c# - indexof array using linq except -



c# - indexof array using linq except -

i have next 2 lists

list<string> list1 = new list<string>(); list1.add("one"); list1.add("two"); list1.add("three"); list1.add("four"); list1.add("five"); list<string> list2= new list<string>(); list2.add("one"); list2.add("two"); list2.add("three"); ienumerable<int> indexofunwantedcolumns; indexofunwantedcolumns = list1.except(list2) .select((p, i) => new { item = p, index = }) .select(p => p.index);

as expected give index of 2 items in new list (0 , 1). want able there index values original list (3 , 4)

any help appreciated

the main thought capture original index before filtering, , check if list2 contains each item. able perform these lookups efficiently (in o(1) aggregated time per lookup), should utilize hashset:

var hashset = new hashset<string>(list2); var unwantedcolindexes = list1 .select((item, idx) => new { item, idx }) // capture original index .where(i => !hashset.contains(i.item)) // filter using hashset .select(i => i.idx) // index .tolist();

c# .net linq

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