C#•16mo ago

❔ Get list of indices from a List

I have a result set called "result". I want to use Linq to get a list of distinct items by name and their indices in the source resultset. How can I accomplish that?

15 Replies

ero•16mo ago

how would this work? first of all, a set already only contains distinct items. if you mean that you have a list, and you then got the items distinct by their name, which distinct item's index would you want? the first? second? nth?

dotnut•16mo ago

Something like collection.DistinctBy(item => item.Name).Select(item, index => new { Name = item.Name, Index = index})

ero•16mo ago

but that's incorrect that would return the index in the new distinct collection

string[] foo =
{
  "Foo",
  "Foo",
  "Bar",
  "Bar"
};

string[] foo =
{
  "Foo",
  "Foo",
  "Bar",
  "Bar"
};

like what would the result look like here?

dotnut•16mo ago

I think they mean collection

ero•16mo ago

dotnut•16mo ago

Just because of the mentioning of Distinct

ero•16mo ago

what about it

dotnut•16mo ago

You said it already a set would have distinct items Hence I believe they just meant a collection of things

ero•16mo ago

sure doesn't really matter your version would be incorrect in any case

their indices in the source resultset

this part just doesn't really make sense

dotnut•16mo ago

Ah good point Yeah it doesnt 😅

ero•16mo ago

it's fine that they want the index in the source collection, but if you expect the item to exist more than once, then you need to have a concept of which one you want

dotnut•16mo ago

Exactly

Alex FrostOP•16mo ago

Hi, thank you for the guidence. I was able to work it out following what was said and some search:

distinctItems = result.Select((value, index) => new { index, value.PropertyID })
                    .DistinctBy(p => p.PropertyID)
                    .ToDictionary(x => x.index, x => x.PropertyID);

distinctItems = result.Select((value, index) => new { index, value.PropertyID })
                    .DistinctBy(p => p.PropertyID)
                    .ToDictionary(x => x.index, x => x.PropertyID);

ero•16mo ago

i mean again like, it's not really clear if this is the desired solution either way you should prefer using a valuetuple

results
.Select((r, i) => (Id: r.PropertyId, Index: i))
.DistinctBy(t => t.Id)
.ToDictionary(t => t.Index, t => t.Id);

results
.Select((r, i) => (Id: r.PropertyId, Index: i))
.DistinctBy(t => t.Id)
.ToDictionary(t => t.Index, t => t.Id);

Accord•16mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Get list of indices from a List