[API Proposal]: Linq method for getting value and index

[terrajobst]

Background and motivation

There is an existing Linq method for returning an enumerable's value and index, but it was added pre-tuples which makes it clunky as it forces the caller to create a type representation for the combination of the value and index, for example using this:

IEnumerable<string> lines = GetLines();

foreach (var (line, lineIndex) in lines.Select((l, i) => (l, i)))
{
    // ...
}

API Proposal

namespace System.Linq;

public static partial class Enumerable
{
    public static IEnumerable<(T Value, int Index)> WithIndex<T>(this IEnumerable<T> source);
}

public static partial class Queryable
{
    public static IQueryable<(T Value, int Index)> WithIndex<T>(this IQueryable<T> source);
}

API Usage

Using WithIndex, one can express this idea much more succinctly:

IEnumerable<string> lines = GetLines();

foreach (var (line, lineIndex) in lines.WithIndex())
{
    // ...
}

Alternative Designs

No response

Risks

None.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-linq
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Background and motivation

There is an existing Linq method for returning an enumerable's value and index, but it was added pre-tuples which makes it clunky as it forces the caller to create a type representation for the combination of the value and index, for example using this:

IEnumerable<string> lines = GetLines();

foreach (var (line, lineIndex) in lines.Select((l, i) => (l, i)))
{
    // ...
}

API Proposal

namespace System.Linq;

public static partial class Enumerable
{
    public static IEnumerable<(T Value, int Index)> SelectIndex<T>(this IEnumerable<T> source);
}

public static partial class Queryable
{
    public static IQueryable<(T Value, int Index)> SelectIndex<T>(this IQueryable<T> source);
}

API Usage

Using SelectIndex, one can express this idea much more succinctly:

IEnumerable<string> lines = GetLines();

foreach (var (line, lineIndex) in lines.SelectIndex())
{
    // ...
}

Alternative Designs

No response

Risks

None.

Author:	terrajobst
Assignees:	-
Labels:	api-suggestion, area-System.Linq, untriaged
Milestone:	-

[eiriktsarpalis]

How about ZipIndex or ZipWithIndex? Select methods typically imply some form of transformation, however this one just zips the source enumerable with its indices (and has a similar output type to Zip).

[KeterSCP]

I also want to point out that name SelectIndex is very unintuitive. It implies that you select only index, which is not true. SelectWithIndex or ZipWithIndex are more verbose, but at least they do not introduce confusion

[marklio]

How about just WithIndex? I agree that 'Select' implies transformation. 'Zip' implies starting with two things. This is just bringing an implicit attribute of the enumerable to the surface.

This is an extension that I have in almost all the code I write, so I'd love it if it were just ambiently available.

[terrajobst]

@eiriktsarpalis that makes sense. Personally, I'd prefer @marklio's suggestion WithIndex over ZipIndex because it seems easier to understand.

[eiriktsarpalis]

Methods starting with With are typically associated with update expressions e.g. in fluent APIs or records. This is not in the same boat. Marking ready for review, I'm sure more than ample debate will be expended on the naming.

[aloraman]

1. How about AsIndexed or just Indexed?
2. What's the intended meaning of EventualNameOfThisMethod in IQueryable, e.g. should it be translated into ROW_NUMBER()?

[viceroypenguin]

The same method in MoreLinq and SuperLinq is just Index(). This is likely not a good choice for the name, but raising awareness of the community implementations of this.

[terrajobst]

I'm sure more than ample debate will be expended on the naming.

[y0ung3r]

What about IndexableSelect or IndexedSelect? 😂

[terrajobst]

• We decided against WithIndex because it sounds like a wither which this one isn't. F# calls it Indexed; following nomenclature in the rest of Linq we chose a verb version (akin to the recently added Order)
• We decided to rename Value to Item because Value implies a singleton vs Item which is used as a collection indexer
• We decided to return Index first, Value second because it feels more natural despite the fact that the existing Select() method provides value first and index second.

  IEnumerable<string> lines = GetLines();
  
  foreach (var (i, line) in lines.Index())
  {
      // ...
  }

• We should match the existing Select's method behavior when enumerating a source that returns more than int.MaxValue items

namespace System.Linq;

public static partial class Enumerable
{
    public static IEnumerable<(int Index, T Item)> Index<T>(this IEnumerable<T> source);
}

public static partial class Queryable
{
    public static IQueryable<(int Index, T Item)> Index<T>(this IQueryable<T> source);
}