How can I implement the behavior of unique in matlab using C++?

Question

In Matlab, the [C, ia, ic] = unique(A) function returns an sorted array C in which the duplicates in A are removed, and the ia and ic arrays contains the indices such that C = A(ia) and A = C(ic). For example:

A = [1, 2, 3, -1, -2, 0, 0, 0, 4]; 
[C, ia, ic] = unique(A); 
C = [-2, -1, 0, 1, 2, 3, 4]; 
% The indices are incremented by 1 to accomodate the C++ convection.
ia = [4, 3, 7, 0, 1, 2, 8]; % size(ia) = size(C) 
ic = [3, 4, 5, 1, 0, 2, 2, 2, 6]; % size(ic) = size(A)

Here is my implementation:

  std::vector<int> A = {1, 2, 3, -1, -2, 0, 0, 0, 4};
  std::set<int> C(A.begin(), A.end());

  std::vector<int> ic;
  ic.reserve(A.size());

  std::transform(A.begin(), A.end(), std::back_inserter(ic),
                 [&](int x) { return (std::distance(C.begin(), C.find(x))); });

I now can correctly get the array C and ic, but I don't know how to get the array ia.

Can someone help me with this?

cigien · Accepted Answer · 2020-08-22 15:09:35Z

3

The answer is very similar to how you calculate ic. Just swap A and C in the transform call:

std::vector<int> ia;
ia.reserve(C.size());

std::transform(C.begin(), C.end(), 
               std::back_inserter(ia),
               [&](int x) { return std::distance(A.begin(), 
                                     std::find(A.begin(), A.end(), x)); 
});

You also need to use std::find since std::vector doesn't provide a .find() member function.

edited Aug 22, 2020 at 15:09

answered Aug 22, 2020 at 15:06

cigien

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2 Comments

bayesNie Over a year ago

I tried this but the STL doesn't provide a find member function for std::vector.

cigien Over a year ago

Aah, right, you'll need std::find. Edited the answer.

Mansoor · Accepted Answer · 2020-08-22 15:13:47Z

3

Another approach which is similar in syntax to your Matlab:

#include <algorithm>
#include <vector>

std::vector<int> A = {1, 2, 3, -1, -2, 0, 0, 0, 4};
std::vector<int> B (A.begin(), A.end());
std::sort(B.begin(), B.end());
auto last = std::unique(B.begin(), B.end());
B.erase(last, B.end());

Compiler explorer link

If you want to find the indices for the locations of entries, you can use find_first_of.

edited Aug 22, 2020 at 15:13

answered Aug 22, 2020 at 15:12

Mansoor

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