How do I add a value to a struct vector, for example in some vector field, because it is from a struct

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#include <iostream>
#include <vector>
using namespace std;


struct dados {
    int pessoas;
    int consumo;
};

int main()
{



    int totalx=0, totaly=0;
    int n,x,y;
     vector<dados> info;
    //int cidade = 1;
    cout << "Insira valor para N: ";
    cin >> n;

    for(int i = 0; i < n; i++){
        cin>> x;
        info[i].consumo.push_back(x); // como  add um valor para o campo do vector, pois ele é do tipo 'dados',uma struct ? 
    }





return 0;
}
c++ c++11

  • The field consumo is the type int. What would that method be push_back?

  • As already said by @Andersoncarloswoss, you are making trouble with the fields. The push_back can’t be done on a int how it is doing but yes on the vector which is your info

  • then I should do something like info[i]. consumption = x ?

1 answer

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There are some misconceptions in your code, which probably comes from your knowledge about arrays. The container std::vector grows and decreases dynamically, unlike arrays, that have a fixed size and the storage space for each element already exists. With std::vector, the storage space of the elements is allocated as elements are inserted. Therefore, you cannot access an element without it existing in an object std::vector. For example, the Code below shows this misconception:

std::vector<int> v;
v[0] = 42; // errado

If v were just one array of the kind int[N], there would be no problem in accessing element 0 (of course, assuming that 0 < N). Like v is an object of the type std::vector<int>, that was initialized by default (default initialization in English), so it contains no elements, that is, it is emptiness.

In order to access an element, we need to enter it first. As you have already discovered, the member function std::vector::push_back does just that: inserts an element at the end of the vector, correctly allocating enough space for the new element. There is also the member function std::vector::emplace_back (since ), which inserts a new element by passing the arguments passed to it to the element type constructor. Anyway, the example code piece above would be correct as follows:

std::vector<int> v;
v.push_back(42); // ok
int i = v[0]; // também ok: elemento na posição 0 existe.

I’m not sure I understood your intent in your code, but I believe this would fix:

for (int i = 0; i < n; ++i) {
    cin >> x;
    info.push_back(dados{0, x});
}

If we add a constructor in dados that starts its members, we can take advantage of the function emplace_back (for she builds the element in place by calling the builder):

struct dados {
    int pessoas;
    int consumo;
    dados(int pessoas, int consumo)
        : pessoas(pessoas)
        , consumo(consumo) {}
};

// ...

for (int i = 0; i < n; ++i) {
    cin >> x;
    info.emplace_back(0, x); // passa-se apenas os argumentos pra construir o novo elemento.
}

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