Re: RandomReal gets stuck

*To*: mathgroup at smc.vnet.net*Subject*: [mg100382] Re: RandomReal gets stuck*From*: Bas Straatman <bastraat at ucalgary.ca>*Date*: Tue, 2 Jun 2009 06:44:46 -0400 (EDT)*Organization*: The University of Calgary*References*: <h00d18$pis$1@smc.vnet.net>

By the way, Bill, there is nothing wrong with the use of square brackets to index a variable. Check out the array function: In[1]:= Array[y, 3] Out[1]= {y[1], y[2], y[3]} In[2]:= Do[y[i] = 3 - i, {i, 3}] In[3]:= ?y Global`y y[1]=2 y[2]=1 y[3]=0 In[4]:= Head[y] Out[4]= Symbol In[5]:= Head[y[1]] Out[5]= Integer Bas Bill Rowe wrote: > 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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**Re: RandomReal gets stuck**

**Re: RandomReal gets stuck**