Debugging lapply/sapply calls

Question

Code written using lapply and friends is usually easier on the eyes and more Rish than loops. I love lapply just as much as the next guy, but how do I debug it when things

Answer 1

I've faced the same problem and have tended to make my calls with (l)(m)(s)(t)apply to be functions that I can debug().

So, instead of blah<-sapply(x,function(x){ x+1 })

I'd say,

 myfn<-function(x){x+1}
 blah<-sapply(x,function(x){myfn(x)})

and use debug(myfn) with options(error=recover).

I also like the advice about sticking print() lines here and there to see what is happening.

Even better is to design a test of myfn(x) that it has to pass and to be sure it passes said test before subjecting it to sapply. I only have patience to to this about half the time.