Using vectors in C++ program is not printing anything - c++

Question

I have written a C++ program to find fibonacci numbers. It's running successfully but is not printing anything.
#include<bits/stdc++.h>
using namespace std;
int main(){
int n = 5;
vector<int> fib;
fib[0] = 0;
fib[1] = 1;
for (int i = 2; i <= n; i++){
fib[i] = fib[i-1] + fib[i-2];
}
cout<<fib[n];
}
If I do the same thing using array instead of vector it prints successfully.
#include<bits/stdc++.h>
using namespace std;
int main(){
int n = 5;
int fib[10];
fib[0] = 0;
fib[1] = 1;
for (int i = 2; i <= n; i++){
fib[i] = fib[i-1] + fib[i-2];
}
cout<<fib[n];
}
I have tested this on sublime text and onlinegdb.

Answer 1

int fib[10];
This creates an array of 10 integers.
vector<int> fib;
This creates a vector of size 0, with 0 integers.
For these two snippets to match, you need to initialize the vector with 10 integers like the array. So:
vector<int> fib(10);
Some notes on vectors
One of the big differences between std::vector and primitive arrays is that std::vector is resizable! So while initializing the vector with 10 ints like above will work, you could also add them on the fly using push_back() or by calling resize().
Likewise, std::vector has a .at() function for access. It's marginally slower than the subscript operator ([]), so I would not suggest using it in production-level code. But while you're learning, I would strongly suggest using .at(), as it will do bounds checking for you. So this program would've told you you were trying to access locations in the vector that don't exist--instead of just running with Undefined Behavior.

Answer 2

Note that when you wrote :
vector<int> fib;//creates an empty vector that is a vector of size 0
fib[0] = 0; //UNDEFINED BEHAVIOR
fib[1] = 1; //UNDEFINED BEHAVIOR
In the above code snippet, you created an empty std::vector. That is a vector of size 0.
Next when you wrote fib[0] = 0; you're trying access the first element of the vector. That is, the element at index 0. But note that there is no element inside the vector and so you have undefined behavior in your program.
Similarly when you wrote fib[1] = 1;, you're trying to access the second element of the vector. But since there is no element inside the vector, this again leads to undefined behavior.
To solve this you can create a vector of some size(say 10) and then access its elements as shown below:
vector<int> fib(10);//create vector of size 10
fib[0] = 0;//OK NOW
fib[1] = 1;//OK NOW
Since you're using indices to access the elements of the vector, i have not suggested push_back. std::vector::push_back is a member function that can be used to add an element into the vector.