Inserting elements in multidimensional Vector

Question

vector> sort_a;
vector v2;
vector v3;

for (int i=0; i<4; ++i) {
v2.push_back(i);

  for (int j=0; j<4; ++j) {
           


        

Answer 1

Don't think of it as a multidimentional vector, think of it as a vector of vectors.

int n = 4;
std::vector> vec(n, std::vector(n));

// looping through outer vector vec
for (int i = 0; i < n; i++) {
  // looping through inner vector vec[i]
  for (int j = 0; j < n; j++) {
    (vec[i])[j] = i*n + j;
  }
}

I included parentheses in (vec[i])[j] just for understanding.

Edit:

If you want to fill your vector via push_back, you can create a temporary vector in the inner loop, fill it, and then push_back it to your vector:

for (int i = 0; i < n; i++) {
  std::vector temp_vec;

  for (int j = 0; j < n; j++) {
    temp_vec.push_back(j);
  }

  vec.push_back(temp_vec);
}

However, push_back calls result in slower code, since not only you need to reallocate your vector all the time, but also you have to create a temporary and copy it.