RE: RE: Re: Is it possible to access internal variables?

*To*: mathgroup at smc.vnet.net*Subject*: [mg34786] RE: [mg34749] RE: [mg34709] Re: [mg34705] Is it possible to access internal variables?*From*: "Wolf, Hartmut" <Hartmut.Wolf at t-systems.com>*Date*: Thu, 6 Jun 2002 01:55:14 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

Bobby, yes, this can be read at Allan's post I cited. Indeed we can do arbitrary funny things In[1]:= (pr =.; {Print[pr], pr = Print, pr[2]}; Evaluate[{Hold[Print[1]], pr[3]}] )//ReleaseHold >From In[1]:= pr >From In[1]:= 2 >From In[1]:= 3 >From In[1]:= 1 Out[1]= {Null, Null} In[2]:= In[1] >From In[2]:= 3 >From In[2]:= pr >From In[2]:= 2 >From In[2]:= 1 Out[2]= {Null, Null} -- Hartmut > -----Original Message----- > From: DrBob [mailto:majort at cox-internet.com] To: mathgroup at smc.vnet.net > Sent: Wednesday, June 05, 2002 4:26 PM > Cc: 'David Park'; BobHanlon at aol.com > Subject: [mg34786] RE: [mg34749] RE: [mg34709] Re: [mg34705] Is it possible to > access internal variables? > > > >>The expressions e1, e2, up to en are executed in order > > No, unfortunately. > > (If we can't say what the rules are, we're permanently in trouble.) > > pr=.; > {Print[pr],pr=Print,pr[2]};Evaluate[{Print[1],pr[3]}] > > 1 > pr > 2 > 3 > {Null,Null} > > "Evaluate" is taken out of sequence so that "1" is printed first. > > I can adjust to this if I know in advance which statements > will be taken > out of order. Is Evaluate the only case? > > For now, I'll remember that I don't want to enclose in > Evaluate anything > that might have side-effects. Even if I WANT those side-effects to > occur first in a compound expression, it's smarter to put them at the > beginning so that they occur when a na=EFve reader would expect it. > > Bobby Treat > > -----Original Message----- > From: Wolf, Hartmut [mailto:Hartmut.Wolf at t-systems.com] To: mathgroup at smc.vnet.net > Sent: Wednesday, June 05, 2002 4:09 AM > Cc: 'David Park'; BobHanlon at aol.com > Subject: [mg34786] RE: [mg34749] RE: [mg34709] Re: [mg34705] Is it possible to > access internal variables? > > Hello Bobby, > Two things: (1) as Hob Hanlon has pointed out, I missed to assign to v > at definiton of h[x_], which of course explains everything. Aside from > being this my standard blunder mixing up Append with AppendTo > in my mind > from my first day on with Mathematica, I did not look close > enough, and > finally: there are no mysteries. Sorry for disturbing you. > (2) What considers CompoundExpression: this is no bug for > shure; instead > we simply have to recognize the precise semantics of (e1; e2; ...; en) > The expressions e1, e2, up to en are executed in order, then > the result > of en is assigned to CompoundExpression[e1, e2, ..., en] and then this > is evaluated again. Normaly, this has no visible effect since the > execution environment of en when evaluated within the > CompoundExpression > is the same as that of the CompoundExpression when it > returns; yet this > is not enough. Here, since FindMinimum fails it returns itself > unevaluated (a standard behaviour), this now becomes the value of > CompoundExpression and such FindMinimum will start anew. > This all conforms to the execution sequence as specified for > Mathematica. We certainly should have internalized this precisely and > permanently (as a morning prayer). However, we are human beings, and > build up our own rules in our heads and take many shortcuts. > It's there > where we often become irritated. We just have concede that > In[1]:= expr1 > In[2]:= expr2 > is *not* the same as > In[1]:= expr1; > expr2 > As I mentioned I've seen a prior thread to this theme with a prominent > contribution of Allan Hayes, e.g. > http://library.wolfram.com/mathgroup/archive/1999/Feb/msg00460.html or > see also my reply to Allan at another thread > http://library.wolfram.com/mathgroup/archive/2000/Jul/msg00115.html . > BTW esp. with Block, things are quite the same, and quite common here, > since the execution > environment within Block almost always differs form that one > outside: we > have to anticipate the transformations to the _result_ of > Block, as here > to the _result_ of CompoundExpression. > -- > Hartmut > > --snipped--