Re: [R] how to rewrite this without a loop ?
Thomas Lumley wrote:
On Thu, 18 Nov 2004, Stijn Lievens wrote:
add.fun <- function(perf.data) {
ss <- 0
for (i in 0:29) {
ss <- ss + cor(subset(perf.data, dataset == i)[3],
subset(perf.data, dataset == i)[7], method = "kendall")
}
ss}
As one can see this function uses a for-loop. Now chapter 9 of 'An
introduction to R' tells us that we should avoid for-loops as much as
possible.
You don't say whether `dataset' is the name of a column in `perf.data'.
Assuming it is, and assuming that 0:29 are all the values of `dataset'
sum(by(perf.data, list(perf.data$dataset),
function(d) cor(d[,3],d[,7], method="kendall")))
would work.
Indeed, this works. The 'by' command is exactly what I was looking for.
As far as I can tell, this useful command it isn't mentioned in 'An
introduction to R'.
If this is faster it will be because you don't call
subset() twice per iteration, rather than because you are avoiding a
loop. However it has other benefits: it doesn't have the variable `i',
it doesn't have to change the value of `ss', and it doesn't have the
range of `dataset' hard-coded into it. These are all clarity
optimisations.
In fact I don't care too much about speed at the moment, but a one-line
statement is more convenient to type (and recall) in the command line
interface then a multi-line statmement.
Your solution really does the trick for me. Thanks,
Stijn.
-thomas
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Re: [R] how to rewrite this without a loop ?
Stijn Lievens wrote:
Dear Rexperts,
First of all let me say that R is a wonderful and useful piece of software.
The only thing is that sometimes it takes me a long time to find out how
something can be done, especially when aiming to write compact (and
efficient) code.
For instance, I have the following function (very rudimentary) which
takes a (very specific) data frame as input and for certain subsets
calculates the rank correlation between two corresponding columns.
The aim is to add all the rank correlations.
add.fun <- function(perf.data) {
ss <- 0
for (i in 0:29) {
ss <- ss + cor(subset(perf.data, dataset == i)[3],
subset(perf.data, dataset == i)[7], method = "kendall")
}
ss
}
As one can see this function uses a for-loop. Now chapter 9 of 'An
introduction to R' tells us that we should avoid for-loops as much as
possible.
Is there an obvious way to avoid this for-loop is this case ?
Using the lapply function in the e-mail of James, I came up with the
following.
sum (as.numeric( lapply( split(perf.data, perf.data$dataset),
function(x) cor(x[3],x[7],method="kendall") ) ))
So, first I split the dataframe into a list of dataframes using split,
and using lapply I get a list of correlations, which I convert to
numeric and finally sum up.
I definitely avoided the for-loop in this way, although I am not sure
whether this is more efficient or not.
Cheers,
Stijn.
I would like to see something in the lines of
(maple style)
add( seq(FUN(i), i = 0..29) )
Greetings
Stijn.
--
==
Dept. of Applied Mathematics and Computer Science, University of Ghent
Krijgslaan 281 - S9, B - 9000 Ghent, Belgium
Phone: +32-9-264.48.91, Fax: +32-9-264.49.95
E-mail: [EMAIL PROTECTED], URL: http://allserv.ugent.be/~slievens/
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[R] how to rewrite this without a loop ?
Dear Rexperts,
First of all let me say that R is a wonderful and useful piece of
software.
The only thing is that sometimes it takes me a long time to find out how
something can be done, especially when aiming to write compact (and
efficient) code.
For instance, I have the following function (very rudimentary) which
takes a (very specific) data frame as input and for certain subsets
calculates the rank correlation between two corresponding columns.
The aim is to add all the rank correlations.
add.fun <- function(perf.data) {
ss <- 0
for (i in 0:29) {
ss <- ss + cor(subset(perf.data, dataset == i)[3],
subset(perf.data, dataset == i)[7], method = "kendall")
}
ss
}
As one can see this function uses a for-loop. Now chapter 9 of 'An
introduction to R' tells us that we should avoid for-loops as much as
possible.
Is there an obvious way to avoid this for-loop is this case ?
I would like to see something in the lines of
(maple style)
add( seq(FUN(i), i = 0..29) )
Greetings
Stijn.
--
==
Dept. of Applied Mathematics and Computer Science, University of Ghent
Krijgslaan 281 - S9, B - 9000 Ghent, Belgium
Phone: +32-9-264.48.91, Fax: +32-9-264.49.95
E-mail: [EMAIL PROTECTED], URL: http://allserv.ugent.be/~slievens/
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