以相反顺序c ++从文本文件中读取行 [英] Reading in lines from a text file in reverse order c++

问题描述

我需要找到一种从文件中读取最后6行数据的方法。

I need to find a way of reading in the last 6 lines of data from a file.

例如，如果我有
1
2
3
4
5
6
7
8
9
10

For example if I have 1 2 3 4 5 6 7 8 9 10

我需要阅读才能获得
10,9 ，8,7,6,5,4。

I need to read be able to get 10, 9, 8, 7, 6, 5, 4.

这些需要放入变量或字符串中以便稍后输出。目前我已经设法读取文件的最后一行，但我不知道如何读取其他5个数字。

These need to then be put into variables or strings to be outputted later. Currently I have managed to read in the last line of the file but I have no idea how to then read in the other 5 numbers.

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

using namespace std; 

int main()
{
    std::ifstream in("test.txt");

    if (in.is_open())
    {
        std::vector<std::string> lines_in_reverse;

        std::string line, line2;

        while (std::getline(in, line))
        {
            // Store the lines in reverse order.
            lines_in_reverse.insert(lines_in_reverse.begin(), line);



        }
        cout << line << endl;
        while (std::getline(in, line2))
        {
            // Store the lines in reverse order.
            lines_in_reverse.insert(lines_in_reverse.begin(), line2);



        }
        cout << line2 << endl;
    }

    cin.get();
    return 0;
}

任何人都可以建议这样做吗？我不知道任何可以提供帮助的功能或方法。

Can anyone advise a way to this? I do not know of any functions or methods that can help.

编辑

此方法输出文件中的最后6个数字，但它们是向后的，我需要一种方法来反转它们并摆脱它打印出的空白。

This method outputs the last 6 numbers from the file however they are backwards and I need a way to reverse them and get rid of the whitespace it prints out.

我不确定如何使用反向以及需要哪些参数 - http://en.cppreference.com/w/cpp/algorithm/reverse

I'm unsure on how to use reverse and which arguments are required from this - http://en.cppreference.com/w/cpp/algorithm/reverse

#include <iostream>
#include <fstream>
#include <string>
#include <algorithm>
using namespace std;
int main()
{
    char x;
    ifstream f("test.txt", ios::ate);
    streampos size = f.tellg();
    for (int var = 1; var <= size; var++){
        f.seekg(-var, ios::end);
        f.get(x);
        reverse(x);
        cout << x; 
    }
    cin.get();
    return 0;
}

很多回复告诉我如何使用向量反转文本文件但不是最后6个数字是我需要的唯一信息。

Alot of the responses show me how to reverse the text file using vectors but not the last 6 numbers which is the only information I need.

问候

推荐答案

如何读取文件并反向打印，仅在三个代码陈述中（不包括声明和其他样板）：

How to read a file and print it reverse, in only three statements of code (excluding declarations and other boilerplate):

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>

void read_and_print_reverse_n(std::istream& is, const int n)
{
    std::vector<std::string> v;

    // This call reads all whitespace-delimited "words" from the input stream
    // and appends them to the vector
    std::copy(std::istream_iterator<std::string>(is),
              std::istream_iterator<std::string>(),
              std::back_inserter(v));

    // Output the last `n` lines from the input
    for (const auto i = v.rbegin();
         i < v.rend() && i < v.rbegin() + n;
         ++i)
    {
        std::cout << *i << '\n';
    }
}

int main()
{
    read_and_print_reverse_n(std::cin, 6);
}

参考文献

• std :: copy


• std :: istream_iterator


• std :: back_inserter

• std::copy
• std::istream_iterator
• std::back_inserter

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