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hi evrybody!

I wan't to multiply two square matrixes, and i don't understand why it
doesn't work.
Could you explain me?

def multmat(A,B):
"A*B"
if len(A)!=len(B): return "error"
D=[]
C=[]
for i in range(len(A)): D.append(0)
for i in range(len(A)): C.append(D)
for i in range(len(A)):
for j in range(len(A)):
for k in range(len(A)):
C[i][j]+=A[i][k]*B[k][j]
print C[i][j]
print C[i]
return C

when i use it on :
>>> A=[[2,3,4],[5,8,6],[4,5,7]]
>>> B=[[1,0,0],[0,1,0],[0,0,1]]

I get :
2
2
2
0
3
3
0
0
4
[2, 3, 4]
7
7
7
3
11
11
4
4
10
[7, 11, 10]
11
11
11
11
16
16
10
10
17
[11, 16, 17]
[[11, 16, 17], [11, 16, 17], [11, 16, 17]]

thank you

"Sssasss" wrote:

> I wan't to multiply two square matrixes, and i don't understand why it
> doesn't work.
>
> def multmat(A,B):
> "A*B"
> if len(A)!=len(B): return "error"
> D=[]
> C=[]
> for i in range(len(A)): D.append(0)
> for i in range(len(A)): C.append(D)

append doesn't copy data, so you're basically adding len(A) references to
the same D list to C. for more on this, see:

http://pyfaq.infogami.com/how-do-i-c...mensional-list

</F>

Fredrik Lundh wrote:
> "Sssasss" wrote:
>
> > I wan't to multiply two square matrixes, and i don't understand why it
> > doesn't work.
> >
> > def multmat(A,B):
> > "A*B"
> > if len(A)!=len(B): return "error"
> > D=[]
> > C=[]
> > for i in range(len(A)): D.append(0)
> > for i in range(len(A)): C.append(D)
>
> append doesn't copy data, so you're basically adding len(A) references to
> the same D list to C. for more on this, see:
>
> http://pyfaq.infogami.com/how-do-i-c...mensional-list
>
> </F>

Ok!! Tank you very much, i understand now.

ciao

On 2006-10-18 14:15:17 +0200, Sssasss wrote:
> Fredrik Lundh wrote:
> > "Sssasss" wrote:
> >
> > > I wan't to multiply two square matrixes, and i don't understand why it
> > > doesn't work.
> > >
> > > def multmat(A,B):
> > > "A*B"
> > > if len(A)!=len(B): return "error"
> > > D=[]
> > > C=[]
> > > for i in range(len(A)): D.append(0)
> > > for i in range(len(A)): C.append(D)
> >
> > append doesn't copy data, so you're basically adding len(A) references to
> > the same D list to C. for more on this, see:
> >
> > http://pyfaq.infogami.com/how-do-i-c...mensional-list
> >
> > </F>
>
> Ok!! Tank you very much, i understand now.

You might also want to look at numpy/numarray.

Gerrit.

Il 18 Oct 2006 04:17:29 -0700, Sssasss ha scritto:

> hi evrybody!
>
> I wan't to multiply two square matrixes, and i don't understand why it
> doesn't work.
Can I suggest a little bit less cumbersome algorithm?

def multmat2(A,B):
"A*B"
if len(A)!=len(B): return "error" # this check is not enough!
n = range(len(A))
C = []
for i in n:
C.append([0]*len(A)) # add a row to C
for j in n:
a = A[i] # get row i from A
b = [row[j] for row in B] # get col j from B
C[i][j] = sum([x*y for x,y in zip(a,b)])
return C

regards
D.

Sssasss wrote:
> hi evrybody!
>
> I wan't to multiply two square matrixes, and i don't understand why it
> doesn't work.
> Could you explain me?
>
> def multmat(A,B):
> "A*B"
> if len(A)!=len(B): return "error"

Wrong validation here: you _can_ multiply two matrices with a different
number of rows! And instead of returning "error" you should raise an
exception.

[...]

I suggest using a linear algebra package, but if you insist in using lists
of lists:

>>> b = [[1, 2, 3, 4],
.... [4, 5, 6, 7],
.... [7, 8, 9, 10]]
>>>
>>> a = [[1, 2, 3],
.... [4, 5, 6]]
>>>
>>> ab = [[sum(i*j for i, j in zip(row, col)) for col in zip(*b)] for row in a]
>>> ab
[[30, 36, 42, 48], [66, 81, 96, 111]]

Straightforward from the definition of matrix multiplication.
--
Roberto Bonvallet

David wrote:
> Il 18 Oct 2006 04:17:29 -0700, Sssasss ha scritto:
>
> > hi evrybody!
> >
> > I wan't to multiply two square matrixes, and i don't understand why it
> > doesn't work.
> Can I suggest a little bit less cumbersome algorithm?
>
> def multmat2(A,B):
> "A*B"
> if len(A)!=len(B): return "error" # this check is not enough!
> n = range(len(A))
> C = []
> for i in n:
> C.append([0]*len(A)) # add a row to C
> for j in n:
> a = A[i] # get row i from A
> b = [row[j] for row in B] # get col j from B
> C[i][j] = sum([x*y for x,y in zip(a,b)])
> return C
>
> regards
> D.

This one is really nice, i didn't knew the zip function, thank you
ciao

Roberto Bonvallet wrote:
> Sssasss wrote:
> > hi evrybody!
> >
> > I wan't to multiply two square matrixes, and i don't understand why it
> > doesn't work.
> > Could you explain me?
> >
> > def multmat(A,B):
> > "A*B"
> > if len(A)!=len(B): return "error"
>
> Wrong validation here: you _can_ multiply two matrices with a different
> number of rows! And instead of returning "error" you should raise an
> exception.
>
> [...]
>
> I suggest using a linear algebra package, but if you insist in using lists
> of lists:
>
> >>> b = [[1, 2, 3, 4],
> ... [4, 5, 6, 7],
> ... [7, 8, 9, 10]]
> >>>
> >>> a = [[1, 2, 3],
> ... [4, 5, 6]]
> >>>
> >>> ab = [[sum(i*j for i, j in zip(row, col)) for col in zip(*b)] for row in a]
> >>> ab
> [[30, 36, 42, 48], [66, 81, 96, 111]]
>
> Straightforward from the definition of matrix multiplication.
> --
> Roberto Bonvallet

Thank you, this one is very short!
yes of course we can multiply different kinds of matrices, bu since
I'm starting with python i started with something quick.

ciao