Linewise lexicographic sort of a file with a catch

Question

Although I am not happy with the title of this question and this might be an odd question; bear with me, please.

So I have text files with content as follows:

 & AAABBAB

 this

 & AAAAAAB 

 is

 & BCAAAA

 an

 & BBBBBA

 example

 & BABABAB

 text

where every other line starts with an identifier ('&'). Lines with said identifier should be lexicographically sorted, but I need it in a way such that the next line is dragged along to the new position in the output file with it.

This is what I am hoping to be the content of the output file.

 & AAAAAAB 

 is

 & AAABBAB

 this

 & BABABAB

 text

 & BBBBBA

 example

 & BCAAAA

 an

With this, I can get the file content line-by-line:

#include <iostream>

#include <fstream>

#include <string>

#include <algorithm>



using namespace std;



int main()

{

    ifstream is("test.txt");

    string str;

    while(getline(is, str))

    {

        cout<<str<<endl;

    }

    return 0;

}

Is there an easy way to accomplish what I am looking for? Thanks for your help!

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Answer 1

I'd bundle the pairs together while reading, making them easy to sort:

vector<pair<string, string>> vec; // first is identifier
vec.reserve(1000);

bool first = true;
while(getline(is, str))
{
    if (first)
        vec.emplace_back(str, string());
    else
        vec.back().second = str;
    first = !first;
}

sort(vec.begin(), vec.end());

Linewise lexicographic sort of a file with a catch

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