selecting from a group in c++
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Thread: selecting from a group in c++

  1. #1
    Join Date
    Jul 2013
    Posts
    1

    Question selecting from a group in c++

    Hi,

    I have N vectors which look like this:
    [1→m] [m+1→2m] [2m+1→3m] [3m+1→4m] [4m+1→5m]..... [{(N-1)m}+1→Nm]

    I want to select 1 element from each vector without duplication of any combinations.
    Essentially only when all combinations are done with 1st element in first vector ,only then it should move to next element in first vector
    say i have elements :[123] [456] [789]

    my combinations should be like
    147
    148
    149
    157
    158
    159
    167
    168
    169
    247.

    Also, i cant have any repetitions and only after all combinations of 1 are done only then the loop has to move to next combination ie 247 combination and so on.

    i tried NCK (n choose k) command but it gave me random combinations.
    how should i go about it with using minimal for loops?
    thanks!

  2. #2
    Join Date
    Jul 2005
    Location
    Netherlands
    Posts
    2,014

    Re: selecting from a group in c++

    Quote Originally Posted by san1234 View Post
    my combinations should be like
    147
    148
    149
    157
    158
    159
    167
    168
    169
    247….

    Also, i cant have any repetitions and only after all combinations of 1 are done only then the loop has to move to next combination ie 247 combination and so on.
    So, how did you generate that list? Write down the exact steps you took that, if followed, would produce the entire list. Then you can translate that into C++ code, but you need to write the steps down on paper first.
    Cheers, D Drmmr

    Please put [code][/code] tags around your code to preserve indentation and make it more readable.

    As long as man ascribes to himself what is merely a posibility, he will not work for the attainment of it. - P. D. Ouspensky

  3. #3
    Join Date
    Oct 2008
    Posts
    1,153

    Re: selecting from a group in c++

    do you mean something like:

    Code:
    #include <iostream>
    #include <vector>
    
    template< typename BiIter >
    bool next_indeces( BiIter from, BiIter to, typename std::iterator_traits<BiIter>::value_type extent )
    {
        if( from != to )
        {
            for( --to; ++*to == extent && to != from; --to )
            {
                *to = typename std::iterator_traits<BiIter>::value_type();
            }
    
            return *to != extent;
        }
        else
            return false;
    }
    
    int main()
    {
        int N = 3;
        int M = 3;
        std::vector<int> indeces( N );
    
        do
        {
            for( int i = 0; i < N; ++i  )
            {
                std::cout << 1 + M * i + indeces[i] << ' ';
            }
    
            std::cout << std::endl;
        }
        while( next_indeces( indeces.begin(), indeces.end(), M ) );
    }
    which gives

    Code:
    1 4 7 
    1 4 8 
    1 4 9 
    1 5 7 
    1 5 8 
    1 5 9 
    1 6 7 
    1 6 8 
    1 6 9 
    2 4 7 
    2 4 8 
    2 4 9 
    2 5 7 
    2 5 8 
    2 5 9 
    2 6 7 
    2 6 8 
    2 6 9 
    3 4 7 
    3 4 8 
    3 4 9 
    3 5 7 
    3 5 8 
    3 5 9 
    3 6 7 
    3 6 8 
    3 6 9
    ? clearly, note that if N and M are fixed at compile time and small, you can write this with just a bunch of for loops and/or some modulo arithmetic ...

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