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Re: RandomReal gets stuck

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100350] Re: RandomReal gets stuck
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Mon, 1 Jun 2009 07:11:47 -0400 (EDT)

On 5/31/09 at 6:34 AM, sjoerd.c.devries at gmail.com (Sjoerd C. de
Vries) wrote:

>I can confirm this bug.

>Executing an even simpler version, namely

>Maximize[Plus @@ Table[y[i], {i, 31}], Table[y[i], {i, 31}]];

>will also set the random generator to a fixed starting point. The
>number 31 is crucial, as lower values do not appear to cause the
>bug, while higher values do. Changing Plus to Times appears to
>prevent the bug.

>In[394]:= Table[
>Maximize[Plus @@ Table[y[i], {i, 31}], Table[y[i], {i, 31}]];
>RandomReal[], {20}
>]

When I execute the code above, I do get the same result as you
report. Clearly, there is a problem here somewhere. But it isn't
clear the issue is with RandomReal.

The code above uses Maximize in a non-sensical way, particularly
when the function y hasn't been defined. This usage asks
Mathematica to find the maximize the sum of n things with
respect to each of the n things. It appears there is an attempt
to use the notation y[n] to mean an array indexed by n rather
than a function of n. So, Maximize correctly generates error
messages. What is unexpected is poor input to Maximize appears
to cause a problem for RandomReal.

On my system, changing Plus to Times eliminates the effect on
RandomReal even though there is still a non-sensical input to
Maximize. Alternatively, defining y then executing the code does

In[5]:= y[n_] := n^2 - n

In[6]:= Table[
  Maximize[Plus @@ Table[y[i], {i, 31}], Table[y[i], {i, 31}]];
  RandomReal[], {20}]

During evaluation of In[6]:= Maximize::ivar: 0 is not a valid
variable. >>

During evaluation of In[6]:= Maximize::ivar: 0 is not a valid
variable. >>

During evaluation of In[6]:= Maximize::ivar: 0 is not a valid
variable. >>

During evaluation of In[6]:= General::stop: Further output of
Maximize::ivar will be suppressed during this calculation. >>

Out[6]= {0.877054,0.833103,0.703786,0.0482118,0.226066,0.230746,0.0738072=
,0.680963,0.53264,0.989333,0.418793,0.951114,0.963168,0.870439,0.926361,0.267113,0.195084,0.810066,0.875896,0.579076}

>This is very nasty, as a lot of people critically depend on random
>functions to be random. I would urge you to report this to wolfram
>support.

I agree many people including myself depend on the random
functions to be random. And I would also like Mathematica to
fail gracefully when given non-sensical input and provide useful
error messages. But I am never surprised when software does
something other than fail gracefully given non-sensical input or
provides less than useful error messages.

Entering

Table[
  Maximize[Plus @@ Table[y[i], {i, 31}], Table[y[i], {i, 31}]];
  RandomReal[],
  {20}
  ]

into a new session and expecting useful output simply isn't a
reasonable expectation.

Note, this in no way says what the original poster was doing is
unreasonable or non-sensical. The rest of the information needed
to determine whether what the original poster was attempting is
sensible hasn't been provided. I would also note, that sending a
bug report to Wolfram without the additional information is
probably pointless.



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