C++ - Define Vector size with class argument

Question

Hello I'm trying to create a genetic algorithm with C++ and I tried to use vector as the container the problem is I didn't know how to set the size of the vector because the vector have a class argument like this

class population
{
    friend class chromosome;
private:
    int population_number;
    int best_index[2];
    vector <chromosome *> chromosome_population;
public:
    population(int numberOfPopulation);
    population(int numberOfPopulation,int numberOfSynapse);
    ~population();
    int worst_chromosome();
    void mating();
    void crossover(int parent_1,int parent_2);
};

this is the population class and here's the chromosome class

class chromosome
{
    friend class population;
private:
    int chromosome_id;
    float fitness;
    vector <gen *> gen_chromosome;
public:
    chromosome();
    ~chromosome();
    void fitness_function();
    void mutation_translocation();
    int get_chromosome_size();
};

how can I set the vector length in the population class constructor? I've tried to use vector.pushback and vector.resize but both will give me error because the argument doesn't match. Actually I'm understand why it become error but I didn't know how to match the argument inside the vector pushback here's my population constructor

population::population(int numberOfPopulation)
{
    srand (time(NULL));
    population_number = numberOfPopulation;
    for(int a=0;a<population_number;a++)
    {
        chromosome_population.push_back();
    }
    cout<<chromosome_population.size();
    for(int i=0;i<population_number;i++)
    {
        chromosome_population[i]->chromosome_id = i;
        int chromosome_length = rand() % 10 + 1;
        for(int j=0;j<chromosome_length;j++)
        {
            chromosome_population[i]->gen_chromosome[j]->basa_biner = rand()%1;
            chromosome_population[i]->fitness = (rand()%99)+1;
        }
    }
}

If is there any other information you want you can tell me in the comment and I'll add the information you needed. Thanks before.

[vector::reserve](http://www.cplusplus.com/reference/stl/vector/reserve/) — sgarizvi, Nov 19 '12 at 09:26

score 6 · Accepted Answer · edited May 23 '17 at 10:27

std::vector has several constructors and one of the variants accepts the initial number of elements to be stored in the vector.

Specify the size of the vector in the population constructor's initializer list:

population::population(int numberOfPopulation) :
    population_number(numberOfPopulation),
    chromosome_population(numberOfPopulation)
{
}

Give this approach, the population_number member variable is unnecessary as it can be obtained by chromosome_population.size().

Specifying an initial size on the vector will mean that it contains numberOfPopulation null pointers. Before accessing elements in the vector you need to create objects, in this case using new. If the elements are copyable and polymorphic behaviour is not required then recommend using vector<chromosome> instead. If you must use dynamically allocated elements in the vector then you must allocate first:

chromosome_population[i] = new chromosome();

and remember to delete when no longer required.

It also desirable to use a form of smart pointer instead of raw pointers. An advantage of using the smart pointer is that when the vector<unique_ptr<chromosome>> goes out of scope the elements will be destructed for you, without having to explicitly call delete on each of the elements. See What C++ Smart Pointer Implementations are available? for a useful listing of the available smart pointers.

Note that vector::push_back() accepts an argument, with same type as its element. So the correct invocation of push_back() is:

chromosome_population.push_back(new chromosome());

If you specify an initial size of the vector at construction, calling push_back() will add elements after the initial (null pointers in this case) elements in the vector.

I think this one more suitable -> chromosome_population[i] = new chromosome(); with this I can use the vector pushback? — Niko Adrianus Yuwono, Nov 19 '12 at 09:42
@NAYOSO, update answer. Use either an initial size constructor and `chromosome_population[i] = new chromosome();` _or_ just `chromosome_population.push_back(new chromosome());`. — hmjd, Nov 19 '12 at 09:47
I use this -> chromosome_population[i] = new chromosome(); and it works! thank you very much!and if I may ask one more where I must declare the vector gen_chromosome; size? gen is another class and the vector placed in the chromosome class but it'll be called in the population constructor like this chromosome_population[i]->gen_chromosome[j]->basa_biner = rand()%1; is it possible? by the way I already vote up and accept your answer :) — Niko Adrianus Yuwono, Nov 19 '12 at 09:52
@NAYOSO, if the number of elements to be stored in `gen_chromosome` is known at the point of construction of the `chromosome` instance pass it as a constructor argument and use the same approach. Otherwise, you can use [`vector::resize()`](http://en.cppreference.com/w/cpp/container/vector/resize). — hmjd, Nov 19 '12 at 09:56
OK thank you very much hmjd! actually I'm also very interested with smart pointer but I think I'll go with this implementation for now and study the smart pointer later, once again thank you! — Niko Adrianus Yuwono, Nov 19 '12 at 10:07

C++ - Define Vector size with class argument

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