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Re: Is this a bug?
*To*: mathgroup at smc.vnet.net
*Subject*: [mg13439] Re: Is this a bug?
*From*: "Allan Hayes" <hay at haystack.demon.cc.uk>
*Date*: Fri, 24 Jul 1998 01:45:29 -0400
*References*: <6p6oa9$56m@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
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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