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MathGroup Archive 2001

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Re: Is this a bug?

  • To: mathgroup at
  • Subject: [mg32056] Re: [mg32047] Is this a bug?
  • From: Andrzej Kozlowski <andrzej at>
  • Date: Fri, 21 Dec 2001 03:57:24 -0500 (EST)
  • Sender: owner-wri-mathgroup at

No, there is no bug here. If you think carefully about this you'll see 
that k[5] has two be evaluated twice 9at 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 


hallo world

hallo world

hallo world


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

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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