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MathGroup Archive 1992

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Timing of a simple table operation

  • To: mathgroup at yoda.physics.unc.edu
  • Subject: Timing of a simple table operation
  • From: Jeffrey K. MacKie-Mason <jmm at umich.edu>
  • Date: Fri, 7 Aug 92 13:19:40 EDT

I created a list of numbers with Table, then calculated the product  
of 1 plus each element in the list.  I want to calculate many such  
products, so I nested this inside another Table, but found that it  
took *forever* to calculate.  I can illustrate this but having the  
outer Table create only one product of an inner Table, and then do  
the same thing without the outer Table.  The Table-nested approach  
tool 5 minutes (NeXT '040-25mHz), the piecemeal approach took about  
0.3 seconds!  


I'm pretty new to Mma, and I understand recursion is a tricky beast,  
but since I can create this massive delay when the outer Table is  
only one element long I'm baffled.  Suggestions?

home/jmm.12> math
Mathematica 2.0 for NeXT
Copyright 1988-91 Wolfram Research, Inc.
 -- NeXT graphics initialized -- 


In[1]:= <<Statistics`ContinuousDistributions`

In[2]:= rdist = NormalDistribution[(1.1^(1/240)-1),(0.2/240)]

Out[2]= NormalDistribution[0.000397205, 0.000833333]

In[3]:= annualReturns=Timing[Table[
                (Product[
                        (1+Table[Random[rdist],{240}][[i]]),
                        {i,1,240}
                        ] - 1),
                {1}
                ]]

Out[3]= {312.933 Second, {0.109928}}

In[4]:= y = Timing[Table[Random[rdist],{240}]][[2]]

Out[4]= {-0.000684265,  < list of 240 numbers suppressed> }

In[5]:= Timing[Product[(1+y[[i]]),{i,1,240}]-1]

Out[5]= {0.266667 Second, 0.114892}


So, the nested Tables took 5 minutes, the direct (two-step) approach  
less than 0.3 seconds.

What dumb thing am I doing?





---
Prof. Jeff MacKie-Mason	    	  internet:    jmm at umich.edu
Dept. of Economics                bitnet:   usergc76 at umichum
Univ. of Michigan                 phone:        313-764-7438      
Ann Arbor, MI 48109-1220          fax:          313-763-9181      





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