[R] Positions in datasets

[Uwe Ligges]

David Winsemius wrote:
> 
> 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



Completely right, but just for completeness let me add that you do never 
want to say 1:length(p) in your code, because it might happen that p 
becomes empty and your loop will iterate over 1:0, hence better always 
(then you'll never run into that problem) use seq_along as in:

for (i in seq_along(p)) if(all.equal(p[i],1)) print(i)

Best,
Uwe Ligges


>> Anders B Falk PhD
>> Uppsala Sweden
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide 
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
> 
> David Winsemius, MD
> Heritage Laboratories
> West Hartford, CT
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide 
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.