[R] Positions in datasets

[David Winsemius]

On Dec 27, 2009, at 12:11 PM, Anders Falk wrote:

> Could someone help me understand this
>
> Basically I want to know the position of certain numbers in large  
> output
> data sets. First consider the following simple example where we get  
> the
> postions of ones (1) in the vector q.
>
>> q <- c(5,1,1,3,1,1,1,1)
>> q
> [1] 5 1 1 3 1 1 1 1
>> for (i in 1:length(q)) if(q[i]==1) print(i)
> [1] 2
> [1] 3
> [1] 5
> [1] 6
> [1] 7
> [1] 8
>
> Well done!
>
>
> But now consider the following case where the input consists of  
> different
> combinations of the three numbers 2, 3 and 7. They are put into a  
> function
> 1/x +1/y +1/z +1/(x*y*z). All different combinations will yield the  
> same
> result namely precisely 1, which is also clearly seen in the output.
> However when I try to get information on the position of the ones in  
> the
> output, there are only two that are recognized, although all six  
> items in
> the output are indeed ones.
>
>> x <- c(2,2,3,3,7,7); y <- c(3,7,2,7,2,3); z <- c(7,3,7,2,3,2)
>> data.frame(x,y,z)
>  x y z
> 1 2 3 7
> 2 2 7 3
> 3 3 2 7
> 4 3 7 2
> 5 7 2 3
> 6 7 3 2
>> p <- numeric(length(x))
>> for (i in 1:length(x)) p[i] <- ((1/x[i]) + (1/y[i]) + (1/z[i]) +
> (1/(x[i]*y[i]*z[i])))
>> p
> [1] 1 1 1 1 1 1
>> for (i in 1:length(p)) if(p[i]==1) print(i)
> [1] 4
> [1] 6
>
>
> I suppose there must exist some better way of accessing the position  
> of
> certain numbers in a large data output.

"data output" is a bit vague, but if you are talking about vectors then:

?which

> So apart from getting advice on
> that I would certainly also like to understand why the above only  
> seems to
> work in some cases.

It's a FAQ, 7.31:
http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f

 > for (i in 1:length(p)) if(all.equal(p[i],1)) print(i)
[1] 1
[1] 2
[1] 3
[1] 4
[1] 5
[1] 6
>
> Anders B Falk PhD
> Uppsala Sweden
>
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> and provide commented, minimal, self-contained, reproducible code.

David Winsemius, MD
Heritage Laboratories
West Hartford, CT