Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
1999
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 1999

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Evaluate/Module

  • To: mathgroup at smc.vnet.net
  • Subject: [mg16119] Re: Evaluate/Module
  • From: Paul Abbott <paul at physics.uwa.edu.au>
  • Date: Sat, 27 Feb 1999 03:22:58 -0500
  • Organization: University of Western Australia
  • References: <7atq9s$7kr@smc.vnet.net> <7b32nl$1ss@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Allan Hayes wrote:

> Neither problem invoves Module; both are due to how expressions like
>     e1;e2;e3;e4        (FullForm CompoundExpression[e1,e2, e3,e4]
> evaluate
> 
> Normally, CompoundExpression evaluates its entries in sequence and then
> gives the value of the last one. However, since it has the attribute HoldAll
> this evaluation is under the control of the programmer and if some entries
> are wrapped in Evaluate[  ] they will be evaluated first *and* the the
> resulting expression will then be evaluated as usual
> So, writing  ei* for the value of ei, we get the following steps
> 
> CompoundExpression[e1, Evaluate[e2], Evaluate[e3], e4]
> CompoundExpression[e1, e2*, Evaluate[e3], e4]
> CompoundExpression[e1, e2*, e3*, e4]       (*restart!*)
> CompoundExpression[e1*, e2*, e3*, e4]
> CompoundExpression[e1*, e2**, e3*, e4]
> CompoundExpression[e1*, e2**, e3**, e4]
> CompoundExpression[e1*, e2**, e3**, e4*]
> e4*

I'm not convinced by your explanation.  Consider the following
CompoundExpressions:

In[1]:= a = 1; a = a - 1; Evaluate[a]
Out[1]= 0

In[2]:= a = 1; a = a - 1; a
Out[2]= 0

Now, compare these results with a simple Module:

In[3]:= Module[{a = 1}, a = a - 1; Evaluate[a]]
Out[3]= 1

In[4]:= Module[{a = 1}, a = a - 1; a]
Out[4]= 0

Note that both Module and CompoundExpression have the HoldAll Attribute:

In[5]:= Attributes[Module]
Out[5]= {HoldAll, Protected}

In[6]:= Attributes[CompoundExpression]
Out[6]= {HoldAll, Protected, ReadProtected}

Using Trace does give some idea as to what is going on:

In[7]:= Trace[a = 1; a = a - 1; Evaluate[a]]
Out[7]= {{a, 0}, a = 1; a = a - 1; 0, {a = 1, 1},  
  {{{a, 1}, 1 - 1, 0}, a = 0, 0}, 0}

In[8]:= Trace[a = 1; a = a - 1; a]
Out[8]= {a = 1; a = a - 1; a, {a = 1, 1}, 
  {{{a, 1}, 1 - 1, 0}, a = 0, 0}, {a, 0}, 0}

In[9]:= Trace[Module[{a = 1}, a = a - 1; a]]
Out[9]= {Module[{a = 1}, a = a - 1; a], {a$9 = 1, 1},  
  {a$9 = a$9 - 1; a$9, {{{a$9, 1}, 1 - 1, 0}, a$9 = 0, 0},  
   {a$9, 0}, 0}, 0}

In[10]:= Trace[Module[{a = 1}, a = a - 1; Evaluate[a]]]
Out[10]= {Module[{a = 1}, a = a - 1; Evaluate[a]], {a$10 = 1, 1},  
  {{a$10, 1}, a$10 = a$10 - 1; 1,  
   {{{a$10, 1}, 1 - 1, 0}, a$10 = 0, 0}, 1}, 1}

Cheers,
	Paul

____________________________________________________________________ 
Paul Abbott                                   Phone: +61-8-9380-2734
Department of Physics                           Fax: +61-8-9380-1014
The University of Western Australia           
Nedlands WA  6907                     mailto:paul at physics.uwa.edu.au 
AUSTRALIA                        http://www.physics.uwa.edu.au/~paul

            God IS a weakly left-handed dice player
____________________________________________________________________


  • Prev by Date: Using Utilities`Notation` in a package
  • Next by Date: Re: Reset In[ ] & Out[ ]
  • Previous by thread: Re: Evaluate/Module
  • Next by thread: How can I plot for array variable?