using python 3.5 code in python 2.7 gives different answer

Question

a=[[2,5],[1,3]]
b=[5,1]
r=len(a)
x=[1,1]

def func():
    tol=.00000000001;l=0;ite=10000
    fnd=0
    while(l<ite and fnd==0):
        j=0
        while(j<r):
            temp=b[j]
            k=0
            while(k<r):
                if (j!=k):
                    temp=temp-a[j][k]*x[k]
                k=k+1
            c=temp/a[j][j]
            if (abs(x[j]-c)<=tol):
                fnd=1
                break
            x[j]=c
            j=j+1
        l=l+1
        if l==ite:
            print ("Iterations are over")
    print(x)

func()

running this code in python 3.5 gives correct answer [9.999999999858039, -2.9999999999432156] but gives answer [7,-2] in python 2.7. Why? This is implementation of Gauss-Seidel method to find solution of system of equations using matrix.

try putting at the top of the 2.7 source file this: from __future__ import division and see how that goes. — Keith
– Keith, Commented Nov 28, 2015 at 8:12

Mureinik · Accepted Answer · 2015-11-28 08:02:35Z

3

The difference is in the way python 2 and python 3 handle the / operator in the line c=temp/a[j][j]. In python 2, / performs integer division, while in python 3 it performs floating point division.

Converting temp to a float before the division (i..e, c=float(temp)/a[j][j]) will ensure you get the right answer in both versions.

answered Nov 28, 2015 at 8:02

Mureinik

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