On Fri, Apr 17, 2015 at 7:19 PM, Paddy <paddy3...@gmail.com> wrote: > Having just seen Raymond's talk on Beyond PEP-8 here: > https://www.youtube.com/watch?v=wf-BqAjZb8M, it reminded me of my own recent > post where I am soliciting opinions from non-newbies on the relative > Pythonicity of different versions of a routine that has non-simple array > manipulations. > > The blog post: > http://paddy3118.blogspot.co.uk/2015/04/pythonic-matrix-manipulation.html > > The first, (and original), code sample: > > def cholesky(A): > L = [[0.0] * len(A) for _ in range(len(A))] > for i in range(len(A)): > for j in range(i+1): > s = sum(L[i][k] * L[j][k] for k in range(j)) > L[i][j] = sqrt(A[i][i] - s) if (i == j) else \ > (1.0 / L[j][j] * (A[i][j] - s)) > return L > > > The second equivalent code sample: > > def cholesky2(A): > L = [[0.0] * len(A) for _ in range(len(A))] > for i, (Ai, Li) in enumerate(zip(A, L)): > for j, Lj in enumerate(L[:i+1]): > s = sum(Li[k] * Lj[k] for k in range(j)) > Li[j] = sqrt(Ai[i] - s) if (i == j) else \ > (1.0 / Lj[j] * (Ai[j] - s)) > return L > > > The third: > > def cholesky3(A): > L = [[0.0] * len(A) for _ in range(len(A))] > for i, (Ai, Li) in enumerate(zip(A, L)): > for j, Lj in enumerate(L[:i]): > #s = fsum(Li[k] * Lj[k] for k in range(j)) > s = fsum(Lik * Ljk for Lik, Ljk in zip(Li, Lj[:j])) > Li[j] = (1.0 / Lj[j] * (Ai[j] - s)) > s = fsum(Lik * Lik for Lik in Li[:i]) > Li[i] = sqrt(Ai[i] - s) > return L > > My blog post gives a little more explanation, but I have yet to receive any > comments on relative Pythonicity.
I prefer the first version. You're dealing with mathematical formulas involving matrices here, so subscripting seems appropriate, and enumerating out rows and columns just feels weird to me. That said, I also prefer how the third version pulls the last column of each row out of the inner loop instead of using a verbose conditional expression that you already know will be false for every column except the last one. Do that in the first version, and I think you've got it. -- https://mail.python.org/mailman/listinfo/python-list
