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

Created by Oliver Woodford

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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.

Solve

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534 Solutions
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Last Solution submitted on Apr 01, 2024

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8 Comments

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Elmar Zander on 30 Mar 2017

The test case is buggy: the beginning of the for loop is on the same line as the %%. Problem is not solvable that way...

Elmar Zander on 2 Apr 2017

Hmm, ok the test works. However, there's no output at all, neither from inside the function nor from the passed/failed test cases. Very annoying...

David Verrelli on 3 Jul 2018

Every test case should begin with "%%". If it doesn't, then it is treated in Cody as a Shared Variables section (variables available to every test case: intentionally utilized in Problem 44655).
Eventually I have found that this causes problems when the "%%" is unintentionally omitted from test cases, because it causes a very confusing suppression of test case output if an assertion in the nominal SV section fails, but suppresses the nominal Shared Variables section's output if all the SV section is passed without error!! Unfortunately there doesn't appear to be any official public documentation of this, and there's no Cody forum to post about it, but I have left a comment trail at Problems 520, 753 & 970. —DIV

David Verrelli on 3 Jul 2018

I forgot to mention: if the author of the problem omits "%%" at the start of a Test Suite, Cody displays the Test Suite to the public as if "%%" were inserted before the actual start of the Test Suite. It's a little bit like the way consecutive spaces are hidden in default HTML rendering [or the opposite of that, because Cody is displaying characters that did not exist in the original!].

goc3 on 31 Jul 2018

The "%%" problem here should be fixed now.

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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