Specifying the size of a vector in declaration vs using reserve

Question

What is the difference between specifying the size of vector in declaration vs using reserve? My code seems to produce different results.

int main() {
    int n = 10;
    std::vector<int> arr(n);
    arr.push_back(0);
    arr.push_back(1);
    for (int i = 2; i <= n; i++)
    {
        int a = arr[i - 1] + arr[i - 2];
        arr.insert(arr.begin() + i, a);
    }
   std::cout << arr[3] << std::endl;
}

The above code prints 0

int main() {
    int n = 10;
    std::vector<int> arr;
    arr.reserve(n);
    arr.push_back(0);
    arr.push_back(1);
    for (int i = 2; i <= n; i++)
    {
        int a = arr[i - 1] + arr[i - 2];
        arr.insert(arr.begin() + i, a);
    }
   std::cout << arr[2] << std::endl;
}

This code prints 2 which is correct

`std::vector arr(n); arr.push_back(0); arr.push_back(1);` -- Now print `arr.size()`, and that should give you a hint on what is going on. — PaulMcKenzie, May 26 '20 at 15:29
Does this answer your question? [Vector: initialization or reserve?](https://stackoverflow.com/questions/8928547/vector-initialization-or-reserve) — Nicolas Dusart, May 26 '20 at 15:30
By doing this: `std::vector arr(n);`, you're initiating 10 empty slots inside the vector; `.push_back()` doesn't fill said spots, it appends a value after them. So with aforesaid syntax, the 10 first slots will be empty and unreserved unless you decide to reserve them. For instance: `arr[0] = 5`, or: `arr.insert(arr.begin(), 5);` — Skiller Dz, May 26 '20 at 15:37
@SkillerDz So is insert the only keyword that can be used to insert elements into those empty slots? — Kshitiz Chaudhary, May 26 '20 at 16:00
@NicolasDusart It did help, but my doubt was better explained better in this thread. Thanks — Kshitiz Chaudhary, May 26 '20 at 16:02
@FrançoisAndrieux It is here, my original code was different and I just modified that code to make it a bit simpler. — Kshitiz Chaudhary, May 26 '20 at 16:03

Specifying the size of a vector in declaration vs using reserve

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