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Kotlin like lazy Grouping API #457
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Hi there! How easy would it be to express these operations in terms of simpler iterator operations? From what I've tried, there may be some value in these functions, but wouldn't it be easier (and more in line with existing methods) to generalize I would imagine something where the caller could customize Line 18 in 710d9f2
Of course, the new method would not require |
Yes, it would be possible. What you're proposing is essentially merging the The biggest problem of your approach IMO is that we would have to choose between implementing it like However, I agree we could find a name more in line with the existing ones. Edit: formatting |
465: Add into_grouping_map for efficient group-and-fold operations r=jswrenn a=SkiFire13 Adds two functions on the `Itertools` trait, `into_grouping_map` and `into_grouping_map_by`. `into_grouping_map` expects an iter of `(K, V)` where the `K` will be used as key and `V` as value. `into_grouping_map_by` expect only an iter of `V` as values, the keys will be calculated using the provided functions. Both of them return a `GroupingMap`, which is just a wrapper on an iterator. Since it introduces a lot of related methods I thought it would be better to separate them from the `Itertools` trait. This also prevents duplicating every method for the `_by` version. All of these functions have in common the fact they perform efficient group-and-fold operations without allocating temporary vecs like you would normally do if you used `into_group_map` + `into_iter` + `map` + `collect::<HashMap<_, _>>()`. Here's the possible issues I can see, I would like to hear some feedback before trying to fix any of them: - name: I initially though of `grouping_by` which is the java/kotlin equivalent, but later changed it to `into_grouping_map` to match the already existing `into_group_map` and to differentiate from `group_by`; - `fold_first`: the equivalent function in the `Itertools` trait is `fold1` but there's an unstable stdlib function that does the same thing and is called `fold_first`. I decided to be consistent with the stdlib; - `minmax` return type is the already existing `MinMaxResult`, but the `NoElements` variant is never returned. I didn't want to duplicate that struct thinking it could cause confusion (and if I did what name could I have chosen?); - `sum` and `product`: They dont' use the `Sum` and `Product` traits but instead require `V: Add<V, Output=V>` and `V: Mul<V, Output=V>`. They're pretty much a wrapper around `fold_first`. I don't really know if they're meaningful or if I should just remove them; - almost every closure takes as one of the parameters a reference to the key. It bloats them a bit, but could be useful if computing the key is a relatively expensive operation. - no `scan` function. Even though it is an iterator adapter I could sort of "collect" it into a `Vec` (more like extend) but I don't really see an use for this; - ~~no integration tests for the `_by` and `_by_key` versions of `min`, `max` and `minmax`. To be fair I was a bit lazy, but I also couldn't find any integration test for the normal `minmax_by` and `minmax_by_key` so I though it was fine;~~ added; - no benchmark (I don't think it is required but probably would be cool to compare this with `into_group_map`; - I didn't want to touch the rest of the library, but I guess we could implement `into_group_map` in terms of `into_grouping_map`? Related issues: #457, #309 Related PR: #406 (in particular relates to the `into_group_map_by_fold` function that was proposed but later removed) Co-authored-by: SkiFire13 <skifire13.1@gmail.com>
@SkiFire13 I see this issue and I'm wondering if there is anything left to do after #465 or if I can close this. |
I'm opening this to request/discuss a possible Grouping API like Kotlin's
Grouping
/groupingBy
API. See Kotlin's Grouping reference and its source code.It would provide an easy way to perform efficient group-and-fold operations without allocating additional
Vec
s (unlikeinto_group_map
), while also maintaning unique keys (unlikegroup_by
). The disadvantage is that it can only supportfold
-like operations.The implementation should be pretty simple, I've already written a pretty minimal working sample that includes:
fold
function, like a normal fold except it associate each key with the result of folding each element that maps to that key;fold_first
function, likefold
but without an initial value, instead the first element acts as the first value;count
function that associate each key with the number of elements that maps to that key;aggregate
function that provides the basic functionality we can use to write all the other functionshttps://gist.github.com/SkiFire13/a0010ff658d905dcbc1e1f5f6ae910e0
The downsides of my implementation are:
HashMap
. This could be solved with aMap
trait but we're still waiting for GATs for that. It is required because I don't onlyextend
the map, but alsoremove
elements from it;Map
trait then it would break more things);fold
's initial value must beClone
but I can't see any way around that;group_by
although Kotlin has bothgroupBy
andgroupingBy
and I haven't seen someone complaining;fold_first
is consistent with the stdlib corrispondent but inconsistent with Itertools'fold1
.I'm also thinking of other possible extensions with functions like:
fold
;max
/min
/maxmin
and friends;sum
but I found it would be pretty hard to use theSum
trait since it requires a whole iterator andGrouping
can only fold.The text was updated successfully, but these errors were encountered: