c++ - 对 double 的二维 vector 进行排序

Question

我有一个数据列表(4 列)，我想按特定列排序。它从一个文件读入一个 2D vector 。我使用了 std::sort 方法并编写了我的比较仿函数。程序编译并运行，但是当我打印前 10 个元素时，它没有排序，但肯定与它添加到 2D vector 的顺序不同。

代码如下:

#include <iostream>
#include <fstream>
#include <stdio.h>
#include <string>
#include <vector>
#include <algorithm>
#include <iomanip>

using namespace std;

typedef vector<double> Row;
typedef vector<Row> Matrix;

bool sortByFourthColumn(const Row& row1, const Row& row2){
    return (double) row1[3] < (double) row2[3];
}

int main(){
    std::ifstream infile;
    infile.open("Test8_Output.txt");

    double x,y,z,E;
    char line[200];
    int line_count=0; 

    ofstream outfile;
    outfile.open("WO_crp.txt");

    if (infile.is_open()){

        while (!infile.eof()){
            infile.getline(line,170);

            if (line[0] != '%'){
            outfile<<line<<"\n";         
            line_count++;
            }
            else{
            }
    }

    Matrix data(line_count,Row(4));

    outfile.close();

    std::ifstream myfile;
    myfile.open("WO_crp.txt");

    int i = 0;
    while(myfile >> x >> y >> z >> E){
        data[0][i] = x;
        data[1][i] = y;
        data[2][i] = z;
        data[3][i] = E;
        i++;
    }
    myfile.close();

    std::sort(data.begin(), data.end(), sortByFourthColumn);

    for (int u = 0; u <20; u++){
        cout << setprecision(5) << data[0][u] << "\t" << setprecision(5)<< data[1][u] << "\t" << setprecision(5)<< data[2][u] << "\t" << setprecision(5)<< data[3][u] << endl;
    }

    }
    else{
        cout << "Error: File is invalid.\n";

    }
    return(0);
}

编辑 - 输入文件的样例:
编辑 2 - 在 Matrix data(4,Row(line_count));

中交换了 4 和 line_count

% Model:              CDS_Test8.mph
% Version:            COMSOL 5.2.0.220
% Date:               Jul 13 2016, 14:33
% Dimension:          3
% Nodes:              86183
% Expressions:        1
% Description:        Electric field norm
% Length unit:        m
% x                       y                        z                        es.normE (V/m)
0.13774675805195374       0.05012986567931247      0.20735                  67.35120820901535
0.13870000000000005       0.04957489750396299      0.20735000000000003      102.8772500513651
0.13870000000000002       0.050800000000000005     0.20735                  87.56008679032011
0.13792733849817027       0.050131465727838186     0.20801419247484804      73.55192534768238
0.13674627634411463       0.04992349737428063      0.20735                  63.23018910026428
0.13750191177019236       0.0508                   0.20735000000000003      67.26176884022838
0.13827743496772454       0.05193409099097887      0.20734999999999998      73.35474409597487
0.13803618792088135       0.05134931748395268      0.20841988134890965      75.3712126982815
0.13905949760011943       0.05141879754884912      0.20734999999999998      83.70739713476813
0.13896970815034013       0.05092428105421264      0.208142746399683        84.73571510992372
0.1390220807917094        0.0501245422629353       0.20817502757007986      85.57119242707628
0.13944867847480893       0.05161480113017738      0.2081969878426443       89.65643851109644

以此类推，还有 87k 行左右。

Answer 1

I have a list of data (in 4 columns) that I would like to sort by a certain column.

问题在于OP程序中用于存储数据的vector of vector的维数在声明和使用之间不一致。

一个小问题是 while(!infile.eof()){... 的使用 should be avoided .

固定版本是这样的:

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <vector>
#include <array>
#include <algorithm>
#include <iomanip>

using Row = std::array<double,4>;     // instead of typedefs
using Matrix = std::vector<Row>;
using std::cout;

bool sortByFourthColumn(const Row& row1, const Row& row2){
    return row1[3] < row2[3];
    //     ^  The cast is unnecessary
}

int main(){

    std::string file_name{"Test8_Output.txt"};
    std::ifstream infile{file_name, std::ios_base::in};
    if ( !infile ) {
        cout << "Error: unable to open file " << file_name << '\n';
        return EXIT_FAILURE;
    }

    Matrix data;
    data.reserve(90000);  // if you are afraid of the reallocations
    int count = 0;
    std::string line;

    // instead of two loops you can use one loop and read the file once
    // I'll use std::getline to extract a row in a std::string
    while ( std::getline(infile, line) ) {
        // skip comments and empty lines
        if ( line.empty() || line[0] == '%' )
            continue;
        ++count;

        // extract data from the string using a stringstream
        std::stringstream ss{line};
        Row r;
        ss >> r[0] >> r[1] >> r[2] >> r[3];
        if ( !ss ) {
            cout << "Format error in line " << count << " of file.\n";
            break;
        }
        data.push_back(std::move(r));
    }

    std::sort(data.begin(), data.end(), sortByFourthColumn);
    cout << std::setprecision(5) << std::fixed;

    for ( const auto & r : data ) {
        for ( auto const &x : r ) {
            cout << std::setw(10) << x;
        }
        cout << '\n';
    }
    return EXIT_SUCCESS;
}

给定示例数据的输出是:

   0.13675   0.04992   0.20735  63.23019
   0.13750   0.05080   0.20735  67.26177
   0.13775   0.05013   0.20735  67.35121
   0.13828   0.05193   0.20735  73.35474
   0.13793   0.05013   0.20801  73.55193
   0.13804   0.05135   0.20842  75.37121
   0.13906   0.05142   0.20735  83.70740
   0.13897   0.05092   0.20814  84.73572
   0.13902   0.05012   0.20818  85.57119
   0.13870   0.05080   0.20735  87.56009
   0.13945   0.05161   0.20820  89.65644
   0.13870   0.04957   0.20735 102.87725