Friday Finking: Contorted loops

[jak]

Il 11/09/2021 22:29, Avi Gross ha scritto:
> 
> Alan and others,
> 
> I think human languages used to make computer languages will often cause
> confusion.
> 
> Some languages have an IF .. ELSE construct but also an EITHER ... OR and a
> NEITHER ... NOR and other twists and turns like words that sometimes come
> apart and you end up having to dangle a part that was in the front of a word
> to later in the sentence and so on.
> 
> But I suspect many languages do NOT naturally have a construct like:
> 
> WHILE ... ELSE.
> 
> The might have a sentence like "While it is sunny you should use sunscreen
> but when it rains use an umbrella." It probably is even a tad deceptive to
> use WHILE in one part and not in the other. Perfectly valid sentences are
> "When going outside if it is sunny use sunscreen but if it is rainy use an
> umbrella" or skip the while and use a more standard if/else. The world
> "while" just does not feel like a partner for "else".
> 
> So say you want to have a loop starting with WHILE and FOLLOWED by a single
> ELSE clause. Arguably you could make WHILE as a construct return a status of
> sorts if it runs at all or perhaps if it exits after at least one iteration
> because the condition evaluates to FALSE. It would either return false if
> you exit with a BREAK or by an error or perhaps not exit at all if you do a
> return from within.
> 
> So if you made up a syntax like:
> 
> IF (WHILE condition {...})
> ELSE {...}
> 
> Then what would that mean? Again, this is a make-believe construct. In the
> above, if WHILE returned a True of some sort, the else is skipped.
> Otherwise, no matter what has been done within the while loop, it is done.
> 
> But as noted we have odd choices here potentially. Could we differentiate
> between a BREAK statement within and something like BREAK OK variant that
> means the while is to be treated as succeeded and please do not do the
> trailing ELSE? I can see many possible ways to design things and cannot
> expect humans to automatically assume the specific nomenclature will be
> meaningful to them.
> 
> There is an alternative that people who are not damn sure what the meaning
> is can do. Create a variable that is set to False or True to represent
> something before the WHILE is entered. Then make sure your code flips that
> value in cased you want to make sure a trailing statement is run. Then
> following the while, you place an IF statement that tests that variable and
> does what the ELSE cluse would have done, or not.
> 
> Looking at other constructs, look at this code with a try:
> 
> i=0
> while i<5:
>      try:
>          assert(i!=3) #Raises an AssertionError if i==3
>          print("i={0}".format(i))
>      except:
>          continue
>      finally:
>          i+= 1; #Increment i
> 
> Now attach an ELSE clause to the WHILE, LOL!
> 
> At some point, van some humans decide just not to write the code this way?
> 
> What  about code that uses CONTINUE to the point where you enter the WHILE
> statement and get a secondary IF or something that keeps triggering a
> CONTINUE to start the next iteration. Arguably, this can effectively mean
> the WHILE loop did nothing. An example would be evaluating the contents of a
> data structure like a list and adding all numeric items together and
> ignoring any that are character strings. Given all characters, no summation
> is done. The first statement in the loop tests a list item and does a
> CONTINUE. But by the rules as I see them, the loop was entered. Yet, a
> similar loop written where the WHILE condition simply tests if ANY item is
> numeric, might drop right through to an ELSE clause.
> 
> Bottom line is humans do not all think alike and language constructs that
> are clear and logical to one may be confusing or mean the opposite to
> others.
> 
> I can even imagine designing an interface like this:
> 
> WHILE (condition):
>      ...
> IF_NOT_RUN:
>      ...
> IF_EXITED_EARLY:
>      ...
> IF_ERROR_THROWN:
>      ...
> ON_PREMATURE_RETURN_DO_THIS:
>      ...
> 
> I am not suggesting we need critters like that, simply that ELSE is a grab
> bag case that can mean many things to many people.
> 
> But if the specific meaning is clearly documented, use it. Lots of people
> who program in languages like Python do not necessarily even speak much
> English and just memorize the keywords.
> 
> We can come up with ever more interesting or even bizarre constructs like
> multiple WHILE in a row with each one being called only if the previous one
> failed to process the data. An example might be if each tests the data type
> and refuses to work on it so the next one in line is called. That could
> perhaps be done by having multiple ELSE statements each with another WHILE.
> But is that an ideal way to do this or perhaps instead use some variant of a
> switch statement or a dictionary pointing to functions to invoke or
> something.
> 
> Time to go do something lese of even minor usefulness!
> 
> -----Original Message-----
> From: Python-list <python-list-bounces+avigross=verizon.net at python.org> On
> Behalf Of Alan Gauld via Python-list
> Sent: Saturday, September 11, 2021 3:59 AM
> To: python-list at python.org
> Subject: Re: Friday Finking: Contorted loops
> 
> On 10/09/2021 19:49, Stefan Ram wrote:
>> Alan Gauld <alan.gauld at yahoo.co.uk> writes:
>>> OK, That's a useful perspective that is at least consistent.
>>> Unfortunately it's not how beginners perceive it
>> ...
>>
>>    Beginners perceive it the way it is explained to them by
>>    their teacher.
> 
> I'm not sure that's true. Most  beginners, in my experience, learn the
> syntax from their teachers and then go off and play.
> What they observe happening is what sticks. And python loop 'else'
> constructs appear inconsistent to them.
> 
> As teachers we like to think we are passing on our wisdom to our students
> but in reality everyone learns from their own experience. The teachers
> advice is just the starting point. Hopefully, that starting point sends them
> in the right direction but that's the best we can hope for.
> 
> --
> Alan G
> Author of the Learn to Program web site
> http://www.alan-g.me.uk/
> http://www.amazon.com/author/alan_gauld
> Follow my photo-blog on Flickr at:
> http://www.flickr.com/photos/alangauldphotos
> 
> 

An inconsistency that I have been able to notice is this:
someone suggests to remedy the absence of the do-while with:
while True:
      ...
      if condition:
          break
the problem arises if the while has an else of its own because the break
not only blocks the while loop but will also ignore the relative else.