为什么 R 说没有循环中断/下一个，跳转到顶层

Question

Why does R throw the error "Error in value[3L]: no loop for break/next, jumping to top level" instead of going to the next iteration of a loop? I'm on R version 2.13.1 (2011-07-08)

for (i in seq(10)) { 
   tryCatch(stop(), finally=print('whoops'), error=function(e) next) 
}

This problem came up because I wanted to create a different image or no image at all when plot failed. The code, using joran's approach, would look like this:

for (i in c(1,2,Inf)) { 
   fname = paste(sep='', 'f', i, '.png')
   png(fname, width=1024, height=768) 
   rs <- tryCatch(plot(i), error=function(e) NULL)
   if (is.null(rs)){
    print("I'll create a different picture because of the error.")
   }
   else{
    print(paste('image', fname, 'created'))
    dev.off()
    next
   } 
}

Answer 1

Maybe you could try:

for (i in seq(10)) { 
   flag <- TRUE
   tryCatch(stop(), finally=print('whoops'), error=function(e) flag<<-FALSE)
   if (!flag) next
}

Answer 2

Unfortunately, once you get inside your error function you're no longer in a loop. There's a way you could hack around this:

for (i in seq(10)) { 
   delayedAssign("do.next", {next})
   tryCatch(stop(), finally=print('whoops'),
        error=function(e) force(do.next))
}

Though that is... well, hacky. Perhaps there is a less hacky way, but I don't see one right off.

(This works because delayedAssign happens every loop, canceling out the efforts of force )

EDIT

Or you could use continuations:

for (i in seq(10)) { 
   callCC(function(do.next) {
       tryCatch(stop(), finally=print('whoops'),
           error=function(e)    do.next(NULL))
       # Rest of loop goes here
       print("Rest of loop")
   })
}

EDIT

As Joris points out, you probably shouldn't actually use either of these, because they're confusing to read. But if you really want to call next in a loop, this is how:).

Answer 3

Wouldn't it make more sense to put the next outside the tryCatch based on an if check? Something like this:

for (i in c(1,2,Inf)) { 
   rs <- tryCatch(seq(i), finally=print('whoops'), error=function(e) NULL)
   if (is.null(rs)){
    print("I found an error!")
   }
   else{
    next
   } 
}

although I'm not sure this is what you want, since I'm a little unclear on what you're trying to do.

EDIT

Based on the OP's revisions, this formulation works for me:

plotFn <- function(fname,i){
    png(fname, width=400, height=200)
    plot(i)
    dev.off()
}


for (i in c(1,Inf,3)) { 
   fname = paste('f', i, '.png',sep="")
   rs <- tryCatch(plotFn(fname,i), error=function(e){dev.off(); return(NULL)})
   if (is.null(rs)){
    print("I'll create a different picture because of the error.")
   }
   else{
    print(paste('image', fname, 'created'))
    next
   } 
}

I'm certain that not having a dev.off() call in the case of an error needed to be fixed. I'd have to dig a little deeper to figure out exactly why separating png and plot was causing problems. But I think it's probably cleaner to keep the png(); plot(); dev.off() png(); plot(); dev.off() png(); plot(); dev.off() sequence self contained anyway. Also note that I put a dev.off() in the error function.

I haven't tested what will happen if plotFn throws an error on png() , never creates the device and then reaches the error function and calls dev.off() . Behavior may depend on what else you have going on in your R session.