[racket] clarification for beginners please

Ah, yes, right.



On Fri, Apr 26, 2013 at 4:09 AM, Robby Findler
<robby at eecs.northwestern.edu>wrote:

> Yes, except that we do know something about what happens when it returns
> #t in the "in all other cases" case.
>
> Robby
>
>
> On Thu, Apr 25, 2013 at 9:03 PM, Pierpaolo Bernardi <olopierpa at gmail.com>wrote:
>
>> The traditional way of explaining eq? is that it works when one of the
>> arguments is a symbol or a mutable data structure.  In all other cases the
>> result is implementation dependent.  Is this explanation still valid for
>> racket?
>>
>>
>>
>> On Fri, Apr 26, 2013 at 1:37 AM, Robby Findler <
>> robby at eecs.northwestern.edu> wrote:
>>
>>> I don't think that either of these explanations are really the right way
>>> to think about eq?. The only way to really understand eq? on immutable
>>> values is to understand that it is exposing low-level details of the
>>> implementation to you (ostensibly for performance reasons). That is, if
>>> (eq? a b) returns #true (where a and b are immutable things like the
>>> structs in the previous messages), then that tells you that equal? will
>>> return also return #true. When it returns #false all you know is that
>>> equal? may or may not return #true (ie, you know nothing).
>>>
>>> This may seem like a strange primitive (and it is), but it can be very
>>> useful for performance reasons, since it takes constant time but equal? may
>>> take up to linear time in the size of its inputs.
>>>
>>> When the arguments are mutable objects, then Macros and Danny's
>>> explanations are a good way to start understanding them.
>>>
>>> (After all, if you punch one crab named "Joe", the other one doesn't get
>>> a bruise.)
>>>
>>> Robby
>>>
>>>
>>> On Thu, Apr 25, 2013 at 3:45 PM, Marco Morazan <morazanm at gmail.com>wrote:
>>>
>>>> Joe,
>>>>
>>>> Perhaps a more pedestrian explanation might help. There is a difference
>>>> between two items having the same value and two items being the same item.
>>>>
>>>> equal? tests if its two items have the same value.
>>>>
>>>> eq? tests if two items are the same item.
>>>>
>>>> Using your example:
>>>>
>>>>
>>>> (define A (make-posn 4 5))
>>>> (define B (make-posn (+ 3 1) (- 6 1)))
>>>>
>>>> Both A and B are items (in this case posns) that have the same value.
>>>> Thus, (equals? A B) is true.
>>>>
>>>> A and B, however, are not the same item (they are two different posns
>>>> that happen to have the same value). Thus, (eq? A B) is false.
>>>>
>>>> Consider:
>>>>
>>>> (define C B)
>>>>
>>>> Now, C and B are the same item (the same posn). Thus, (eq? C B) is
>>>> true. As C and A are not the same posn, we have (eq? C A) is false. As you
>>>> would expect (equal? C A) is true (because the have the same value).
>>>>
>>>> eq? returning true suffices to know that equal? returns true (if two
>>>> items are the same item, then clearly they have the same value). equal?
>>>> returning true does not suffice to know what eq? returns (given that two
>>>> different items may or may not have the same value).
>>>>
>>>> This difference is not relevant for most of HtDP, because it is not
>>>> until your reach the material with assignment that you need to
>>>> understand/know if two items are the same item or not. Before the
>>>> assignment material, you are always testing for value equality.
>>>>
>>>> Finally, eqv? is not used in HtDP and my advice would be to ignore its
>>>> existence with beginners.
>>>>
>>>> I hope this helps.
>>>>
>>>> Marco
>>>>
>>>>
>>>>
>>>>
>>>> On Thu, Apr 25, 2013 at 2:31 PM, Ford, Joe <jford at hollandhall.org>wrote:
>>>>
>>>>> I have a group of high school students with a question... can someone
>>>>> please explain to beginner Racket users the differences between these three
>>>>> boolean functions:   eq?   equal?   eqv?
>>>>>
>>>>> We have read the help menu verbage visible from DrRacket, but simply
>>>>> don't understand what it is saying.  Maybe that's lack of context or
>>>>> vocabulary... but we're struggling a bit.  To test simple variations of
>>>>> application, we wrote some simple code (shown below) but don't understand
>>>>> why the results are what they are:
>>>>>
>>>>> (define FOUR "four")
>>>>> (define A (make-posn 4 5))
>>>>> (define B (make-posn (+ 3 1) (- 6 1)))
>>>>> "-------------"
>>>>> (equal?  FOUR  "four")
>>>>> (equal?  4  (+ 1 3))
>>>>> (equal?  4 (posn-x (make-posn 4 3)))
>>>>> (equal? A B)
>>>>> "-------------"
>>>>> (eq?  FOUR  "four")
>>>>> (eq?  4  (+ 1 3))
>>>>> (eq?  4 (posn-x (make-posn 4 3)))
>>>>> (eq? A B)
>>>>> "---------------"
>>>>> (eqv?  FOUR  "four")
>>>>> (eqv?  4  (+ 1 3))
>>>>> (eqv?  4 (posn-x (make-posn 4 3)))
>>>>> (eqv? A B)
>>>>>
>>>>>
>>>>> Why in the world would the above-defined A and B be considered
>>>>> "equal?" but not "eq?" or "eqv?"?
>>>>>
>>>>> --
>>>>> *Joe Ford
>>>>> Technology Resources, Scheduling & Yearbook
>>>>> Holland Hall
>>>>> jford at hollandhall.org
>>>>>
>>>>> *
>>>>>
>>>>>
>>>>> ____________________
>>>>>   Racket Users list:
>>>>>   http://lists.racket-lang.org/users
>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>>
>>>> Cheers,
>>>>
>>>> Marco
>>>>
>>>> Have a´¨)
>>>> ¸.·´¸.·*´¨) ¸.·*¨)
>>>> (¸.·´ (¸.·´ * wonderful day! :)
>>>>
>>>> ____________________
>>>>   Racket Users list:
>>>>   http://lists.racket-lang.org/users
>>>>
>>>>
>>>
>>> ____________________
>>>   Racket Users list:
>>>   http://lists.racket-lang.org/users
>>>
>>>
>>
>
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