c++ - 查找文本文件中的行数，并使用此维度构建数据点

Question

这是直接来 self 的程序的代码。

 // count how many lines the read text document has, so the datastructure can have the correct dimension.
        int number_of_lines = 0;
        std::string line;
        while (getline(myfile, line)) {
            ++number_of_lines;
        }
        std::cout << "Number of lines in text file: " << number_of_lines;
        // the first line won't have a new line before it, so add one to number_of_lines
        rows = number_of_lines + 1;
        return 0;
    }

    // create a data point structure with "rows" number of rows
    datapoint mypoints[rows];

[行]下方有一条红色波浪线，表示:

double rows Error: expression must have a constant value.

值不应该是常量，因为添加到 number_of_lines 的 while 循环在行使用 number_of_lines 之前完成？

Answer 1

您不能在 C++ 中创建可变长度数组 (VLA)。

datapoint mypoints[rows];
//                 ^^^^
//                 this is a variable (i.e. not a compile-time constant)

你可以做的是创建一个动态数组:

datapoint* mypoints = new datapoint[rows];
// ...
delete [] mypoints;

或者使用 vector :

std::vector<datapoint> mypoints(rows); // will default-construct all of them