Position of first element in list which is repeated

Question

I have the list

t1 = {{-1, 0}, {-2, 0}, {-3, 0}, {0, 0}, {-2, 0}, {1, 1}}

How do I find the position where an element repeats? In this case it would be element {-2, 0} at position 5, because {-2, 0} first came up at postion 2. So the answer would be 5.

I made it to comparisons with the first element but don't know how to procede.

How to efficiently find position of duplicates (https://mathematica.stackexchange.com/questions/21341/how-to-efficiently-find-positions-of-duplicates) — LouisB, Apr 07 '20 at 18:36

kglr · Accepted Answer · 2020-04-08T20:29:48.037

3

firstDup = Min @* Map[Rest] @* Values @* PositionIndex;;

firstDup @ {{-1, 0}, {-2, 0}, {-3, 0}, {0, 0}, {-2, 0}, {1, 1}}

firstDup @ {{-1, 0}, {-2, 0}, {-3, 0}, {0, 0}, {-2, 0}, {1, 1}, {-2, 0}, {-2, 0}}

firstDup @ {{-1, 0}, {-2, 0}, {-3, 0}, {-1, 0}, {0, 0}, {-2, 0}, {1, 1}}

edited Apr 08 '20 at 20:29

answered Apr 08 '20 at 18:51

kglr

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score 3 · Answer 2 · answered Apr 08 '20 at 19:08

If you just want the position of the first duplicate element you could use something like this,

firstDuplicatePosition[list_]:=With[{ds = CreateDataStructure @ "HashSet"},
    Catch[
        MapIndexed[If[!ds["Insert",#1],Throw[#2]]&, list];
        {}
    ]
];
firstDuplicatePosition @ {{-1, 0}, {-2, 0}, {-3, 0}, {0, 0}, {-2, 0}, {1, 1}}
(* {5} *)

Position of first element in list which is repeated

2 Answers2