Question

Here is my sample code. It is meant to be an iterative procedure for gauss seidel (matrix solver). Essentially when the error is small enough it breaks out of the while loop.

i=1 while (i>0):     x_past = x_present      j=0     while(j<3):         value=0         k=0         while(k<3):             if(k!=j):                 if(i==1):                     if(k>j):                         value=value+0                     else:                         value=value+x_present[k]*eqn[j][k]                     else:                     value=value+x_present[k]*eqn[j][k]             else:                 value=value+eqn[j][k]             k=k+1         x_present[j:j+1]=[value]         j=j+1     print 'X_PAST'     print x_past     print 'X_PRESENT'     print x_present         if(error(x_past, x_present)<10**-2):         break;     i=i+1 

I’ve reduced the code so its more manageable. if you don’t understand what its doing its not really that important to solving this problem.

Here is the issue. Everytime

x_present[j:j+1]=[value] 

is run, x_past is made equal to x_present. I don’t know why this is the case as the only place i have set x_past equal to x_present is at the top of the loop. If I take away the

x_past=x_present 

sentence, x_past is never made equal to x_present. This leads me to think it is some combination of the two statements that is causing the issue.

This is a big problem cause if x_past = x_present the error = 0 every time and the loop breaks after the first iteration. The code does work, for example if I tell the code to run for 8 iterations and the break it gives me the answer its supposed to.

I’ve been trying to figure this out for the last 4 hours and am completely stumped. I haven’t been working with python long, so my trouble shooting skills syntax wise aren’t that great. Any help would be appreciated!!

Answer 1

Yes, I think the answers here show your problem. Just to try and clarify a little bit.

You’re referencing a list, so when the list changes any reference to that list will reflect that change. To demonstrate:

>>> x_present = [4,5,6] >>> >>> x_past = x_present >>> >>> x_past [4, 5, 6] >>> >>> x_present.append(7) >>> >>> x_past [4, 5, 6, 7] >>> 

If you want a copy of the list you have to do do this, listcopy = mylist[:]. (or import copy;listcopy = copy.copy(mylist)

>>> x_past = x_present[:] >>> x_past [4, 5, 6, 7] >>> >>> x_present.append(8) >>> >>> x_past [4, 5, 6, 7]