Pitfalls of using Vectors incorrectly

I was implementing binary search and I run into an interesting problem in C++

        vector<int> nums;
        nums.reserve(30);
        nums[0] = 1;
        for (int index = 1; index < 30; index++)
        {
            nums[index] = // assign a random number
        }

        bool flag = binarySearch(v1, 57);


The problem with this code was:

I reserved memory for 30 elements and wrote into the memory. It looked like nums had 30 numbers written into memory. But the size was zero.




When I implemented a binary search function the size returned zero

bool binarySearch(vector<int>& nums, int k) 

{

...

high = nums.size();

...

}