How to generate a list from a list vector by concatenating subsequences of positives and negatives

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How to generate a list from a list vector by concatenating subsequences of positives and negatives

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#1 by (1 votes)

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Implement a function void sign_join(vector< list<int> > &VL,list<int> &L) that, given a vector of lists VL generate a L list where they are First concatenated all the first non-negative subsequence of VL [0], VL [1] , ... After the negatives. Then again the non-negatives. So continuing until all the VLs are finished

Examples: VL: [[1,2, -1, -2,3,4, -3, -4], [5,6, -5, -6,7,8, -7, -8], [- 9 , -10]],

= > L: [1,2,5,6, -1, -2, -5, -6, -9, -10,3,4,7,8, -3, -4, -7, -8],

The code I have is the following:

int sign(int &x){
    if(x>=0){
        return 1;
    }else{
        return 0;
    }
}

bool vectorempty(vector<list<int>> &VL) {
    for(int i=0; i<VL.size(); i++) {
        if(!VL[i].empty()) {
            return false;
        }
    }
    return true;
}

int main(int argc, char *argv[]) {

    vector<list<int>> vl = {{1,2,-3}, {3,4,-5,-1}, {5,6,-8}};
    list<int> l;

    int s = 1;
    int i=0;
    while(vectorempty(vl)==false){
        auto it = vl[i].begin();
        while(it != vl[i].end() && sign(*it)==s) {
            l.push_back(*it);
            it=vl[i].erase(it);
        }
        if(i == (vl.size()-1)) {
            i=0;
            s=-s;
        }else{
            ++i;
        }
    }

    for(int &x:l) cout<<x<<" ";

    return 0;
}

But it remains in an infinite loop and I do not know how to solve it.

c++ c++11

asked by facundo rotger 10.09.2018 в 17:09

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1 answer

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score 1 · Answer 1

Without debugging I can not see where the program stays in an infinite loop (by the way, have you tried to debug?).

I still think that you complicate your life unnecessarily and reinvent the wheel: There is already a function to determine if a number is negative, it belongs to the header <cmath> and it's the function std::signbit .

Proposal.

Create a separate function (to follow the principle of sole responsibility ) to make the operation, my advice is that it is not a destructive operation (that does not alter the original data).

The algorithm to follow, in pseudocode, would be:

Access a list.

Extract numbers while the numbers are of the appropriate sign (and numbers remain).

Follow the previous step to the end or until you find an inappropriate sign number.

Go to the next list, repeat until all the lists are empty.

In code it could look like this:

using ilist = std::list<int>;
using vlist = std::vector<ilist>;

ilist procesa(const vlist &vl)
{
    auto negativo = false;
    auto temp = vl;
    auto datos = [](const vlist &vl)
    {
        for (const auto &l : vl)
            if (l.size())
                return true;
        return false;
    };
    ilist resultado{};

    while (cuenta(temp))
    {
        for (auto &lista : temp)
//                ~~~~~~ <--- Accede a la lista
        {
            for (auto v = lista.begin(), e = lista.end();
                (v != e) && (std::signbit(*v) == negativo);)
//              ~~^~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
//                 \--- Mientras queden números     /
//                                                 /
// Mientras los números sean del signo adecuado --/
            {
                resultado.push_back(*v);
                v = lista.erase(v);
//              ~~~~~~~~~~~~~~~~~~~~~~~~ <--- Extrae números
            }
        }
        negativo = !negativo;
    }

    return resultado;
}