Filter Lists

[Benjamin Darfler]

It looks like there is no List filter(List, Predicate) method. Am I
missing something?

[Kevin Bourrillion]

Sounds like you're aware that you can get the result as an Iterable or as a Collection using the corresponding methods in the Iterables and Collections2 classes.  The reasons to specifically want a List to be returned would be the indexed-based methods like get(n).  However, the performance characteristics of these methods on a filtered list would be... sad.  We think that if you really want a list, you should use ImmutableList.copyOf(Iterables.filter()).

This is going into the FAQ that I swear I'm working on!

On Wed, Nov 4, 2009 at 1:23 PM, Benjamin Darfler <b...@locamoda.com> wrote:

It looks like there is no List filter(List, Predicate) method. Am I
missing something?

--
Kevin Bourrillion @ Google
internal:  http://go/javalibraries
external: guava-libraries.googlecode.com

[Ben Darfler]

Thanks!

--
Software Engineer
LocaModa
Connecting People & Places

[Johan Van den Neste]

Don't want to start a new thread for this one even though it is not
very related:

I'm missing find/filter Iterable methods that return indices rather
than the objects themselves. For lists, this is more natural.
Implementing an index-based filter on top of the existing filter is
very clumsy compared to doing it the other way around. (again, only
for lists, that is)

But I understand that this could be regarded as convenience and would
conflict with 'keeping things minimal'.

--
Johan Van den Neste

[Kevin Bourrillion]

Please file an enhancement request for a Lists.find() method returning
the index.

[Jared Levy]

How about creating Lists.indexOf(List, Predicate) and Lists.lastIndexOf(List, Predicate) methods?

[Kevin Bourrillion]

That's what I meant, just didn't want to spend though on what's the best name yet.  Maybe indexOfFirstMatch(), or maybe indexOf() is fine.  Not sure the lastIndexOf() version is needed as much.

[Rogan Creswick]

On Thu, Nov 5, 2009 at 9:52 AM, Kevin Bourrillion <kev...@google.com> wrote:
> That's what I meant, just didn't want to spend though on what's the best
> name yet.  Maybe indexOfFirstMatch(), or maybe indexOf() is fine.  Not sure
> the lastIndexOf() version is needed as much.

Just tossing this out because it came to mind:

Iterator<Integer> Lists.indexes(List, Predicate)

The other variations can be implemented on top of that, it
encapsulates the common (in my experience) loop over all indices, and
it can be implemented lazily.

I don't think I feel strongly about the name ;) but returning an
Iterator would be more useful to me than returning atomic values.

--Rogan

[Kevin Bourrillion]

It makes sense, but I have some difficulty imagining it really getting used.  Feel free -- whoever does actually file this request :-) -- to include this option.

It might as well be Iterable, not Integer.  You still wouldn't want to implement lastIndexOf() atop that, tho.

[fishtoprecords]

> I'm missing find/filter Iterable methods that return indices rather
> than the objects themselves. For lists, this is more natural.
> Implementing an index-based filter on top of the existing filter is
> very clumsy compared to doing it the other way around. (again, only
> for lists, that is)

A related approach would be a transforming function that applies the
filter on the object and returns a List of indecies. But you would
expect a sublist, where the Predicate matches, rather than a complete
list.

Should be fairly easy to implement in specific, but I'm not sure how
to extend it in general