# Numerical methods challenge: Day 4

During October (2017) I will write a program per day for some well-known numerical methods in both Python and Julia. It is intended to be an exercise then don't expect the code to be good enough for real use. Also, I should mention that I have almost no experience with Julia, so it probably won't be idiomatic Julia but more Python-like Julia.

## Newton's method: vector case

Today we have Newton's method one more time. In this case, the function is vector-valued. This implies a slight modification over the original post. The new approximation is computed from the old one using

\begin{equation*} \mathbf{x}_k = \mathbf{x}_{k-1} - J(\mathbf{x}_k)^{-1} \mathbf{F}(\mathbf{x_k}) \end{equation*}

where we need to use the Jacobian matrix $J$.

We will test the method with the function $\mathbf{F}(x, y) = (x + 2y - 2, x^2 + 4x - 4)$ with solution $\mathbf{x} = (0, 1)$.

Following are the codes.

### Python

from __future__ import division, print_function
from numpy import array
from numpy.linalg import solve, norm, det

def newton(fun, jaco, x, niter=50, ftol=1e-12, verbose=False):
msg = "Maximum number of iterations reached."
for cont in range(niter):
J = jaco(x)
f = fun(x)
if det(J) < ftol:
x = None
msg = "Derivative near to zero."
break
if verbose:
print("n: {}, x: {}".format(cont, x))
x = x - solve(J, f)
if norm(f) < ftol:
msg = "Root found with desired accuracy."
break
return x, msg

def fun(x):
return array([x[0] + 2*x[1] - 2, x[0]**2 + 4*x[1]**2 - 4])

def jaco(x):
return array([
[1, 2],
[2*x[0], 8*x[1]]])

print(newton(fun, jaco, [1.0, 10.0]))


### Julia

function newton(fun, jaco, x, niter=50, ftol=1e-12, verbose=false)
msg = "Maximum number of iterations reached."
for cont = 1:niter
J = jaco(x)
f = fun(x)
if det(J) < ftol
x = nothing
msg = "Derivative near to zero."
break
end
if verbose
println("n: $(cont), x:$(x)")
end
x = x - J\f
if norm(f) < ftol
msg = "Root found with desired accuracy."
break
end
end
return x, msg
end

function fun(x)
return [x[1] + 2*x[2] - 2, x[1]^2 + 4*x[2]^2 - 4]
end

function jaco(x)
return [1.0 2.0;
2*x[1] 8*x[2]]
end

println(newton(fun, jaco, [1.0, 10.0]))


### Comparison

Regarding number of lines we have: 31 in Python and 33 in Julia. The comparison in execution time is done with %timeit magic command in IPython and @benchmark in Julia.

For Python:

%timeit newton(fun, jaco, [1.0, 10.0])


with result

1000 loops, best of 3: 284 µs per loop


For Julia:

@benchmark newton(fun, jaco, [1.0, 10.0])


with result

BenchmarkTools.Trial:
memory estimate:  10.44 KiB
allocs estimate:  192
--------------
minimum time:     6.818 μs (0.00% GC)
median time:      7.167 μs (0.00% GC)
mean time:        9.607 μs (16.53% GC)
maximum time:     2.953 ms (97.40% GC)
--------------
samples:          10000
evals/sample:     4


In this case, we can say that the Python code is roughly 40 times slower than the Julia one. This is an improvement compared to the previous examples, where the ratio was around 100. The reason for this "improvement" might be in the inversion of the Jacobian, that calls a numpy routine, doing the weight-lifting for us.