Homework 5: Root finding of polynomials

This homework should test your ability to use the knowledge of benchmarking, profiling and others to improve an existing implementation of root finding methods for polynomials. The provided code is of questionable quality. In spite of the artificial nature, it should simulate a situation in which you may find yourself quite often, as it represents some intermediate step of going from a simple script to something, that starts to resemble a package.

How to submit?

Put the modified root_finding.jl code inside hw.jl. Zip only this file (not its parent folder) and upload it to BRUTE. Your file should not use any dependency other than those already present in the root_finding.jl.

Homework (2 points)

Use profiler on the find_root function to find a piece of unnecessary code, that takes more time than the computation itself. The finding of roots with the polynomial

\[p(x) = (x - 3)(x - 2)(x - 1)x(x + 1)(x + 2)(x + 3) = x^7 - 14x^5 + 49x^3 - 36x\]

should not take more than 50μs when running with the following parameters

atol = 1e-12
maxiter = 100
stepsize = 0.95

x₀ = find_root(p, Bisection(), -5.0, 5.0, maxiter, stepsize, atol)
x₀ = find_root(p, Newton(), -5.0, 5.0, maxiter, stepsize, atol)
x₀ = find_root(p, Secant(), -5.0, 5.0, maxiter, stepsize, atol)

Remove obvious type instabilities in both find_root and step! functions. Each variable with "inferred" type ::Any in @code_warntype will be penalized.

HINTS:

running the function repeatedly 1000x helps in the profiler sampling
focus on parts of the code that may have been used just for debugging purposes

Voluntary exercise

Use Plots.jl to plot the polynomial $p$ on the interval $[-5, 5]$ and visualize the progress/convergence of each method, with a dotted vertical line and a dot on the x-axis for each subsequent root approximation x̃.

HINTS:

plotting scalar function f - plot(r, f), where r is a range of x values at which we evaluate f
updating an existing plot - either plot!(plt, ...) or plot!(...), in the former case the plot lives in variable plt whereas in the latter we modify some implicit global variable
plotting dots - for example with scatter/scatter!
plot([(1.0,2.0), (1.0,3.0)], ls=:dot) will create a dotted line from position (x=1.0,y=2.0) to (x=1.0,y=3.0)