kgotlele...@galmail.co.za wrote:
Hi,
I am trying to write a program in python that solves a system of nonlinear
equations using newton's method. I don't know what I am doing wrong.
Please help
First thing wrong is posting two messages in the mailing list in rapid
succession (7 minutes) with same content and different titles. Not the
best way to get sympathy.
Next: paste your error log, or if there is none, show the results and
explain how they differ from what you wanted. Also show versions of
both Python and the scipy library you're importing.
Third: you have lots of obvious things wrong with the code itself, but
it's usually best to start fixing it where the error message points;
I'm not familiar with scipi, but I'll try to give you a few pointers anyway.
Fourth: you use the "import *" form, which is generally frowned upon
for all but trivial scripts. It pollutes the global namespace, and
possibly masks built-in functions and variables.
Fifth: your names aren't very mnemonic, so somebody not familiar with
the code has to spend longer just guessing what things are for.
Now for specifics in the code:
from scipy import*
x = array([0.0,0.0,0.0])
n=len(x)
tol= 0.00001
N=30
k=1
while k <= N:
Nowhere in this loop do you modify k or N, so it'll never terminate.
def f(x):
Defining a function inside a loop is seldom useful, and not a technique
for beginners. In fact, a beginner should expect all functions to be at
module level (left margin), and all methods to be at class level (one
indent in). Besides, you never call it or the nested function J.
f= zeros((len(x)),float)
It's bad form to re-use the function name as a local within the
function. It works, but probably contributes to your confusion below.
f[0][k]= x[0][k]**2 + x[1][k]-37
f[1][k]=x[0][k]- x[1][k]**2- 5
f[2][k]= x[0][k] + x[1][k]+ x[2][k]- 3
return f[k]
def J(x):
This definition is never reached, so the code is dead.
J= zeros((n,n),float)
for i in range(n):
ei=zeros(n,float)
ei[i]=1.0
J[:i]=(f(x[k]+tol*ei)-f(x[k]))/tol
return J
y[k] = -(J.I)*f[k]
Here, you're misusing J and f. If you're trying to call those
functions, your syntax is way-off. But if you're trying to use the
local variables by those names, realize that the local J is not in scope.
x[k+1]=x[k]+y[k]
You'll never get to these next lines, because the previous while loop
never terminates. Perhaps an indentation problem?
if sqrt(dot(f0,f0)/len(x)) < tol: print x
else:
k=k+1
Not much point in incrementing a loop variable outside the loop.
print 'Too many iterations'
There are other things to ask, such as whether you intended to quit
after printing x. But the fix for that depends on other things you'll
fix first.
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