python - Nested list doesn't work properly

Question

import re
def get_number(element):
    re_number = re.match("(d+.?d*)", element)

    if re_number:
        return float(re_number.group(1))
    else:
        return 1.0

def getvalues(equation):
    elements = re.findall("([a-z0-9.]+)", equation)
    return [get_number(element) for element in elements]


eqn = []
eqn_no = int(raw_input("Enter the number of equations: "))

for i in range(eqn_no):
    eqn.append(getvalues(str(raw_input("Enter Equation %d: " % (i+1)))))
print "Main Matrix: "
for i in range((eqn_no)):
    for j in range((eqn_no+1)):
        print "	%f" %(eqn[i][j]),
    print
print
equation=[]
equation=eqn
for k in range((eqn_no-1)):
    for i in range((k+1),eqn_no):
        for j in range((eqn_no+1)):
            if(eqn[i][j]!=0):
                eqn[i][j]=eqn[i][j]-(eqn[k][j]*(equation[i][k]/eqn[k][k]))



    print "Matrix After %d step: " %(k+1)
    for i in range(eqn_no):
            for j in range((eqn_no+1)):
                print "	%f"%(eqn[i][j]),
                equation[i][j]=eqn[i][j];

            print
    print

for input:

25x+5y+z=106.8
64x+8y+z=177.2
144x+12y+z=279.2

output is:

Main Matrix: 
    25.000000   5.000000    1.000000    106.800000
    64.000000   8.000000    1.000000    177.200000
    144.000000  12.000000   1.000000    279.200000

Matrix After 1 step: 
    25.000000   5.000000    1.000000    106.800000
    0.000000    8.000000    1.000000    177.200000
    0.000000    12.000000   1.000000    279.200000

Matrix After 2 step: 
    25.000000   5.000000    1.000000    106.800000
    0.000000    8.000000    1.000000    177.200000
    0.000000    0.000000    1.000000    279.200000

But it should be like

Main Matrix: 
    25.000000   5.000000    1.000000    106.800000
    64.000000   8.000000    1.000000    177.200000
    144.000000  12.000000   1.000000    279.200000

Matrix After 1 step: 
    25.000000   5.000000    1.000000    106.800000
    0.000000    -4.80000    -1.56000    -96.208000
    0.000000    -16.8000    -4.76000    -335.968000

Matrix After 2 step: 
    25.000000   5.000000    1.000000    106.800000
    0.000000    -4.80000    -1.56000    -96.208000
    0.000000    0.000000    0.699999    0.759981

First of all this is a partial code for solving root of n number of equations using Naive Guass elemination method. Does anyone have any idea why the hell on earth is this happening? Why the zero parts are changing and others aren't? I have done this code in c++ and it works there perfectly but here I'm facing many problem. Maybe I'm newbie to python. I'm using python 2.7.....

Answer 1

I think the problem is the assignment equation = eqn. Since eqn is a list, it is a mutable and thus passed as a reference, when you assign a mutable, the variable actually contains a pointer to that object. This means that equation and eqn are the same list.

You should

from copy import deepcopy
equation = deepcopy(eqn)

You need deepcopy instead of copy because you have a list of lists, also the inner list needs to be copied.