Le 10/05/2014 17:24, Albert van der Horst a écrit :
I have the following code for calculating the determinant of
a matrix. It works inasfar that it gives the same result as an
octave program on a same matrix.
/ ----------------------------------------------------------------
def determinant( mat ):
''' Return the determinant of the n by n matrix mat
i row j column
Destroys mat ! '''
#print "getting determinat of", mat
n=len(mat)
nom = 1.
if n == 1: return mat[0][0]
lastr = mat.pop()
jx=-1
for j in xrange(n):
if lastr[j]:
jx=j
break
if jx==-1: return 0.
result = lastr[jx]
assert(result<>0.)
# Make column jx zero by subtracting a multiple of the last row.
for i in xrange(n-1):
pivot = mat[i][jx]
if 0. == pivot: continue
assert(result<>0.)
nom *= result # Compenstate for multiplying a row.
for j in xrange(n):
mat[i][j] *= result
for j in xrange(n):
mat[i][j] -= pivot*lastr[j]
# Remove colunm jx
for i in xrange(n-1):
x= mat[i].pop(jx)
assert( x==0 )
if (n-1+jx)%2<>0: result = -result
det = determinant( mat )
assert(nom<>0.)
return result*det/nom
/-----------------------------------------
Now on some matrices the assert triggers, meaning that nom is zero.
How can that ever happen? mon start out as 1. and gets multiplied
with a number that is asserted to be not zero.
Any hints appreciated.
Groetjes Albert
I know it's not the question, but if you want a replacement for octave
did you try numpy (and scipy) ? The determinant would be computer faster
and with less memory than with your function.
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