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
______________________________________________
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT
______________________________________________
R-help@r-project.org mailing list
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PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
______________________________________________
R-help@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.
