Re: Re: Re: Is this a bug?

*To*: mathgroup at smc.vnet.net*Subject*: [mg13473] Re: [mg13445] Re: Re: Is this a bug?*From*: Carl Woll <carlw at fermi.phys.washington.edu>*Date*: Sun, 26 Jul 1998 02:33:34 -0400*Sender*: owner-wri-mathgroup at wolfram.com

Hi, I still don't understand why you need to wrap your ToExpression with Evaluate. What's wrong with If[ test, ToExpression["valid statement"], ToExpression["invalid statement"]] Carl Woll Dept of Physics U of Washington On Fri, 24 Jul 1998 jfreeze at lexmark.com wrote: > 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: [mg13473] [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 > > > > >