[R] which LETTERS?

[Duncan Murdoch]

On 10/11/2014 7:50 AM, Charles Stangor wrote:
> I'm confused:
>
> Thanks in advance.
>
> > which(LETTERS == c("A"))

This computes

LETTERS == c("A")

then returns the indices where it is TRUE.  Since LETTERS has 26 
elements, but "A" has only one, the "A" is repeated 26 times.  Only the 
first one matches LETTERS.
> [1] 1
> > which(LETTERS == c("A","B"))

The c("A", "B") needs to be repeated 13 times to get to length 26. Only 
the first two match.
> [1] 1 2
> > which(LETTERS == c("A","B","C"))

c("A", "B", "C") can't be repeated a whole number of times to extend to 
length 26, so it is repeated 8 2/3 times, and you get a warning.
> [1] 1 2 3
> Warning message:
> In LETTERS == c("A", "B", "C") :
>    longer object length is not a multiple of shorter object length

You probably want to use

which(LETTERS %in% c("A","B","C"))

instead.

Duncan Murdoch


>
> Charles Stangor
> Professor
> Dept of Psychology
> University of Maryland
> Academic Achievement Research Group <http://www.charlesstangor.com/AARG>
>
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>
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