# Bug in Vector class? **URL:** \ **Category:** Ruby\ **Created:** [October 9, 2008, 11:54am UTC](https://www.ruby-forum.com/t/bug-in-vector-class/150519 "2008-10-09T11:54:16Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![daz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/87869e/32.png) [@daz](https://www.ruby-forum.com/u/daz)\ **Post date:** [October 9, 2008, 11:54am UTC](https://www.ruby-forum.com/t/bug-in-vector-class/150519/1 "2008-10-09T11:54:16Z")
require ‘matrix’ m = Matrix.identity(4) v = Vector[5, 6, 7, 1] puts m \* v gives the expected answer of [5, 6, 7, 1] but puts v \* m gives ExceptionForMatrix::ErrDimensionMismatch: Matrix dimension mismatch because the Vector#\* method promotes self to a column vector when the argument is found to be a Matrix. I think it should be promoting self to a row vector and making this change removes the exception and gives the correct result. The updated method is: def \*(x) case x when Numeric els = @elements.collect{|e| e \* x} Vector.elements(els, false) when Matrix Matrix.row\_vector(self) \* x else s, x = x.coerce(self) s \* x end end Dave. ---
**Author:** ![daz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/87869e/32.png) [@daz](https://www.ruby-forum.com/u/daz)\ **Post date:** [October 9, 2008, 5:15pm UTC](https://www.ruby-forum.com/t/bug-in-vector-class/150519/2 "2008-10-09T17:15:43Z")
Dave B. wrote: > because the Vector#\* method promotes self to a column vector when the > argument is found to be a Matrix. I think it should be promoting self > to a row vector and making this change removes the exception and gives > the correct result. I think it should not. Either the programmer knows what he or she is doing and is able to take care of the matrices dimensions, or no magic auto-transpositions or other tricks are going to save their project from falling apart sooner or later. Vector is a column vector, and let it be this way. TPR. ---
**Author:** ![daz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/87869e/32.png) [@daz](https://www.ruby-forum.com/u/daz)\ **Post date:** [October 9, 2008, 5:51pm UTC](https://www.ruby-forum.com/t/bug-in-vector-class/150519/3 "2008-10-09T17:51:44Z")
On 9 Oct 2008, at 10:40, Dave B. wrote: > gives > els = @elements.collect{|e| e \* x} > Dave. Further modification so it returns a Vector instead of a matrix (Matrix.row\_vector(self) \* x).row(0) Dave. ---
**Author:** ![daz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/87869e/32.png) [@daz](https://www.ruby-forum.com/u/daz)\ **Post date:** [October 9, 2008, 7:03pm UTC](https://www.ruby-forum.com/t/bug-in-vector-class/150519/4 "2008-10-09T19:03:24Z")
On 9 Oct 2008, at 16:13, Thomas B. wrote: > auto-transpositions or other tricks are going to save their project > from > falling apart sooner or later. Vector is a column vector, and let it > be > this way. The Vector class doesn’t seem to have any idea that it is representing a row or column vector and as the class stands now a vector \* matrix will always fail. The only way to achieve this operation is to manually convert the vector into a Matrix with one row but now you are really doing a matrix \* matrix operation and not using the failing path in Vector. If the intent was for vector \* matrix to fail then it would be better to raise an exception directly rather than rely on the Matrix class to catch a dimensional error. I still think it is better to treat it as a row vector in this case. Dave. ---
**Author:** ![daz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/87869e/32.png) [@daz](https://www.ruby-forum.com/u/daz)\ **Post date:** [October 9, 2008, 7:14pm UTC](https://www.ruby-forum.com/t/bug-in-vector-class/150519/5 "2008-10-09T19:14:40Z")
Dave B. wrote: > The Vector class doesn’t seem to have any idea that it is representing > a row or column vector and as the class stands now a vector \* matrix > will always fail. Vector.elements([1,2,3])\*Matrix.identity(1) #=\> Matrix[[1], [2], [3]] Of course it’s not much of a useful operation, but for me it’s evident from it that a vector assumes itself to be a vertical matrix each time it is required to behave like a matrix. And now, what will be the result of the above operation if, as you want, a vector would auto-transpose when sent \* with a matrix? A row or a column matrix? For me, this proves that your idea is inconsistent. It is not true that vector\*matrix operation is never possible, so we shouldn’t automagically make it possible even if it is not. It wouldn’t make debugging any easier. TPR. ---
**Author:** ![daz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/87869e/32.png) [@daz](https://www.ruby-forum.com/u/daz)\ **Post date:** [October 10, 2008, 9:43am UTC](https://www.ruby-forum.com/t/bug-in-vector-class/150519/6 "2008-10-10T09:43:24Z")
On 9 Oct 2008, at 18:12, Thomas B. wrote: > from it that a vector assumes itself to be a vertical matrix each time > it is required to behave like a matrix. > > And now, what will be the result of the above operation if, as you > want, > a vector would auto-transpose when sent \* with a matrix? A row or a > column matrix? For me, this proves that your idea is inconsistent. It > is not true that vector\*matrix operation is never possible, so we > shouldn’t automagically make it possible even if it is not. It > wouldn’t > make debugging any easier. OK, so you demonstrated a degenerate case that is not much use as you point out. How would you do something more useful such as Vector[1,2,3]\*Matrix.identity(3)? I don’t think you can unless you convert the vector to a row vector aka a matrix with one row and do it as a matrix \* matrix operation. Also a Vector \* Matrix should return a Vector and not a Matrix (as the Matrix \* Vector operation does). Thankfully with open classes I can get the behaviour I think is desirable. Dave.