Re: Newbie question about numpy
On Thu, 24 Aug 2006 17:23:49 GMT, Dennis Lee Bieber <[EMAIL PROTECTED]> wrote: >On Thu, 24 Aug 2006 16:38:45 +0100, Paul Johnston ><[EMAIL PROTECTED]> declaimed the following in >comp.lang.python: > >> I know its a long time since my degree but that's not matrix >> multiplication is it ? > > Define "matrix multiplication"... > > What you see appears to be multiplication of corresponding elements. > > Were you expecting a dot product, or a cross product, or something >else? > That as explained in http://people.hofstra.edu/faculty/stefan_Waner/RealWorld/tutorialsf1/frames3_2.html As I say its been a long time :-) Thanks to everyone for the help. Paul -- http://mail.python.org/mailman/listinfo/python-list
Re: Newbie question about numpy
Avell Diroll wrote: > For matrices multiplication, you could get a hint by typing the > following in the interpreter : > import numpy dir(numpy) help(numpy.matrixmultiply)#type "q" to exit Note that the name matrixmultiply() has been deprecated in favor of dot() for many, many years now even in Numeric, numpy's predecessor. It has finally been removed in recent versions of numpy. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco -- http://mail.python.org/mailman/listinfo/python-list
Re: Newbie question about numpy
Paul Johnston wrote: (snip) > I noted the lack of matrices so installed numpy (snip) > _ > from numpy import * > > a = array([[1,2,3],[4,5,6],[1,2,3]]) > b = array([[1,3,6],[2,5,1],[1,1,1]]) (snip) > print "a * b is \n", a * b > _ (snip) > a * b is > [[ 1 6 18] > [ 8 25 6] > [ 1 2 3]] > _ > > > I know its a long time since my degree but that's not matrix > multiplication is it ? You consider that a and b are matrices, but for the python interpreter they are arrays so a*b returns the multiplication of 2 arrays. For matrices multiplication, you could get a hint by typing the following in the interpreter : >>> import numpy >>> dir(numpy) >>> help(numpy.matrixmultiply)#type "q" to exit which could make you want to try the following code : >>> from numpy import * >>> a = array([[1,2,3],[4,5,6],[1,2,3]]) >>> b = array([[1,3,6],[2,5,1],[1,1,1]]) >>> print matrixmultiply(a,b) ... output : ... array([[ 8, 16, 11], [20, 43, 35], [ 8, 16, 11]]) ... HIH, avell -- http://mail.python.org/mailman/listinfo/python-list
Re: Newbie question about numpy
Paul Johnston wrote: > Hi I'm new to python and have just been taking a look at what it has > to offer. > I noted the lack of matrices so installed numpy You will want to ask numpy questions on the numpy list. http://www.scipy.org/Mailing_Lists numpy arrays are not matrices; they are arrays. All of the arithmetic operations on them are done element-wise. The dot() function will do matrix multiplication. There is a matrix class (with the constructor numpy.mat(some_array)) that derives from arrays and overrides the * operator to do matrix multiplication if that is what you want. I prefer using dot() on regular arrays, myself. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco -- http://mail.python.org/mailman/listinfo/python-list
Re: Newbie question about numpy
At Thursday 24/8/2006 12:38, Paul Johnston wrote: from numpy import * a = array([[1,2,3],[4,5,6],[1,2,3]]) b = array([[1,3,6],[2,5,1],[1,1,1]]) print "a * b is \n", a * b I know its a long time since my degree but that's not matrix multiplication is it ? No, it's plain element-by-element multiplication. You want matrixmultiply: http://numpy.scipy.org/numpydoc/numpy-9.html#pgfId-36542 Gabriel Genellina Softlab SRL __ Preguntá. Respondé. Descubrí. Todo lo que querías saber, y lo que ni imaginabas, está en Yahoo! Respuestas (Beta). ¡Probalo ya! http://www.yahoo.com.ar/respuestas -- http://mail.python.org/mailman/listinfo/python-list
Newbie question about numpy
Hi I'm new to python and have just been taking a look at what it has to offer. I noted the lack of matrices so installed numpy I know the documentation is chargable so wanted a quick play to see if I should buy it However _ from numpy import * a = array([[1,2,3],[4,5,6],[1,2,3]]) b = array([[1,3,6],[2,5,1],[1,1,1]]) print 'a = \n',a,"\n" print 'b = \n',b,"\n" print 'a has shape ', a.shape print 'b has shape ', b.shape, "\n" print "a * b is \n", a * b _ Gives me _ a = [[1 2 3] [4 5 6] [1 2 3]] b = [[1 3 6] [2 5 1] [1 1 1]] a has shape (3, 3) b has shape (3, 3) a * b is [[ 1 6 18] [ 8 25 6] [ 1 2 3]] _ I know its a long time since my degree but that's not matrix multiplication is it ? TIA Paul -- http://mail.python.org/mailman/listinfo/python-list
