Using Next in a Control Flow

Question

There doesn't seem to be any examples of 'next' usage in the control flow help page. I'd like it to skip to the next iteration based on a condition within the script.

Using the example below, let's say I don't want it to print, unless x[i] > 5, the expected output would be 5 through 10 on screen:

x <- 1:100
for(i in 1:10) {
# next(x[i] < 5) # Just for conceptualizing my question.
print(x[i])
}

How would I go about implementing the use of next to accomplish something like what's shown above?

It would be nice if SO had a [tag:missing-documentation] tag. For all those little things where the R manual is incomplete. :) — Iterator, Feb 14 '12 at 00:37
@Iterator: the documentation isn't _missing_ (`?"next"`), so maybe a "documentation-missing-example" tag? — Joshua Ulrich, Feb 14 '12 at 01:26
@JoshuaUlrich Errm, I added a link to the documentation for this case. However, [as you know](http://stackoverflow.com/a/7327565/805808) there are other cases where the documentation is ... left as an exercise. :) However, there is a "code-example" (and "code-examples") tag. — Iterator, Feb 14 '12 at 01:47
@JoshuaUlrich the problem was that there's not really a clear way to see an example of usage of `next` from the documentation. `example(next)` is similarly useless. — Brandon Bertelsen, Feb 14 '12 at 01:49
Hmm, this makes me think of cases where I have a dataset, but elements are `NA`. Is my dataset half full or half empty? Same with documentation. Anyway, maybe the "code-examples" tag is better here. I still think there's a lot of undocumented functionality in R. :) — Iterator, Feb 14 '12 at 01:59
@Iterator: That's what I meant to say. It _is_ documented but an example would really help because `next` and `break` aren't functions with arguments. I would also like if `?continue` took you to `?"next"`. — Joshua Ulrich, Feb 14 '12 at 04:34
@BrandonBertelsen Cheesy? You should meet my friend "multithreader" and his jokes. ;-) "OS: You've got nice threads." "Process: Wanna spawn?" "OS: Go fork yourself." — Iterator, Feb 14 '12 at 17:35
@Iterator 7 years later, I just giggled at this horrible joke... — Brandon Bertelsen, Mar 07 '19 at 12:41

score 16 · Accepted Answer · answered Feb 14 '12 at 00:10

16

I will give you a complete example and a 'yes' but I am unsure what your questions is:

R> for (i in 1:10) {
+     if (i < 5) next
+     print(i)
+ }
[1] 5
[1] 6
[1] 7
[1] 8
[1] 9
[1] 10
R>

answered Feb 14 '12 at 00:10

Dirk Eddelbuettel

360,940
56
644
725

Josh O'Brien · Answer 2 · 2012-02-14T00:20:05.950

10

To make this work, you need to test whether x < 5 and, if it is, go to next. next will, in turn (to quote the help page), "[halt] the processing of the current iteration and [advance] the looping index", starting back through the loop again.

x <- 1:100
for(i in 1:10) {
    if(x[i] < 5) next
    print(x[i])
}
[1] 5
[1] 6
[1] 7
[1] 8
[1] 9
[1] 10

edited Feb 14 '12 at 00:20

answered Feb 14 '12 at 00:10

Josh O'Brien

159,210
26
366
455

Using Next in a Control Flow

2 Answers2