[R] function and apply

[Gabor Grothendieck]

On 6/5/05, jose silva <chess.player at oninet.pt> wrote:
> Dear all
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> I think my problem is not complicated but I'm having difficulties to solve it.
> v is a vector: v=c(p1 , p2 , p3 , p4), and f is a function: f : v -> w , where
> w=c(p1 , p2*(1-p1) , p3*(1-p2)*(1-p1) , p4*(1-p3)*(1-p2)*(1-p1))
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> I write the function f as:
> f<- function(w,x,y,z) {c(w,x*(1-w),y*(1-x)*(1-w),z*(1-y)*(1-x)*(1-w))}
> f(a,b,c,d) it works well.

> But now I want to apply f to each row of a data frame with 4 columns: 
> d<-data.frame(a=seq(1,10), b=seq(11,20), c=seq(21,30),d=seq(31,40))
> t(apply(d,1,f)) is not working?
> I think each element of each row is not corresponding to w,x,y,z and now I'm lost?
> Can someone help me?

apply passes each of row of d to f as a single vector, not 4 individual
elements, so in terms of your f define fv which is like f but takes a 
vector argument:

fv <- function(x) f(x[1], x[2], x[3], x[4])
apply(d,1,fv)  # or maybe you want t(apply(d,1,fv))

# In fact, since your f works with vectors as arguments this would do:
as.data.frame(fv(d))