c++ - 从文件中的特定位置读取 C++

Question

我有一个 C++ 程序需要返回出现特定单词的行。例如，如果我的文件如下所示:

the cow jumped over
the moon with the
green cheese in his mouth

我需要打印带有“with”的行。程序获取的所有内容都是距文件开头的偏移量(在本例中为 24，因为“with”是距文件开头的 24 个字符)。

如何仅使用偏移打印整行“带月亮的月亮”？

非常感谢!

Answer 1

您可以通过单独读取每一行并记录读取前后的文件位置来做到这一点。然后它只是一个简单的检查，看看单词的偏移量是否落在该行的范围内。

#include <iostream>
#include <fstream>
#include <string>

std::string LineFromOffset(
    const std::string &filename,
    std::istream::pos_type targetIndex)
{
    std::ifstream input(filename);

    //  Save the start position of the first line. Should be zero of course.
    std::istream::pos_type  lineStartIndex = input.tellg();

    while(false == input.eof())
    {
        std::string   line;

        std::getline(input, line);

        //  Get the end position of the line
        std::istream::pos_type  lineEndIndex = input.tellg();

        //  If the index of the word we're looking for in the bounds of the
        //  line, return it
        if(targetIndex >= lineStartIndex && targetIndex < lineEndIndex)
        {
            return line;
        }

        // The end of this line is the start of the next one. Set it
        lineStartIndex = lineEndIndex;
    }

    //  Need a better way to indicate failure
    return "";
}

void PrintLineTest()
{
    std::string str = LineFromOffset("test.txt", 24);

    std::cout << str;
}