R help - Mar 2012 - beginner's loop issue

Dear All,

I hope you don't mind helping me with this small issue. I haven't been
using
R in years and I'm trying to fill in a matrix 
with the output of a function (I'm probably using the Matlab logic here and
it's not working). 
Here is my code:

for (i in 1:length(input)){
  out[i,1:3] <- MyFunction(input[i,1],input[i,2], input[i,3]) 
    out[i,4:6] <- MyFunction(input[i,5],input[i,7], input[i,6]) 
      out[i,7:9] <- MyFunction(input[i,8],input[i,10], input[i,9]) 
}

'input' is a matrix 
> dim(input)
[1] 46 10

and each raw corresponds to a different subject. 
The error I get here is

/Error in out[i, 1:3] <- get.vaTer(input[i, 2], input[i, 4], input[i, 3],  : 
  object 'out' not found/

So I wonder, what's wrong in the assignment to the variable out? 
Should I define the variable before the loop?

Thanks for your help
Best

Ale

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Yes, the short answer is that you need to define out before running
the loop. The most effective way to do so will be to set up a matrix
with the exact right dimensions (if you know them up front); something
like out <- matrix(NA, nrow = length(input), ncol = 9)

Michael



On Tue, Mar 13, 2012 at 12:27 PM, aledanda <danda.galli at gmail.com>
wrote:
> Dear All,
>
> I hope you don't mind helping me with this small issue. I haven't
been using
> R in years and I'm trying to fill in a matrix
> with the output of a function (I'm probably using the Matlab logic here
and
> it's not working).
> Here is my code:
>
> for (i in 1:length(input)){
> ?out[i,1:3] <- MyFunction(input[i,1],input[i,2], input[i,3])
> ? ?out[i,4:6] <- MyFunction(input[i,5],input[i,7], input[i,6])
> ? ? ?out[i,7:9] <- MyFunction(input[i,8],input[i,10], input[i,9])
> }
>
> 'input' is a matrix
>> dim(input)
> [1] 46 10
>
> and each raw corresponds to a different subject.
> The error I get here is
>
> /Error in out[i, 1:3] <- get.vaTer(input[i, 2], input[i, 4], input[i,
3], ?:
> ?object 'out' not found/
>
> So I wonder, what's wrong in the assignment to the variable out?
> Should I define the variable before the loop?
>
> Thanks for your help
> Best
>
> Ale
>
> --
> View this message in context:
http://r.789695.n4.nabble.com/beginner-s-loop-issue-tp4469514p4469514.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

On Tue, Mar 13, 2012 at 11:27 AM, aledanda <danda.galli at gmail.com>
wrote:
> Dear All,
>
> I hope you don't mind helping me with this small issue. I haven't
been using
> R in years and I'm trying to fill in a matrix
> with the output of a function (I'm probably using the Matlab logic here
and
> it's not working).
> Here is my code:
>
> for (i in 1:length(input)){
> ?out[i,1:3] <- MyFunction(input[i,1],input[i,2], input[i,3])
> ? ?out[i,4:6] <- MyFunction(input[i,5],input[i,7], input[i,6])
> ? ? ?out[i,7:9] <- MyFunction(input[i,8],input[i,10], input[i,9])
> }
>
> 'input' is a matrix
>> dim(input)
> [1] 46 10
>
> and each raw corresponds to a different subject.
> The error I get here is
>
> /Error in out[i, 1:3] <- get.vaTer(input[i, 2], input[i, 4], input[i,
3], ?:
> ?object 'out' not found/
out has to exist first, as previous commenter said.

Furthermore, suggestions:

Consider making MyFunction accept a vector of 3 arguments, rather than
separate arguments.

Consider making out 3 columns, as in

out <- matrix(0, nrow=N, ncol=3)
for(i ...){
out[i,1:3] <- MyFunction(input[i,1:3])
out[i,1:3] <- MyFunction(input[i,4:6])
out[i,1:3] <- MyFunction(input[i,7:9])
}

If you could re-shape your input "thing" as a list with one element
that needs to go into MyFunction, this could get easier still:

lapply(input, MyFunction)

 or if input were an array with 3 columns, you could revise MyFuntion
to accept a 3-vector.

apply(input, 1, MyFunction)

Hardly ever in R does one need to specify inputs as you have done in
your example.
pj



-- 
Paul E. Johnson
Professor, Political Science ? ?Assoc. Director
1541 Lilac Lane, Room 504 ? ? Center for Research Methods
University of Kansas ? ? ? ? ? ? ? University of Kansas
http://pj.freefaculty.org ? ? ? ? ? ?http://quant.ku.edu