I have this array :
Point[] arr = samples.pointsArray; I using this row to retrieve all elements that satisfy condition:
var maxXCol = arr.Where( p => maxX.X == p.X ); Any idea how to modify row above, to get only the indexes of these elements?
Thank you in advance!
EDIT
Use the version of Select that takes both the index and the object and create an anonymous object with the object and index inside it. It would look like this:
someEnumerable.Select((obj, idx) => new {Item = obj, Index = idx}) You'll need to do this before you use Where so that the original index remains intact after the filter operation.
In the following operations you can use the item like so:
x => x.Item and the index like so:
x => x.Index 
var maxXCol = arr.Select((p, inx) => new { p,inx}) .Where(y => maxX.X == y.p.X) .Select(z => z.inx); You may select the value first with the index in anonymous type, later you filter it with your condition and then select the index.
var result = arr.Select((g, index) => new { g, index }) .Where(r => maxX.X == r.X) .Select(t => t.index); You can use Select overload which takes an index, and project that index together with the original row. Then take only the index for the result collection.
var maxXCol = arr .Select((p, index) => new { Item = p, Index = index }) .Where(p => maxX.X == p.Item.X) .Select(x => x.Index); 
Try this:
arr.Select((e,i)=>new{index=i, value=e}).Where(ei=>ei.value.X==maxX.X).Select(ei=>ei.index); 
var maxXCol = arr .Select((a, b) => new { b, a }) .Where(p => maxX.X == p.a.X) .Select(i=>i.b); 