c ++最快的方式只读最后一行文本文件？ [英] c++ fastest way to read only last line of text file?

问题描述

我想只读取文本文件的最后一行（我在UNIX上，可以使用Boost）。我知道的所有方法都需要扫描整个文件，以获得最后一行效率不高。有没有一种有效的方法只获得最后一行？

I would like to read only the last line of a text file (I'm on UNIX, can use Boost). All the methods I know require scanning through the entire file to get the last line which is not efficient at all. Is there an efficient way to get only the last line?

此外，我需要这是足够健壮，即使有问题的文本文件不断被附加

Also, I need this to be robust enough that it works even if the text file in question is constantly being appended to by another process.

推荐答案

使用seekg跳转到文件的结尾，然后回读，直到找到第一个换行符。
下面是使用MSVC的我的头顶部的一些示例代码。

Use seekg to jump to the end of the file, then read back until you find the first newline. Below is some sample code off the top of my head using MSVC.

#include <iostream>
#include <fstream>
#include <sstream>

using namespace std;

int main()
{
    string filename = "test.txt";
    ifstream fin;
    fin.open(filename);
    if(fin.is_open()) {
        fin.seekg(-1,ios_base::end);                // go to one spot before the EOF

        bool keepLooping = true;
        while(keepLooping) {
            char ch;
            fin.get(ch);                            // Get current byte's data

            if((int)fin.tellg() <= 1) {             // If the data was at or before the 0th byte
                fin.seekg(0);                       // The first line is the last line
                keepLooping = false;                // So stop there
            }
            else if(ch == '\n') {                   // If the data was a newline
                keepLooping = false;                // Stop at the current position.
            }
            else {                                  // If the data was neither a newline nor at the 0 byte
                fin.seekg(-2,ios_base::cur);        // Move to the front of that data, then to the front of the data before it
            }
        }

        string lastLine;            
        getline(fin,lastLine);                      // Read the current line
        cout << "Result: " << lastLine << '\n';     // Display it

        fin.close();
    }

    return 0;
}

下面是一个测试文件。它在文本文件中的空行，单行和多行数据成功。

And below is a test file. It succeeds with empty, one-line, and multi-line data in the text file.

This is the first line.
Some stuff.
Some stuff.
Some stuff.
This is the last line.

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