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.
Today we have LU decomposition. That is a factorization of a matrix into a lower (L) and upper (U) matrix. Basically it stores de steps of a Gauss elimination in matrices.
Following are the codes
from __future__ import division, print_function import numpy as np def LU(mat): m, _ = mat.shape mat = mat.copy() for col in range(0, m - 1): for row in range(col + 1, m): if mat[row, col] != 0.0: lam = mat[row, col]/mat[col, col] mat[row, col + 1:m] = mat[row, col + 1:m] -\ lam * mat[col, col + 1:m] mat[row, col] = lam return mat A = np.array([ [1, 1, 0, 3], [2, 1, -1, 1], [3, -1, -1, 2], [-1, 2, 3, -1]], dtype=float) B = LU(A)
function LU(mat) m, _ = size(mat) mat = copy(mat) for col = 1:m - 2 for row = col + 1:m if mat[row, col] != 0.0 lam = mat[row, col]/mat[col, col] mat[row, col + 1:m] = mat[row, col + 1:m] - lam * mat[col, col + 1:m] mat[row, col] = lam end end end return mat end A = [1.0 1.0 0.0 3.0; 2.0 1.0 -1.0 1.0; 3.0 -1.0 -1.0 2.0; -1.0 2.0 3.0 -1.0] B = LU(A)
As an example we have the matrix
And, the answer of both codes is
Regarding number of lines we have: 23 in Python and 22 in Julia. The comparison in execution time is done with %timeit magic command in IPython and @benchmark in Julia.
BenchmarkTools.Trial: memory estimate: 29.25 KiB allocs estimate: 310 -------------- minimum time: 9.993 μs (0.00% GC) median time: 11.725 μs (0.00% GC) mean time: 14.943 μs (15.90% GC) maximum time: 3.285 ms (95.64% GC) -------------- samples: 10000 evals/sample: 1
In this case, we can say that the Python code is roughly 30 times slower than Julia code.