Also ?tapply
But if you are a beginner who has done no homework -- i.e. you have
made no effort to learn basics with The Intro to R tutorial or other
online tutorial -- then you probably won't be able figure it out. We
expect some minimal effort by posters. If you have done such homework,
then it may be an alternative for you.
-- Bert
On Sun, May 19, 2013 at 9:06 AM, David Winsemius <dwinsem...@comcast.net> wrote:
>
> On May 19, 2013, at 4:20 AM, Jess Baker wrote:
>
>> Dear list,
>>
>> I am very new to R and have been struggling with extracting data from a
>> netcdf file. Basically I have read in a file containing vegetation height
>> data organised in 0.5 degree grid cells spanning the whole globe. Each cell
>> contains a histogram representing the height distribution and I need to
>> extract a single value (the mean) from each cell. My data has 720 rows and
>> 240 columns so is pretty large and I’ve been told for loops should be the
>> most straightforward way to do this. Sorry the code below is not
>> reproducible but hopefully it illustrates where I am going wrong.
>>
>> ##The following returns the mean value of a single cell (260,90)
>>
>>> sum(ht.center.vals*(ht.hist[260,90,]))/sum(ht.hist[260,90,])
>>
>> ##but when I try to translate this to a for loop I get NaN
>>
>>> mean.height<-NULL
>>> for(i in 1:nrow(ht.hist)){
>> + for(j in 1:ncol(ht.hist)){
>> + mean.height<-sum(ht.center.vals*(ht.hist[i,j,]))/sum(ht.hist[i,j,])
>> + }
>> + }
>>> mean.height
>> [1] NaN
>
> You appear to be committing a common programming mistake. Assignment on the
> LHS to a non-indexed variable will overwrite all of the previous values. The
> NaN is now simply the last value calculated.
>
>
>> Can anyone tell me how I tell R to look at each cell in turn, extract the
>> mean value and save it in a new array? Clearly I am not doing it correctly!
>
> I would imagine making mean.height be an matrix/array of the desired
> dimensions and then assign to the proper values inside that object using the
> loop indices.
>
> mean.height <- matrix(NA, nrow(ht.hist), ncol(ht.hist) )
> # then the loop would do the assignment:
>
> mean.height[,i,j] <- …
>
>
> --
>
> David Winsemius
> Alameda, CA, USA
>
> ______________________________________________
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
--
Bert Gunter
Genentech Nonclinical Biostatistics
Internal Contact Info:
Phone: 467-7374
Website:
http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm
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