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MathGroup Archive 1998

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Re: Re: Is this a bug?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13452] Re: [mg13386] Re: Is this a bug?
  • From: Carl Woll <carlw at fermi.phys.washington.edu>
  • Date: Fri, 24 Jul 1998 01:45:38 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

This isn't a bug, it's a result of the way Mathematica processes
expressions. Suppose you have an expression

head[ arg1, arg2 , arg3 ]

Normally, Mathematica will evaluate each of the arguments in turn, and
then evaluate head with the evaluated arguments. Of course, this
doesn't make sense for all functions, so some functions have attributes
which prevent this behavior. Hence, If has the attribute HoldRest,
which says evaluate only the first argument. Thus the normal execution
of the If statement

If[ arg1, arg2, arg3]

will proceed by evaluating arg1, and then evaluating If with arg1
evaluated and arg2 and arg3 unevaluated. Depending on the value of
arg1, If will evaluate arg2 or arg3.

However, there are times when it would be convenient to change the
execution order which Mathematica normally uses, and the command that
does this is Execute. When mathematica decides whether or not to
evaluate arguments it checks the attributes of the head, and it also
checks whether any arguments have the head Evaluate. It then evaluates
both arguments which the attributes allow, and arguments with the head
Evaluate. So, in your example, If has the attribute HoldRest, and both
arguments arg2 and arg3 have the head Evaluate, so all three arguments
are evaluated.

By the way, why did you put Evaluate around the arguments?

Carl Woll
Dept of Physics
U of Washington

On Thu, 23 Jul 1998 jfreeze at lexmark.com wrote:

> 
> 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
> 
> 



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