Deleting from array while looping

[n8ta]

Deleting from array while looping

[benhoyt]

Interesting! Yeah, the spec doesn't seem to define this, though I think a reasonable assumption (extrapolating the bit you quoted about adding new elements) would be that it's not specified and the implementation is free to yield the deleted element (gawk, mawk), or not yield it (GoAWK, original-awk).

Note that apart from the question of how delete interactions with iteration, there's also the question of iteration order itself. The POSIX AWK spec says that the implementation can iterate "in an unspecified order". GoAWK does just that, and actually uses Go's map iteration order, which varies from iteration to iteration. In this case for whatever reason Go usually iterates 0 first, then 1 ... but if you run it a dozen times or so, you'll see it iterate the other way:

# case where 0 is iterated first (seems to be more common)
$ goawk -f t.awk
key 0 val zero
# every so often 1 is iterated first
$ goawk -f t.awk
key 1 val one
key 0 val zero

GoAWK also uses Go's delete() semantics, which is that "If a map entry that has not yet been reached is removed during iteration, the corresponding iteration value will not be produced." In other words, if the 0 is iterated first, the 1 will be deleted and not shown at all; but if the 1 is iterated first, GoAWK will try to delete 2, which doesn't exist so it'll do nothing, and then show the 0 after.

Note also that original-awk ("one true awk") seems to iterate the 0 first, and delete the 1:

$ original-awk -f t.awk
key 0 val zero

In any case, given the above, I think GoAWK's behaviour is within spec here.

[n8ta]

Yes I did notice goawk is the only one to iterate in an (obviously) varying order but that's completely fair given the spec and seems natural for a go impl. If onetrueawk agrees with you I'm a lot more confident that this a fair interpretation of the spec (:

I really don't like the gawk/mawk behavior of yielding a deleted key; happy you did not got go that way.