[R] For loop output

[Jan Kim]

On Thu, Aug 08, 2013 at 04:05:57PM +0100, Jenny Williams wrote:
> I am having difficulty storing the output of a for loop I have generated. All I want to do is find all the files that I have, create a string with all of the names in quotes and separated by commas. This is proving more difficult than I initially anticipated.
> I am sure it is either very simple or the construction of the for loop is not quite right
> The result gets automatically printed after the loop but I can't seem to save it.
> I have tried to create the element in advance but the result is the same: NULL

This is a somewhat frequent confusion of the (very different!) concepts
of assignment and printing. The fact that something becomes visible
to the human user via printing has nothing to do really with the fact
that an object is generated and assigned to a variable, thereby becoming
accessible (and, in that allegorical sense, "visible") to the subsequent
code.

The cat function prints values but it does not return them. It returns
NULL, which is what you get.

> individual.proj = Sys.glob("Arabica/proj_current/individual_projections/*.img", dirmark = FALSE)
> individual.proj
> [1] "Arabica/proj_current/individual_projections/proj_current_arabica_pa.data.tmp$pa.tab_Full_GBM.img"
>  [2] "Arabica/proj_current/individual_projections/proj_current_arabica_pa.data.tmp$pa.tab_Full_GLM.img"
>  [3] "Arabica/proj_current/individual_projections/proj_current_arabica_pa.data.tmp$pa.tab_Full_MARS.img"
>  [4] "Arabica/proj_current/individual_projections/proj_current_arabica_pa.data.tmp$pa.tab_Full_RF.img"
>  [5] "Arabica/proj_current/individual_projections/proj_current_arabica_pa.data.tmp$pa.tab_RUN10_GBM.img"
> 
> 
> ##generate loop to create string out of the table of projected files.
> L.ip = length(individual.proj)
>   for (i in 1:L.ip){
>            individual.proj.i <- individual.proj[i]
>            individual.proj.quote = cat(paste('"', individual.proj.i, '"', ',',sep=""))
>            }
> "Arabica/proj_current/individual_projections/proj_current_arabica_pa.data.tmp$pa.tab_Full_GBM.img","Arabica/proj_current/individual_projections/proj_current....
> 
> ##print output string
> individual.proj.quote
> NULL
> 
> #command to be applied to individual.proj.quote to removed the final comma from the string
> substr(individual.proj.quote, 1, nchar(individual.proj.quote)-1)
> 
> Any help or pointers would be greatly appreciated, no amount of extensive google searches have been fruitful so far.

If you don't mind me suggesting this, reading the documentation page
of the function(s) you use is an approach that is more targeted and
therefore often quicker. (Googling "cat" is probably especially bad,
as there are various cat applications and functions in several languages,
not to mention several species of mammals...  ;-)  )

As a further remark, you don't need a loop, one line composed of sprintf
and paste (check the collapse parameter) should do the trick you're
after.

Best regards, Jan
> 
> ******************
> Jenny Williams
> Spatial Information Scientist, GIS Unit
> Herbarium, Library, Art & Archives Directorate
> Royal Botanic Gardens, Kew
> Richmond, TW9 3AB, UK
> 
> Tel: +44 (0)208 332 5277
> email: jenny.williams at kew.org<mailto:jenny.williams at kew.org>
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-- 
 +- Jan T. Kim -------------------------------------------------------+
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