在Eigen庫向量上使用std :: upper_bound獲取錯誤

Question

我試圖將std :: upper_bound與本征庫定義的向量一起使用。 我在Visual Studio 2017上遇到一些錯誤

#include <stdafx.h>
#include <stdio.h>
#include <Eigen/Dense>
#include <algorithm>  

using namespace Eigen;
using namespace std;

int main(int argc, char* argv[])
{
    VectorXd myVector= VectorXd::LinSpaced(20, -1.5, 6.4);

    double trashold= strtod(argv[1], NULL);

    double firstGreaterValue= std::upper_bound(myVector(0), 
    myVector(myVector.size()-1),trashold);
}

我希望程序返回的第一個元素的位置大於輸入的位置（在本例中為1.1）。

我在編譯時遇到這些錯誤

Severity    Code    Description Project File    Line    Suppression State
Error   C2794   'difference_type': is not a member of any direct or indirect base class of 'std::iterator_traits<_FwdIt>'   errorExample    C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\include\algorithm   2347    
Error   C2938   '_Iter_diff_t<double>' : Failed to specialize alias template    errorExample    C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\include\algorithm   2347    
Error   C2672   'std::distance': no matching overloaded function found  errorExample    C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\include\algorithm   2347    
Error   C2893   Failed to specialize function template 'iterator_traits<_Iter>::difference_type std::distance(_InIt,_InIt)' errorExample    C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\include\algorithm   2347    
Error   C2794   'difference_type': is not a member of any direct or indirect base class of 'std::iterator_traits<_FwdIt>'   errorExample    C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\include\algorithm   2351    
Error   C2938   '_Iter_diff_t<double>' : Failed to specialize alias template    errorExample    C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\include\algorithm   2351    
Error   C2672   'std::advance': no matching overloaded function found   errorExample    C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\include\algorithm   2353    
Error   C2893   Failed to specialize function template 'void std::advance(_InIt &,_Diff)'   errorExample    C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\include\algorithm   2353    
Error   C2100   illegal indirection errorExample    C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\include\algorithm   2355    

Answer 1

在3.4版之前， Eigen::VectorXd與STL算法不兼容，因為它不提供迭代器來訪問其元素。

從3.4版開始， Eigen的密集矩陣和數組提供了與STL兼容的迭代器 ，然后可以使用begin() / end()方法獲取迭代器：

auto firstGreaterValueIter = std::upper_bound(myVector.begin(), myVector.end(), trashold);

同樣不要忘記std::upper_bound返回一個迭代器，因此必須取消引用才能獲取該值：

if (firstGreaterValueIter != myVector.end()) {
    double firstGreaterValue = *firstGreaterValueIter;
    // ...
}