[R-sig-Geo] kriging output

[Roger Bivand]

On Mon, 5 May 2008, Edzer Pebesma wrote:

> Dave Depew wrote:
>>  Thanks,
>>  This worked.
>>
>>  I'm still confused why the if else statement didn't work...
>>  If one wanted to do conditional arithmetic would a for statement bee
>>  needed?
>> 
>>
>>  e.g.
>>  meuse.grid[["class"]] = for(i in 1:length(meuse.grid[["dist"]])){
>>  if (meuse.grid[["dist"]]<0.5)
>>  {meuse.grid[["class"]]=10*meuse.grid[["dist]]} else
>>  {meuse.grid[["class"]]=100* meuse.grid[["dist"]]}
>>  }
>> 
> I'd say
> meuse.grid$class = ifelse(meuse.grid$dist < .5, 10, 100) * meuse.grid$dist

Right. ifelse() is vectorised, but if() else is not, so the vector 
condition in ifelse() splits out nicely, but if() else is flow control on 
a scalar condition. The above for loop looks redundant anyway (no use of 
index), but the if() else construction wouldn't work either - as you 
found out earlier.

For things like this, Braun & Murdoch is good (and from Canada!); if you 
read German, Uwe Ligges' book is recommended. It's also in the online 
Introduction to R, section 9.2.1, which explains the previous paragraph 
much more clearly than I can.

Roger

>
> assigning the result from a for statement also does not yield a beauty price 
> if you ask me, but that might be my  backgrounds in C, long ago.
> --
> Edzer
>
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no