function to do it where * stands for any function. Thanks a lot!?
________________________________
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 4:06 PM
Subject: RE: [R] sapply Call Returning " the condition has length > 1"
Error
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]]
[[alternative HTML version deleted]]
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