Confusion about vectors [closed]

Question

I would love to know what the code below means

I just want to know how it works tbh.

vector<int> lotteryNumVect(10); // I do not understand this part.

int lotteryNumArray[5] = {4, 13, 14, 24, 34}; // I understand this part.

lotteryNumVect.insert(lotteryNumVect.begin(), lotteryNumArray,
                      lotteryNumArray + 3); // I do not understand this part.

cout << lotteryNumVect.at(2) << endl; // I understand this part.

Answer 1

This statement

vector <int> lotteryNumVect(10);

declares a vector with 10 elements initialized by zeroes.

That is there is used the constructor

explicit vector(size_type n, const Allocator& = Allocator());

3 Effects: Constructs a vector with n default-inserted elements using the specified allocator.

The second parameter of the constructor have a default argument so you may call the constructor specifying only the number of elements to be created in a vector.

This statements

lotteryNumVect.insert(lotteryNumVect.begin(), lotteryNumArray,
                      lotteryNumArray + 3);

inserts in the beginning of the vector 3 elements from the array.

So as a result the vector will look like

4, 13, 14, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 

Answer 2

Explanation

1. Statement vector <int> lotteryNumVect(10);:

   This is an example of using constructor. According to cplusplus:

   default (1) : explicit vector (const allocator_type& alloc = allocator_type());

   fill (2) : explicit vector (size_type n, const value_type& val = value_type(), const allocator_type& alloc = allocator_type());

   range (3) : template <class InputIterator> vector (InputIterator first, InputIterator last, const allocator_type& alloc = allocator_type());

   copy (4) : vector (const vector& x);

   So, vector <int> lotteryNumVect(10); initializes the vector with ten zeros (see (1) above). vector <int> lotteryNumVect(5, 2); would initialize the vector with five twos (see (2) above). You can check the example here to understand better.

2. Statement lotteryNumVect.insert(lotteryNumVect.begin(), lotteryNumArray, lotteryNumArray + 3);:

   This actually insertion via iterators. Check this out:

   single element (1) : iterator insert (iterator position, const value_type& val);

   fill (2) : void insert (iterator position, size_type n, const value_type& val);

   range (3) : template <class InputIterator> void insert (iterator position, InputIterator first, InputIterator last);

   The term lotteryNumVect.begin() actually points the first element of lotteryNumVect (see vector::begin()). Whereas lotteryNumArray and lotteryNumArray+3 respectively points the first and the third elements of the lotteryNumArray array. So, basically lotteryNumVect.insert(lotteryNumVect.begin(), lotteryNumArray, lotteryNumArray + 3); inserts the first three elements of the lotteryNumArray to the beginning of the vector lotteryNumVect.

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Further reading on std::vector

• cpluscplus
• cppreference
• GeeksforGeeks

How to navigate on cplusplus:

• Header: cplusplus.com/reference/<type header name here>
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• Function/Container/Keyword: cplusplus.com/reference/<the header which contains it>/<function/container/keyword name>
  Example: cplusplus.com/reference/iostream/cin/
• Member function/variable: cplusplus.com/reference/<the header which contains it>/<function/container/keyword name>/<member variable/function name>/
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Alternatively, you could Google it. At which, you will get all of three sites in your search result and perhaps far better outcome.