Re: How to tell Mathematica to stop conditional testing in an If statment if one condition is niether True or False? McCarthy evaluation rules? 'and then' test?

*To*: mathgroup at smc.vnet.net*Subject*: [mg69586] Re: How to tell Mathematica to stop conditional testing in an If statment if one condition is niether True or False? McCarthy evaluation rules? 'and then' test?*From*: dimmechan at yahoo.com*Date*: Sat, 16 Sep 2006 03:50:29 -0400 (EDT)*References*: <eebfu3$5dt$1@smc.vnet.net>

***SameQ works as desired if b is symbolic or numeric with a value different from 5. Indeed In[102]:=Quit In[4]:=xc=Table[i,{i,1,3}] x=5; If[x===b&&xc[[10]]\[Equal]4,Print["True"],Print["False"],Print["Can't decide"]] Out[4]={1,2,3} >From In[4]:= False ***and In[7]:=xc=Table[i,{i,1,3}] x=5; If[x===(b /. b -> 5))&&xc[[10]]\[Equal]4,Print["True"], Print["False"],Print["Can't decide"]] Out[7]={1,2,3} >From In[7]:= False But if you assign the value 5 to b you get again the same error message. In[10]:=xc=Table[i,{i,1,3}] x=5; If[x===(b/.b\[Rule]5)&&xc[[10]]\[Equal]4, Print["True"],Print["False"],Print["Can't decide"]] Out[10]={1,2,3} >From In[10]:= Part::partw: Part 10 of {1, 2, 3} does not exist. More... >From In[10]:= Can't decide ***But you can overcome this problem as it is shown below: In[17]:=b=. In[18]:= xc=Table[i,{i,1,3}] x=5;b=5; If[b\[Equal]5,Print["False"],If[x===b&& xc[[10]]\[Equal]4,Print["True"],Print["False"],Print["Can't decide"]]] Out[18]={1,2,3} >From In[18]:= False In[19]:= xc=Table[i,{i,1,3}] x=5;b=8; If[b\[Equal]5,Print[" False"],If[x===b&&xc[[10]]\[Equal]4,Print["True"], Print["False"],Print["Can't decide"]]] Out[19]={1,2,3} >From In[19]:= False In[20]:= xc=Table[i,{i,1,3}] x=5;b=a; If[b\[Equal]5,Print[" False"],If[x===b&&c[[10]]\[Equal]4, Print["True"],Print["False"]],Print["Can't decide"]] Out[133]={1,2,3} >From In[133]:= Can't decide Dimitris Anagnostou Nasser Abbasi wrote: > I can better describe this with simple example: > > ------------- code ------------ > > Remove["Global`*"]; > xc = Table[i, {i, 1, 3}] > x = 5; > If[x == b && xc[[10]] == 4, Print["True"], Print["False"], Print["Can't > decide"]] > > ----- end code ------------- > In the above, 'x==b' is neither True nor False, since 'b' has no numerical > value. > > But what I want is when this happens, for Mathematica to NOT continue with > the testing if xc[[10]]==4 is True (because even if it is True, it will > not change the outcome, which is can't decide. > > I am looking for something like 'and then' which says to test the next > condition only if the one just tested was true. > > The interesting thing is that if 'b' had a value, say 7, which makes the > first test (the x==b) to be False, then Mathematica does the right thing, > and will not try to check the xc[[10]]==4 condition. I need it to do the > same thing when also the result of the check is 'undecided', not just > 'False' or 'True'. > > Is there a way to do this? Notice in the example above, I get the error > that xc[[10]] is out of bound, but still get the can't decide message. > > It is clear to me that the way Mathematica does it now is not the right way. > I do not see why it tries to check for xc[[10]]==4 when it will not make a > difference to the final result. > > any thoughts? > > thanks, > Nasser