[R] how to rewrite this without a loop ?

[Stijn Lievens]

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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