ThomasNicholas ThomasNicholas - 5 months ago 33
Python Question

Finding the determinant of an N x N matrix entered as list of lists - "list index out of range"

I'm trying to write a program to find the determinant of an NxN matrix in Python 3.3 but it returns a "list index out of range" error. There is a debugging print statement in the det function which shows that in the case of the 2 x 2 matrix example given then it works fine for the first minor, but then A is reduced to [[3]] and I can't see what part of my code is changing it at all? I want the det function to leave A unchanged as it works along the first row.

def minor(matrix,i):
"""Returns the Minor M_0i of matrix"""
minor = matrix
del minor[0] #Delete first row
for b in list(range(len(matrix))): #Delete column i
del minor[b][i]
return minor

def det(A):
"""Recursive function to find determinant"""
if len(A) == 1: #Base case on which recursion ends
return A[0][0]
else:
determinant = 0
for x in list(range(len(A))): #Iterates along first row finding cofactors
print("A:", A)
determinant += A[0][x] * (-1)**(2+x) * det(minor(A,x)) #Adds successive elements times their cofactors
print("determinant:", determinant)
return determinant

data = [[4, 3], [6, 3]]

print(det(data))

Answer

When you do minor = matrix you aren't creating a copy of the matrix, you're just creating a new reference to it. When you next delete a row from minor you're also deleting the row from matrix.

copy.deepcopy will probably do what you need.