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MathGroup Archive 2001

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Re: Is this a bug?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg32057] Re: [mg32047] Is this a bug?
  • From: Andrzej Kozlowski <andrzej at tuins.ac.jp>
  • Date: Fri, 21 Dec 2001 03:57:26 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

No, there is no bug here. If you think carefully about the evaluation 
chain you will see that k[5] has two be evaluated twice-at least. 
Increment[a[k[5]]  has to compute the value of a[0]=a[k[5]](first 
time) , add one to it  and assign it to a[0]=a[k[5]] (second time) then 
return the old value. The last step does not require computing k[5] 
again because  Increment remembers it. This saves you one evaluation of 
k[5] (and one "hallo world") compared with the following code which 
otherwise does the same thing:

In[22]:=
(a[k[5]]=a[k[5]]+1;a[k[5]]-1)

In[22]:=
hallo world

hallo world

hallo world

Out[22]=
0

You see that this time "hallo world" was printed three times. The 
difference is that Increment remembers the "old value" so it does not 
need to compute it again.

Andrzej Kozlowski
Toyama International University
JAPAN
http://platon.c.u-tokyo.ac.jp/andrzej/

On Thursday, December 20, 2001, at 05:42  PM, Paul van Wamelen wrote:

> Is the following a bug?
>
> Mathematica 4.1 for Sun Solaris
> Copyright 1988-2000 Wolfram Research, Inc.
>  -- Motif graphics initialized --
>
> In[1]:= k[x_] := Module[{},Print["hallo world"];x];
>
> In[2]:= a[5] = 0;
>
> In[3]:= a[k[5]]++
> hallo world
> hallo world
>
> Out[3]= 0
>
>
> I would have expected only one "hallo world" and it would seem to be
> more efficient to only evaluate the k[5] once...
>
> The above example is not important but in the form below it had me
> baffled for a while:
>
> In[4]:= tst[n_] := Module[{a},
>   a[0] = a[1] = 0;
>   Do[a[Random[Integer,{0,1}]]++,{n}];
>   {a[0],a[1]}]
>
> In[5]:= tst[50]
>
> Out[5]= {24, 24}
>
> (Does not add up to 50!)
>
> Thanks!
>
>
>
>
>



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