breaking out of for loop when running a function inside a for loop in R
Suppose you have the following function foo
. When I'm running a for
loop, I'd like it to skip the remainder of foo
when foo
initially returns the value of 0
. However, break
doesn't wor开发者_运维问答k when it's inside a function.
As it's currently written, I get an error message, no loop to break from, jumping to top level
.
Any suggestions?
foo <- function(x) {
y <- x-2
if (y==0) {break} # how do I tell the for loop to skip this
z <- y + 100
z
}
for (i in 1:3) {
print(foo(i))
}
Admittedly my R knowledge is sparse and this is drycoded, but something like the following should work:
foo <- function(x) {
y <- x-2
if (y==0) {return(NULL)} # return NULL then check for it
z <- y + 100
z
}
for (i in 1:3) {
j <- foo(i)
if(is.null(j)) {break}
print(j)
}
Edit: updated null check for posterity
As a matter of coding practice, don't do this. Having a function that can only be used inside a particular loop is not a great idea. As a matter of educational interest, you can evaluate the 'break' in the parent environment.
foo <- function(x) {
y <- x-2
if (y==0) {eval.parent(parse(text="break"),1)}
z <- y + 100
z
}
for (i in 0:3) {
print(foo(i))
}
Are we allowed to be a little more creative? Could you recast your problem to take advantage of the following approach, where the operation is based on vectors?
x <- 1:3
y <- x[x-2 < 0] - 2 + 100 # I'm leaving the "- 2" separate to highlight the parallel to your code
y
If, however, a deeper form underlies the question and we need to follow this pattern for now, perhaps tweak it just a bit...
foo <- function(x) {
y <- x - 2
if (y != 0) {
z <- y + 100
z
} # else implicitly return value is NULL
}
for (i in 1:3) {
if (is.numeric(result <- foo(i))) {
print(result)
} else {
break
}
}
An alternative way is to throw an error and catch it with try
, like so:
foo <- function(x) {
y <- x-2
if (y==0) {stop("y==0")}
z <- y + 100
z
}
try(for (i in 0:5) {
print(foo(i))
}, silent=TRUE)
## or use tryCatch:
for (i in 0:5) {
bar <- tryCatch(foo(i),error=function(e) NA)
if(is.na(bar)){ break } else { print(bar) }
}
I have no clue how r
works but I found the question interesting because I could lookup a new language's syntax so excuse my answer if it is totally wrong :)
foo <- function(x) {
y <- x-2
if (y!=0) z <- NULL else z <- y + 100
z
}
for (i in 1:3)
{
a <- foo(i)
if (a == NULL) {next}
print(a)
}
精彩评论