Re: Re: confused about == vs === in this equality
- To: mathgroup at smc.vnet.net
- Subject: [mg103854] Re: [mg103825] Re: confused about == vs === in this equality
- From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
- Date: Fri, 9 Oct 2009 07:17:20 -0400 (EDT)
- References: <20091003104738.LCJ3I.416659.imail@eastrmwml34> <200910040935.FAA07794@smc.vnet.net> <hacmoa$sko$1@smc.vnet.net> <200910081150.HAA13555@smc.vnet.net>
Hi Drago, I have to say I am not convinced about Null (I have not thought about missing). My reason is simply this: in the case of Intermediate it is easy to construct two completely unrelated expressions which will evalute to Intermediate. Having Intermediate==Intermediate evaluate to True would lead to equality between these expressions also evaluating to True. Conceivably this could be "hidden" within some complicated computation and result in a totally misleading final result. On the other hand I don't know of anything but Null that *evaluates" to Null. If you can produce two obviously unrelated expressions both of which evaluate to Null then indeed I will agree with you. As it is I think the current situation is just fine. Andrzej On 8 Oct 2009, at 20:50, Drago Ganic wrote: > Hi Andrzej, > there is one function (Missing) and one symbol (Null) which >>should<< > behave the same as Indetereminate and ComplexInfinity but > unfortunatly does > not. > > In[1]:= Null == Null > Out[1]= True > > In[2]:= Missing[] == Missing[] > Out[2]= True > > Null is Mathematica legacy which has basically the same meaning as > Missing[] > (or maybe Missing["Nonexistent"]). Missing incorporates > Indetereminate via > Missing["Indeterminate"]. > All of those (Indetereminate & ComplexInfinity for numeric data and > Null/Missing for any kind of data), are so called "null values" in > database > systems and for them the Equal and other logical connectivities > (And, Or, > Not, etc.) are overloaded. Unfortunatly this is not the case in > Mathematica. > > Greetings from Croatia, > Drago Ganic > > "Andrzej Kozlowski" <akoz at mimuw.edu.pl> wrote in message > news:hacmoa$sko$1 at smc.vnet.net... >> I amy be taking a bit of a risk here, but I would guess that >> ComplexInfinity and Indeterminate are the only symbols in Mathematica >> with this property, that is we get: >> >> a=ComplexInfinity >> >> TrueQ[Unevaluated[x == x] /. x -> a] >> >> False >> >> a = Indeterminate; >> >> TrueQ[Unevaluated[x == x] /. x -> a] >> >> False >> >> I believe that there are no other symbols for which this happens (?) >> (If I am right and it is the only one that there is no need to be >> seriously concerned or, as you say, "careful" about this issue.) >> >> Why does and Indeterminate and ComplexInfinity behave in this way? Of >> course this is a matter of design and not (for example) mathematics >> so >> the question really is, is this a reasonable and useful thing rather >> than if it is right. I guess it is pretty clear that since >> Indeterminate refers to a magnitude that cannot be determined, you >> would not really want to assert that two expressions, both of which >> evaluate to Indeterminate, are in any sense equal. For example it >> would seem very strange if >> >> Infinity - Infinity == Infinity/Infinity >> >> returned True (as would have to be the case if >> Indeterminate==Indeterminate returned True). Similar considerations >> perhaps apply to ComplexInfinity, which refers to a complex quantity >> with infinite magnitude but with an indeterminate argument. (However, >> I am less convinced of that in the case of ComplexInfinty than in the >> case of Indeterminate, because ComplexInfinity has a natural >> interpretation as a unique point on the Riemann sphere). >> >> (Of course === asks quite a different question and there is no doubt >> that when you have identical expressions on both sides of === the >> answer should always be True.) >> >> Andrzej Kozlowski >> >> >> >> >> >> >> On 4 Oct 2009, at 18:35, Nasser Abbasi wrote: >> >>> ?=== >>> lhs===rhs yields True if the expression lhs is identical to rhs, and >>> yields >>> False otherwise. >>> >>> ?== >>> lhs==rhs returns True if lhs and rhs are identical. >>> >>> But looking at this example: >>> >>> a = ComplexInfinity; >>> If[a == ComplexInfinity, Print["YES"]] >>> >>> Expecting it would print "YES", but it does not. it just returns the >>> whole >>> thing unevaluated? But >>> >>> If[a === ComplexInfinity, Print["YES"]] >>> >>> does return YES. >>> >>> I guess I am a little confused about the "expression" bit in the >>> definition. >>> >>> So, when using the 3"=", it is looking at the _value_ of the >>> expression, but >>> when using the 2"=", it is looking at the expression _as it is_, >>> i.e. >>> without evaluating it? Is this the difference? I've always used >>> the 2"=" >>> for equality, now I have to be more careful which to use. >>> >>> --Nasser >>> >>> >>> __________ Information from ESET NOD32 Antivirus, version of virus >>> signature database 4478 (20091003) __________ >>> >>> The message was checked by ESET NOD32 Antivirus. >>> >>> http://www.eset.com >>> >>> >>> >>> >> >> > >
- References:
- confused about == vs === in this equality
- From: "Nasser Abbasi" <nma@12000.org>
- Re: confused about == vs === in this equality
- From: "Drago Ganic" <dganic@vodatel.net>
- confused about == vs === in this equality