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MathGroup Archive 1998

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Re: Re: Is this a bug?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13445] Re: Re: Is this a bug?
  • From: jfreeze at lexmark.com
  • Date: Fri, 24 Jul 1998 01:45:33 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Thanks for all of the responses. My original question was (basically)
why does
If[True, Evaluate[x=5], Evaluate[y=5]] give
{x,y} as {5,5}.
It was explained by several people that the reason is that Evaluate
overides HoldRest.
Nevertheless, it seems to me that expressions not returned from If
should not be executed,
regardless if Evaluate is used.

One responder suggested that it was returning what I asked it to. Well,
no, I did not ask
it to execute a false expression. Consider the C statement if(condition)
{
     // do if true
}
else
{
     // do if false
}
Here, no matter what assignments to variables are made in "false", they
are not
made if condition == true. I guess some will say that here lies my real
sin, thinking
in C.   : )

Well, we could start a real good thread here about the
evaluation/execution of
all expressions in If regardless of the value of condition, which I am
not opposed to,
but, being a pragmatist at the moment, I am looking for a solution to
the following
(seeing that If will not work now):

If[ test, Evaluate[ToExpression["valid statement"]],
Evaluate[ToExpression["invalid statement"]]]

What this statement does it to test for validity of a string before
converting
it to an expression.
Since If executes all its arguments, regardless of test, can someone
suggest a
method accomplish this task?
Thanks
Jim Freeze
jfreeze at lexmark.com



"Allan Hayes" <hay%haystack.demon.cc.uk at interlock.lexmark.com> on
07/23/98 07:18:20 AM

Please respond to "Allan Hayes"
      <hay%haystack.demon.co.uk at interlock.lexmark.com>

cc:
bcc:
Subject: [mg13445]  Re: Is this a bug?



jfreeze at lexmark.com wrote in message <6p6oa9$56m at smc.vnet.net>...
>
>The following seems to be a bug. In the first example, both the true and
>the false parts of If are being evaluated. In the secon example, only
>the true part. If this is not a bug, could someone please explain this
>behavior to me. Thanks
>
>In[1]:=
>     Clear[x,y];
>
>If[True,Evaluate[ToExpression["x=5"]],Evaluate[ToExpression["y=5"]]];
>     Print[x,y];
>Out[1]:=
>     55
>In[2]:=
>     Clear[x,y];
>     If[True,x=5,y=5];
>     Print[x,y];
>Out[2]:=
>     5y
>

This is not a bug.
To try and explain the evaluation I will modify your example slightly to

(1)
In[1]:=
    Clear[x,y];

In[2]:=
    If[True,x=5,y=7]
Out[2]=
    5

In[3]:=
    {x,y}
Out[3]=
    {5,y}

(2)
In[4]:=
    Clear[x,y];

In[5]:=
    If[True,Evaluate[x=5],Evaluate[y=7]] Out[5]=
    5

In[6]:=
    {x,y}
Out[6]=
    {5,7}

Notice that the If has the attrribute HoldRest

Here is what happens

(1) For If[True,x=5,y=7]
The *attribute* Hold Rest for If prevents evaluation of x=5 and x=7; so
we get
 If[True,x=5,y=7]
The *rules* for If are now used and give
  x=5
This is evaluated to ouput 5 and the rule x = 5 is stored Because of the
stored rule {x,y} gives {5, y}

(2) For If[True,Evaluate[x=5],Evaluate[x=7]] The *attribute* Hold Rest
is overidden at  Evaluate[x=5] which evaluates to 5 and stores x =5;
similarly for Evaluate[y=7]; so we get
 If[True, 5 , 7 ] and the rules x = 5 and y = 7 have been stored The
*rules* for If are now used and give
 5
Because of the stored rule {x,y} gives {5, 7}

You can see thes stages displayed by using TracePrint


Clear[x,y];
If[True,x=5,y=7]//TracePrint


Clear[x,y];
If[True,Evaluate[x=5],Evaluate[y=7]]//TracePrint


------------------------------------------------------------- Allan
Hayes
Training and Consulting
Leicester UK
http://www.haystack.demon.co.uk
hay at haystack.demon.co.uk
voice: +44 (0)116 271 4198
fax: +44(0)116 271 8642






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