[R] moving from loops to apply

[Rui Barradas]

Hello,

Simple question with reproducible example code.
The best way to go is to know to what dimension you want to apply the 
function, the 1st, and to write the function in such a way as to have 
the passed rows as the first argument. If it has other arguments, they 
go after. Since your function is already written like this, there's 
little left to be done.

d2 <- apply(x, 1, distancer, y)  # note the other arg.
dim(d2)
all.equal(distances, t(d2))

Why this transpose? Because apply is passing _row_vectors_ and the 
function's return values are vectors, in R, _columns_.
Were it applying the function to columns and the return value would be 
with the right dims. Like this just assign

d2 <- t(d2) # or d2 <- t(apply(...etc...))

Hope this helps,

Rui Barradas


Em 15-06-2012 12:27, Schumacher, G. escreveu:
> Dear subscribers,
>
> I have made a simulation using loops rather than apply, simply because the loop function seems more natural to me. However, the current simulation takes forever and I have decided - finally - to learn how to use apply, but - as many other people before me - I am having a hard time changing habits. My current problem is:
>
> My current code for the loop is:
> distances <- matrix(NA, 1000, 5)
> distancer <- function(x, y){-(abs(x-y))}
> x <- as.matrix(rnorm(1000, 5, 1.67))
> y <- rnorm(5, 5, 1.67)
>
> for (v in 1:1000){
> distances[v,] <- distancer(x[v,], y)
> }
>
> The goal is to calculate the distances between the preferences of each voter (X) and all parties (Y). This gives a 1000 by 5 matrix (distances).
>
> If I want to transform this to apply, what would be the best way to go? More specifically, I am not sure what to put into the X part of the apply function.
>
> Sorry, for asking this question that is already much debated, I just don't seem to be able to apply to my own case. Many thanks in advance.
>
> Kind regards,
>
> Gijs Schumacher
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> 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.
>