c++ - 从迭代器中提取数字

Question

在this SO 问题，@HowardHinnant 给出了一个绝妙的答案，它非常有效地将文本文件中的数字提取到 vector 中。这是简洁的代码:

#include <fstream>
#include <iostream>
#include <vector>
#include <string>
#include <sstream>
#include <iterator>

// g++ -std=c++11 test.cpp -o t

int main()
{
    std::ifstream theStream("data.txt"); 
    if( ! theStream )
          std::cerr << "data.txt\n";
    while (true)
    {
        std::string line;
        std::getline(theStream, line);
        if (line.empty())
            break;

        std::istringstream myStream( line );
        std::istream_iterator<int> begin(myStream), eof;
        std::vector<int> numbers(begin, eof);

        // process line however you need
        std::copy(numbers.begin(), numbers.end(),
                  std::ostream_iterator<int>(std::cout, " "));
        std::cout << '\n';
    }
}

数据样本是

800 170 84 439 
129 902 103 492 394 140 496 893 
229 645 
164 389 74 208 726 315 
291 421 230 789 246 791 762 
416 241 538 810 605 
714 555 54 863 
288 465 563 831 
0 339 740 427 718 
449 675 545 842 779 607 
274 958 

这段代码我已经研究了两天了，不太明白如何从每一行中提取数字。那么，比方说，我如何访问第二行的第 3 个数字 (103)，或者更一般地说，第 m 行的第 n 个数字？

任何关于此解决方案如何工作的解释也会很有帮助!

Answer 1

使用 vector 的 vector 分别存储每行编号:

#include <fstream>
#include <iostream>
#include <vector>
#include <string>
#include <sstream>
#include <iterator>

int main()
{
    std::ifstream theStream("data.txt");
    if (!theStream)
        std::cerr << "data.txt\n";

    std::vector<std::vector<int>> data;  // vector to hold the numbers of each line seperately
    while (true)
    {
        std::string line;
        std::getline(theStream, line);
        if (line.empty())
            break;

        std::istringstream myStream(line);
        std::istream_iterator<int> begin(myStream), eof;
        std::vector<int> numbers(begin, eof);

        // process line however you need
        data.push_back(numbers); // add numbers of current line to data
    }

    std::cout << data[1][2] << '\n'; // 2nd row, 3rd number: 103
}

Assuming the query will only be made once, you can do it in one pass by counting which row and column you're on while you're extracting (and with each iteration of the while loop); then break from there. This is so that you don't have to read the entire file.

可能看起来像:

#include <cstddef>
#include <fstream>
#include <iostream>
#include <string>
#include <sstream>

int main()
{
    std::ifstream theStream("data.txt");
    if (!theStream)
        std::cerr << "data.txt\n";

    std::size_t target_row = 2;  // 1-based
    std::size_t target_col = 3;  // 1-based

    int value = 0;
    int valid = false;

    for (std::size_t current_row = 1; true; ++current_row)
    {
        std::string line;
        if (!std::getline(theStream, line) || line.empty())
            break;

        if (current_row != target_row)
            continue;

        std::istringstream myStream(line);
        for (std::size_t current_col = 1; myStream >> value; ++current_col)
            if (current_col == target_col) {
                valid = true;
                break;
            }           

        break;
    }

    if (!valid)
        std::cerr << "No such row and column!\n\n";
    else
        std::cout << value;
}