[R] R-help Digest, Vol 91, Issue 21

[David Winsemius]

On Sep 21, 2010, at 3:47 PM, Kurt_Helf at nps.gov wrote:

> All
>     Is there a script in R equivalent to the "if then" transforms  
> one can
> perform in Systat?  For example, I want to create a "Treatment" column
> coded either 1 or 2 for twelve field sites in a large data set.   
> Ideally,
> I'd be able to tell R to code sites a-f as 1 and sites g-l as 2.

?ifelse  # probably the way most normally adjusted people would do it.

?match

 > selection <- letters[1:15][sample( 1:15, 30, replace=TRUE)]
 > selection
  [1] "e" "k" "l" "b" "d" "e" "b" "b" "o" "o" "c" "n" "i" "f" "g" "k"  
"b" "f" "k"
[20] "e" "m" "d" "h" "e" "c" "o" "e" "h" "a" "i"

 > match(selection, letters[1:7])   # over shot "f", guess I cannot  
count.
  [1]  5 NA NA  2  4  5  2  2 NA NA  3 NA NA  6  7 NA  2  6 NA  5 NA   
4 NA  5  3
[26] NA  5 NA  1 NA
 >  # can be used for logical indexing
  [1]  TRUE FALSE FALSE  TRUE  TRUE  TRUE  TRUE  TRUE FALSE FALSE   
TRUE FALSE
[13] FALSE  TRUE  TRUE FALSE  TRUE  TRUE FALSE  TRUE FALSE  TRUE  
FALSE  TRUE
[25]  TRUE FALSE  TRUE FALSE  TRUE FALSE
 > !is.na(match(selection, letters[8:15]) )
  [1] FALSE  TRUE  TRUE FALSE FALSE FALSE FALSE FALSE  TRUE  TRUE  
FALSE  TRUE
[13]  TRUE FALSE FALSE  TRUE FALSE FALSE  TRUE FALSE  TRUE FALSE  TRUE  
FALSE
[25] FALSE  TRUE FALSE  TRUE FALSE  TRUE
 > # testing to see if any fell through the cracks
 > !is.na(match(selection, letters[8:15]) )| !is.na(match(selection,  
letters[1:7]) )
  [1] TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE  
TRUE TRUE
[16] TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE  
TRUE TRUE

Could do this for the case you requested:
Treatment <- 2 - !is.na(match(selection, letters[1:7]) )

TRUE is 1 and FALSE is zero in Boolean arithmetic (which R supports).

-- 
David.

> Cheers
> Kurt
>
> ***************************************************************
> Kurt Lewis Helf, Ph.D.
> Ecologist
> EEO Counselor
> National Park Service
> Cumberland Piedmont Network
> P.O. Box 8
> Mammoth Cave, KY 42259
> Ph: 270-758-2163
> Lab: 270-758-2151
> Fax: 270-758-2609
> ****************************************************************
> Science, in constantly seeking real explanations, reveals the true  
> majesty
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> -Richard Dawkins
>
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> tradition
> in having two layers.  Like the latter it passes on its theories but  
> it
> also passes on a critical attitude towards them.  The theories are  
> passed
> on not as dogmas but rather with the challenge to discuss them and  
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> upon them.
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>
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> as yourself.
> -Wayside at Wilderness Threshold in McKittrick Canyon, Guadalupe  
> Mountains
> National Park, TX
>
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>
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>
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David Winsemius, MD
West Hartford, CT