In Ruby 1.9.3, a vector can't be multiplied by another vector?

Question

I am implementing basic machine learning algorithm in Ruby, 1.9.3.

I try to use class Matrix and Vector for the arithmetics. But when I try to multiply a vector by another vector, it says "ExceptionForMatrix::ErrOperationNotDefined: Operation(*) can't be defined: Vector op Vector".

But the doc of Ruby, 1.9,3 says:

*(x) Multiplies the vector by x, where x is a number or another vector.

My code and output is here:

> a = Vector[1,2,3]
=> Vector[1, 2, 3]
> b = Vector[1,2,3]
=> Vector[1, 2, 3]
> a * b
ExceptionForMatrix::ErrOperationNotDefined: Operation(*) can't be defined: Vector op Vector

Answer 1

Although the documentation clearly states that you can multiply a vector by another vector, it's nonsensical, and, as zisasign points out, the implementation doesn't permit it.

However, you can turn either vector into a one-row matrix using the covector method, which you can then multiply to give a meaningful calculation:

a = Vector[1, 2, 3]
b = Vector[10, 100, 1000]

a.covector * b
# => Vector[3210]

a * b.covector
# => Matrix[[10, 100, 1000], [20, 200, 2000], [30, 300, 3000]]

Answer 2

The documentation is wrong. When you look at the code linked in the doc, there is

def *(x)
  case x
  when Numeric
    els = @elements.collect{|e| e * x}
    Vector.elements(els, false)
  when Matrix
    Matrix.column_vector(self) * x
  when Vector
    Vector.Raise ErrOperationNotDefined, "*", self.class, x.class
  else
    apply_through_coercion(x, __method__)
  end
end

Multiplying (as in matrix-multiplication) a (column-)vector by a (column-)vector does not make sense, anyway. Maybe you want inner_product?