Problem 1896. Index one element in each vector of an array along a given dimension

Created by Oliver Woodford
Appears in Indexing III

Difficulty:Rate

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Functions like min and max can return in their second output argument the index of the element in each vector along a particular dimension which meets the particular criterion being requested. E.g.

   A = rand(3);
   [B, I] = min(A, [], 2);

Write a function which, given an array A, and an index array I (like the one created above), can reconstruct the first output of functions like min and max. E.g.

   C = dimsel(A, I);

such that C is equal to B.

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Last Solution submitted on Feb 20, 2025

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Rafael S.T. Vieira on 24 Jul 2020

Good problem, but, in practice, no one would ever do that. It is better to store the global-linear index and do a quick calculation to find the needed position than to loop through the local indexes. The MATLAB function max has the option 'linear' because of that.

PS: I liked it anyway because it's a good conceptual problem for students to bear down.

GeeTwo on 20 Jul 2022

Is the "given dimension" always 2?
Also, provide a few clean test cases. (Here's the input, here's the expected outputs). Or at least let me copy and paste the test case.

GeeTwo on 31 Jul 2022

* The first [2,6] dimensions of A are non-singleton; A is singleton in all dimensions above first singleton.
* d, the "given dimension", is one of the non-singleton dimensions of A, and the first singleton dimension of I.
* Values in I will be in 1:length(A,d)).
% size(B) should == size(I).
* The problem is that I is the index along the collapsed dimension (d). All other dimensions should use ":".

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Computational Geometry III

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