How to extract pairs from a collection of vectors?

[Claudius7]

Hi all!
I am stuck with the following problem. I have a map m from int to vector<int>, i.e. suppose we have map<int, vector<int> > m.

I want to write a function that constructs all the possible pairs from consecutive vectors of the value of this map. To explain more clearly, suppose the map m is the following:

  {  
    1: [0],  
    3: [3, 4],  
    4: [1, 5, 7],  
    5: [5],  
    7: [2]  
}  

I would like to construct the following pairs: {(0,3); (0,4); (3,1); (3,5); (3,7); (4,1);
(4,5); (4,7); (1,5); (5,5); (7,5); (5,2)}.

[By the way, my real goal is to find the maximum value (in absolute value)
between the differences of such pairs, i.e max(3-0, 4-0, 3-1, ... etc.)]

How should that possibly be done? Thank you a lot for your help!

[integralfx]

Not a very elegant solution, but it works.

#include <iostream>
#include <map>
#include <utility>
#include <vector>

int main()
{
    std::map<int, std::vector<int>> m{
        { 1, { 0 } }, { 3, { 3, 4 } },
        { 4, { 1, 5, 7 } }, { 5, { 5 } },
        { 7, { 2 } },
    };
    
    std::vector<std::pair<int, int>> pairs{};
    for( auto i = m.begin(); i != m.end(); i++ ) {
        for( int n : i->second ) {
            auto temp = i; temp++;
            if( temp == m.end() ) break;
            for( int o : temp->second )
                pairs.push_back( { n, o } );
        }
    }
    
    for( const auto& p : pairs )
        std::cout << '(' << p.first << ", " << p.second << ")\n";
}

(0, 3)
(0, 4)
(3, 1)
(3, 5)
(3, 7)
(4, 1)
(4, 5)
(4, 7)
(1, 5)
(5, 5)
(7, 5)
(5, 2)

[Claudius7]

Thank you for your code, I find it nice. I have only one question: since I am new to C++ and don't yet have studied C++11 could you please rewrite the program only with C++ commands (ie without auto, etc?) thanks a lot!

[integralfx]

I have only one question: since I am new to C++ and don't yet have studied C++11 could you please rewrite the program only with C++ commands (ie without auto, etc?

Being new to C++ shouldn't prevent you from using the newer C++ features. These features are designed to make your life much easier and should be used as much as possible. For example, would you prefer to write this
for( std::map<int, std::vector<int>>::iterator i = m.begin(); i != m.end(); i++ )
or this
for( auto i = m.begin(); i != m.end(); i++ )
It should be evident which method sane programmers would prefer.

I won't replace the auto and range-based for loops with pre-C++11 code as that would be counter-intuitive. If you have an up to date compiler, why not take advantage of the features C++ has to offer?

Anyway, if there's something you're unsure about go ahead and ask and I'll try to answer it to the best of my ability. Hope this helps.