[Tutor] Re: you may regret that comment Magnus!!

[Magnus Lycka]

At 18:49 2003-01-30 -0800, Mic Forster wrote:
>--- Magnus Lycka <magnus@thinkware.se> wrote:
> >I have some trouble distinguishing between 1 (one)
> >and l (ell) in the picture. Could you clarify that?
>
>Thanks for your help, Magnus. They are all ones in the
>algorithm; there are no ells.

Ok. Then I imagine it's something like below.

>I thought the algorithm
>was rather elaborate and could be simplified.

I still don't have a clue what it does... :)
But the main loop is only 12 lines of code.

from random import random as rnd

# This only works in newer Python versions...
class growList(list):
     '''A list that does as in Perl: Grows when
     we assign to new indices. I.e. x[1] = 5 will
     work if x was previously empty.'''
     def __setitem__(self, i, val):
         while i >= len(self):
             self.append(None)
         list.__setitem__(self, i, val)
     def __getitem__(self, i):
         'Return 0 for unused slots'
         if i < len(self):
             return list.__getitem__(self, i)
         else:
             return 0

abund = growList()
abund[1] = 1
cumul = growList()
jM = 10
theta = 5

def nsp():
     '''Number of species must be the same as the size of the
     abundence list (I guess). Note that first position is
     unused to match the 1-based algorithm syntax.'''
     return len(abund) - 1

for j in range(2, jM + 1):
     x = rnd()
     if x < (theta / float(theta + j - 1)):
         abund.append(1)
     else:
         cumul[1] = abund[1] / float(j-1)
         for i in range(2, nsp()+1):
             cumul[i] = cumul[i-1] + abund[i] / float(j-1)
         i = 1
         while x >= cumul[i]:
             i += 1
         abund[i] += 1

print "Abund", abund[1:]
print "Cumulative", cumul[1:]


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