Friday Finking: Contorted loops

[Avi Gross]

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


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