# Error when using std::vector::size to create another vector

I am learning DSA and while practising my LeetCode questions I came across a question-( https://leetcode.com/problems/find-pivot-index/).
Whenever I use vector prefix(size), I am greeted with errors, but when I do not add the size, the program runs fine.
Below is the code with the size:

``````class Solution {
public:
int pivotIndex(vector<int>& nums) {
//prefix[] stores the prefix sum of nums[]
vector<int> prefix(nums.size());
int sum2=0;
int l=nums.size();
//Prefix sum of nums in prefix:
for(int i=0;i<l;i++){
sum2=sum2+nums[i];
prefix.push_back(sum2);
}
//Total stores the total sum of the vector given
int total=prefix[l-1];
for(int i=0; i<l;i++)
{
if((prefix[i]-nums[i])==(total-prefix[i]))
{
return i;
}
}
return -1;
}
};
``````

I would really appreciate if someone could explain this to me.
Thanks!

### >Solution :

If you use vector constructor with the integer parameter, you get vector with `nums.size()` elements initialized by default value. You should use indexing to set the elements:

``````...
for(int i = 0; i < l; ++i){
sum2 = sum2 + nums[i];
prefix[i] = sum2;
}
...
``````

If you want to use `push_back` method, you should create a zero size vector. Use the constructor without parameters. You can use `reserve` method to allocate memory before adding new elements to the vector.