spectral_norm_python.py

# The Computer Language Benchmarks Game
# http://shootout.alioth.debian.org/
#
# Contributed by Sebastien Loisel
# Fixed by Isaac Gouy
# Sped up by Josh Goldfoot
# Dirtily sped up by Simon Descarpentries
# Used list comprehension by Vadim Zelenin

from math      import sqrt
from itertools import izip
from sys       import argv


def eval_A(i, j):
    ij = i+j
    return 1.0 / (ij * (ij + 1) / 2 + i + 1)

def eval_A_times_u(u):
    local_eval_A = eval_A

    return [ sum([ local_eval_A(i, j) * u_j
                   for j, u_j in enumerate(u)
                 ]
                )
             for i in xrange(len(u))
           ]

def eval_At_times_u(u):
    local_eval_A = eval_A

    return [ sum([ local_eval_A(j, i) * u_j
                   for j, u_j in enumerate(u)
                 ]
                )
             for i in xrange(len(u))
           ]

def eval_AtA_times_u(u):
    return eval_At_times_u(eval_A_times_u(u))

def main():
    n = int(argv[1])
    u = [1] * n
    local_eval_AtA_times_u = eval_AtA_times_u

    for dummy in xrange(10):
        v = local_eval_AtA_times_u(u)
        u = local_eval_AtA_times_u(v)

    vBv = vv = 0

    for ue, ve in izip(u, v):
        vBv += ue * ve
        vv  += ve * ve

    print "%0.9f" % (sqrt(vBv/vv))

main()