deleting from a map
Nathan Trimble
for key := range m {
if expired(key) {
delete(m, key)
}
}
Is it safe to delete from m while iterating through it? That's always
been a big no-no in one of the languages (C#) I'm coming from.
Usually this means I need to create a temporary list of items to
remove and then remove them in another loop. If I don't need to do
that anymore that is great news.
BTW, congrats on Go1, and thanks for the great language :).
-Nate
Peter Bourgon
<nathan....@gmail.com> wrote:
> Looking at Effective Go, I see the following example:
>
> for key := range m {
> if expired(key) {
> delete(m, key)
> }
> }
>
> Is it safe to delete from m while iterating through it?
My understanding is "yes".
From http://golang.org/ref/spec#For_statements --
"The range expression is evaluated once before beginning the loop . .
. If map entries that have not yet been reached are deleted during
iteration, the corresponding iteration values will not be produced."
Evan Shaw
that have not yet been reached are deleted during iteration, the
corresponding iteration values will not be produced."
- Evan
Kyle Lemons
The iteration order over maps is not specified and is not guaranteed to be the same from one iteration to the next. If map entries that have not yet been reached are deleted during iteration, the corresponding iteration values will not be produced. If map entries are inserted during iteration, the behavior is implementation-dependent, but the iteration values for each entry will be produced at most once. If the map is nil, the number of iterations is 0.
Jan Mercl
> Looking at Effective Go, I see the following example:
>
> for key := range m {
> if expired(key) {
> delete(m, key)
> }
> }
>
> Is it safe to delete from m while iterating through it? That's always
> been a big no-no in one of the languages (C#) I'm coming from.
> Usually this means I need to create a temporary list of items to
> remove and then remove them in another loop. If I don't need to do
> that anymore that is great news.
3. The iteration order over maps is not specified and is not guaranteed to be the same from one iteration to the next. If map entries that have not yet been reached are deleted during iteration, the corresponding iteration values will not be produced. If map entries are inserted during iteration, the behavior is implementation-dependent, but the iteration values for each entry will be produced at most once. If the map is nil, the number of iterations is 0.
Nathan Trimble
that this is part of the spec.
-Nate
Jonathan Amsterdam
> yet been reached are deleted during iteration, the corresponding iteration
> values will not be produced.
for key := range m {
if expired(key) {
delete(m, key)
}
iteration.
Not saying it's not safe, just saying that you can't tell from the
spec.
roger peppe
given that the spec doesn't say that deleting from the map during
iteration is unsafe (and in fact explicitly mentions a case to
do with deleting when iterating), it *is* necessarily a safe operation.
"The built-in function delete removes the element with key k from a map m."
no caveats.
Norbert Roos
> Yes, it's safe: http://weekly.golang.org/ref/spec#For_statements
Is it thread-safe, too? When one go routine is iterating over the map,
and another one is deleting from it?
Norbert
Jan Mercl
Is it thread-safe, too? When one go routine is iterating over the map,
and another one is deleting from it?
Evan Shaw
> Is it thread-safe, too? When one go routine is iterating over the map, and
> another one is deleting from it?
No, it's not. Reading from multiple goroutines is fine, but if one
wants to write you need to synchronize somehow.
- Evan