std::sort / Works in test code, but cannot deduce template otherwise

Question

I want to sort eigenvectors by their eigenvalues and I'm using Eigen. I tried the following post (Sorting eigenvectors by their eigenvalues (associated sorting)), but it fails to compile with MS VisualStudio 2015.

So I figure it cannot be that hard. I just need to sort by the eigenvalues, keep track of their old and new order and then resort the eigenvector matrix.

This works well in my test code:

#include <algorithm>
struct a
{
    double num1;
    int num2;
};

bool acompare(a lhs, a rhs) { return lhs.num1 < rhs.num1; }

int demoSortWithStructure()
{
    a array[5];

    array[0].num1 = 1;
    array[0].num2 = 1;

    array[1].num1 = 5;
    array[1].num2 = 2;

    array[2].num1 = 3;
    array[2].num2 = 3;

    array[3].num1 = 2;
    array[3].num2 = 4;

    array[4].num1 = 4;
    array[4].num2 = 0;

    cout << "before sort" << endl;
    for (int i = 0; i < 5; i++)
        cout << array[i].num1 << "\t" << array[i].num2 << endl;

    std::sort(array, array + 5, acompare);

    cout << "\n\nafter sort" << endl;

    for (int i = 0; i < 5; i++)
        cout << array[i].num1 << "\t" << array[i].num2 << endl;

    return 1;
}

So, I went on to generalize the code as follows:

void MyClass::SortEigenValuesAndEigenMatrix(VectorXcd eigenvalue, MatrixXcd eigenvector)
{
    VectorXcd eigenvalueOriginal = eigenvalue;
    MatrixXcd eigenvectorOriginal = eigenvector;
    int noValues = eigenvalue.rows();
    a *array = new a[noValues];

    for (int iValue = 0; iValue < noValues; iValue++)
    {
        array[iValue].index = iValue;
        array[iValue].valueReal = eigenvalue(iValue).real();
    }

    sort(array[0], 
        array[noValues], 
        acompare);

    for (int iValue = 0; iValue < noValues; iValue++)
    {
        int indexNew = array[iValue].index;
        eigenvalue(iValue) = eigenvalueOriginal(indexNew);
        eigenvector.col(iValue) = eigenvectorOriginal.col(indexNew);
    }

}

For context, my eigenvalues and eigenvectors are defined as complex, but they are real values. So, I just take the real value and intend to sort based upon the real (signed) values.

The compiler throws the following error:

Severity    Code    Description Project File    Line    Suppression State
Error   C2784   'std::complex<_Other> std::operator -(const _Ty &,const std::complex<_Other> &)': could not deduce template argument for 'const std::complex<_Other> &' from 'a'    testRCWA    d:\program files (x86)\microsoft visual studio 14.0\vc\include\algorithm    3203

I don't understand. How can the compiler be inferring the use of

std::complex

What about my templating is unclear? Am I supposed to explicitly declare the type when using sort? Why does this happen in this code, but not when it is isolated?

did you mean `sort(&array[0], &array[noValues], acompare);` ? — Piotr Skotnicki, May 03 '18 at 13:46
I put the structure into my include file in the code that doesn't compile. That seems to be the root of the problem. — user3533030, May 03 '18 at 13:46
I know complex does not have a < operator... that's why I only sort by the real value of the eigenvalue. — user3533030, May 03 '18 at 13:47
Just use `std::vector` and please edit your question to contain [mcve] — Slava, May 03 '18 at 14:00
&array is not necessary. Code works without that modification — user3533030, May 03 '18 at 14:15
Using std::vector is not practical because my vectors are complex values, not real values. — user3533030, May 03 '18 at 14:15
Your question is unrelated to the error message, the error message is unrelated to the code, the solution is unrelated to the problem. — Piotr Skotnicki, May 03 '18 at 14:22
@user3533030 `std::vector>` is a perfectly reasonable type. You certainly don't need your `a` class. — Caleth, May 03 '18 at 14:25
What it sounds like you are trying to do is best solved [like this](https://stackoverflow.com/a/12399290/2610810) — Caleth, May 03 '18 at 14:29

score 0 · Answer 1 · answered May 03 '18 at 14:14

Somehow, putting the structure definition into the include file wound up causing an ambiguity in the templating. I'm not sure how, but it was resolved by putting the structure (and boolean compare) into the definition of the class.

Not much of an answer...but problem solved.

std::sort / Works in test code, but cannot deduce template otherwise

1 Answers1