From help-octave-request at bevo dot che dot wisc dot edu  Mon Jan 31 14:04:23 2000
Subject: Re: efficiency
From: etienne grossmann <etienne at anonimo dot isr dot ist dot utl dot pt>
To: help-octave at bevo dot che dot wisc dot edu, Daniel.Heiserer@bmw.de
CC: etienne at isr dot ist dot utl dot pt
Date: Mon, 31 Jan 2000 17:44:25 +0000 (WET)


  Hello,


#  Assume I have a few hundred matrices:
#  k_0004711
#  k_0004712
#  .........
#  And I want to loop through a subset of them
#  with a certain function:
#  like:

#   a=myfunc(b,c,K__00047[1-5])   # in shell regexps .... ;-)
#          call for each matrix

#  What is better/more efficient:

#  1) copying the data via a temporary variable
#  for jj=1:5
#          tmp=eval(sprintf('k_%7d',47*1000+jj));
#          a=myfunc(b,c,tmp);
#  end
#  % Which probably means copying data in core each time .......
#  % for big loops not nice .......
#  or 

   What about 

jj=1:5; 
eval(sprintf("a = myfunc(b,c,k_%7d);",47*1000+jj)) ;

  ? There is a single eval, and not much code around it. Timing seems
improved, at least in the simplified case below :


octave:7> mytic(); eval(sprintf("k%04d=eye(3);",0:1000)) ; mytic()
ans = 0.29000
octave:8> mytic(); for i=1:1000, eval(sprintf("k%04d=eye(3);",i)) ; end ; mytic()
ans = 0.66000

 where mytic() returns the time since last call to mytic().

#  3) put all the crap in ONE Monster array, store the indices somewhere
#  and do it like this and making all the code realy ugly .......

#  a=myfunc(a,b,K_monster,K_indices);
#  % is here any data copied?
#  % and does it only make the code unreadible

  Code make the could more complicated indeed.

  Cheers,

  Etienne



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