[R] how to rewrite this without a loop ?
Stijn Lievens
stijn.lievens at ugent.be
Thu Nov 18 16:47:04 CET 2004
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.
>
> <code>
> 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
> }
> </code>
>
> 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.
<code>
sum (as.numeric( lapply( split(perf.data, perf.data$dataset),
function(x) cor(x[3],x[7],method="kendall") ) ))
</code>
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)
>
> <code>
> add( seq(FUN(i), i = 0..29) )
> </code>
>
> Greetings
>
> Stijn.
>
>
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