c++ vector one dimensional and two dimensional arrays value assignment - c++

Question

I am confused about c++ two dimensioanal vectors
#include <vector>
int main()
{
//First Code
std::vector<int> oneDimArray;
oneDimArray.push_back(5);
for (int i = 0; i < oneDimArray.size(); i++)
{
std::cout << oneDimArray[i];
}
//Second Code
std::vector<std::vector<int>> twoDimArray;
twoDimArray[0].push_back(5); //Here giving error
for (int i = 0; i < twoDimArray.size(); i++)
{
for (int j : twoDimArray[i])
{
std::cout << j;
}
}
return 0;
}
when i try to run this code First Code is working. But in Second Code i am getting vector out of range error while trying to push back. I guess at first i should say twoDimArray's first size to it. But i dont know how. Because i am definig this twoDimArray in my .h file so if i do this i must do it there. Please can you help me?

Answer 1

The reason this is happening is because of two reasons.
The vector you are trying to access is empty
You are trying to put an int into a vector that was supposed to hold vector<int>s.
Fixed code:
#include <vector>
#include <iostream>
int main()
{
std::vector<int> oneDimArray;
oneDimArray.push_back(5);
for (int i = 0; i < oneDimArray.size(); i++)
{
std::cout << oneDimArray[i];
}
std::vector<std::vector<int>> twoDimArray;
twoDimArray.push_back(oneDimArray); //You can put a vector of ints in here
for (int i = 0; i < twoDimArray.size(); i++)
{
for (int j : twoDimArray[i])
{
std::cout << j;
}
}
return 0;
}
twoDimArray[0].push_back(5) is invalid because the vector had no size yet. After you have pushed back something, or done twoDimArray.resize(new size here);, then you can do that (as long as it isn't out of range).
Also, once a vector has size, you can access the elements of the vector inside the vector by going like twoDimArray[0][0] = 1;. That would give the first value of the first vector inside of the vector to get the value 1.