What does 'vector <double> hour {vector<double>(24, -1)};' mean?

Question

Is this a vector of 24 vectors all initialised to '-1'? If so, then why isn't the hour vector of type 'vector' instead of 'double'?

I can't visualise the data structure this'll form.

Answer 1

vector<double> hour { vector<double>(24, -1) };

This will result in a single vector of 24 -1s named hour . The vector<double>(24, -1) inside of the {} is a temporary vector that is created in order to create the hour vector.

Visually, with '.0' omitted:

{-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1}

If so, then why isn't the hour vector of type 'vector' instead of 'double'?

The hour vector is of type vector<double> The <> are for templates and determine at compile time what type the vector will contain.

If it had said vector<int> hour instead, this would have been a vector of int values instead of double values.

Answer 2

As per the cppreference page , this is the constructor form

vector(size_type count,
       const T& value,
       const Allocator& alloc = Allocator());
// Constructs the container with <count> copies of elements, each with value <value>.

with the default allocator.

Hence it constructs a vector of size 24 with each element of that vector set to -1 .

And the type of hour is neither double nor vector , it's vector<double> which is, surprisingly, a vector of doubles :-)

Answer 3

it is first constructing a instance of a vector of double with 24 items initialized with -1.0(implicitly), then it is copy constructed to create a new vector of double named hour.

update : modern compiler will have optimizations that uses move semantics for this operation, and will not do a deep copying.

the better way for doing this is simply vector<double> hour(24, -1);

Answer 4

vector<double>(24, -1) creates a vector with with size 24 and each value initialized to -1 . vector <double> hour{vector<double>(24, -1)} uses a copy constructor to make a copy of this vector so it ends up with 24 -1 s as well.

{-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0,-1.0}