Segmentation fault in C++ adding item to a vector - c++

Question

Lately I have set myself to learn C++, and while working on a bigger project, I have tried to use 'vectors'. But every-time I try passing it a value, it exits with a segmentation fault.
Here is my terminal output:
#include <iostream>
#include <vector>
using namespace std;
int main(){
vector<int> test;
cout << "hello world" << endl;
test[0] = 0;
return 0;
}
me#my-MacBook-Pro Desktop % g++ test.cpp -o o && ./o
hello world
zsh: segmentation fault ./o
#include <iostream>
#include <vector>
using namespace std;
int main(){
vector<int> test;
cout << "hello world" << endl;
//test[0] = 0;
return 0;
}
me#my-MacBook-Pro Desktop % g++ test.cpp -o o && ./o
hello world
me#my-MacBook-Pro Desktop %

Answer 1

The segfault is because of out of bound access.
you need to set the size in the ctor
vector<int> test(1);
or push_back:
vector<int> test;
test.push_back(0);

Answer 2

Size it vector<int> test = {0,1,2,3,4}; or vector<int> test(5)
But you might want to use push_back in this situation
#include <iostream>
#include <vector>
using namespace std;
int main(){
vector<int> test;
cout << "hello world" << endl;
test.push_back(0);
cout << test[0];
return 0;
}
Basically adds an item at the end.
Can also use maps with the keys be ints if you want to be able to just [] it or leave in spaces (which from what i saw is what your trying to do)
#include <iostream>
#include <unordered_map>
using namespace std;
int main(){
unordered_map<int, int> test;
cout << "hello world" << endl;
test[0] = 0;
cout << test[0];
return 0;
}