Question

Python: Newston-Raphson
I am getting too many iterations and my answer is off in the millions decimal place. Any ideas?

Answer 1

poly=(-13.39,0.0,17.5,3.0,1.0)
x=0.1
epsilon=0.001

def evaluate_poly(poly, x):
#evaluates a polynomial input as a tuple for a specific x value
value=0
power=0
for i in poly:
value+=i*x**power
power+=1
return value

def compute_derivative(poly):
#returns a tuple representing the derivative
#of the input polynomial as a tuple
power=0
deriv=()
for i in poly:
deriv+=(i*power,)
power+=1
return deriv

def compute_root(poly, x, epsilon):
#find the root of the polynomial poly using
#Newton-Raphson method
iterations=0
deriv=compute_derivative(poly)
derivValue=evaluate_poly(deriv, x)
#evaluates the current x value of the derivative of the polynomial
value=evaluate_poly(poly, x)
while abs(value) > epsilon:
value=evaluate_poly(poly, x)
derivValue=evaluate_poly(deriv, x)
x-=value/derivValue
iterations+=1
return x, iterations
#outputs a the root and number of iterations as a tuple

I get an output of (0.80679329312..., 139)
the answer is supposed to be (0.806790679..., 8)

My answer for the root is darn close, but why so am I getting so many iterations?
I tried debugging it but all the numbers seem normal on every iteration, I'm just not getting there fast enough somehow...

Answer 2

my answer is within epsilon of the given answer, so I'm really just worried about the iterations

Answer 3

A million decimal places.....? or the millionths place? :P

Answer 4

The millionth place, actually I had the wrong value for epsilon, it should have been 0.0001
with that correction I get (0.8067887806273476, 141) and the given answer is (0.80679075379635201, 8)

Since my answer and the given answer differ by a number less than epsilon=0.0001 the answer is not what I'm worried about, it's the fact that my algorithm uses 141 iterations when it's only supposed to use 8 !

I'm going to do this painfully on paper and see if I can debug this to find which variable(s) is/are misbehaving.

Answer 5

is # a comment?

Answer 6

yes
and never mind, problem solved!

Answer 7

if the problem is solved could u pls giv me the soln??