[Edu-sig] a short essay about programming

[kirby urner]

On Sat, Apr 21, 2012 at 6:11 AM, John Zelle <john.zelle at wartburg.edu> wrote:
> Kirby,
>
> There are some nice thoughts here that I don't really disagree with, but your code examples don't use the while conditions well. If you put a condition on the loop, there should be no reason to retest the same condition inside the loop. Think of the loop condition as a guard, inside the loop it is true, outside the loop it has become false.
>


Ah now there's a good rule of thumb maybe.

Whatever sentinel condition let you into the loop should not be
retested inside the loop.

That sounds cogent.

A related but somewhat contrary mental picture is:  only let live fish
into the fish tank, but some die once already inside the tank.

Are there cases where you need a preview, aren't ready to exit yet, so
need to re-test inside?

> That suggests the more elegant (in my eyes) way to write your first example as a sort of sentinel loop:
>
> guess = int(input("Guess? "))
> while(guess != secret):  // as long as the user didn't get it, get another guess

Pausing here:  I see this style quite often, of parens crammed right
up against the while keyword.  I

'm always worried when I see that in beginner Python that they're
imagining while is a callable, and their feeding it some kind of
argument.

Same with if:  I'll get if(x>5):

You're an experienced Pythonista though...

>    print("Nope, try again")
>    guess = int(input("Guess? "))
> // Here we know the condition is false
> print("You guessed it")
>
> There's no reason for the re-test of the loop condition to either break or continue.
>

Or is there sometimes?  I'm thinking this is an excellent rule of
thumb of the kind that needs to be broken.

Python in particular is interesting because it tries to stay simple -
economical in providing control over flow.

Other languages gorge themselves on all imaginable kinds of loop
syntax, bulking up their keyword vocabulary in the process sometimes
(e.g. 'until').

I agree your examples look more elegant and economical than mine
featuring a re-test.

> This applies to the second example as well, but a post-loop conditional will still be required to figure out why the loop quit:
>
> allowed = 5
>
> guess = int(input("Guess? "))
> tries = 1
>
> while guess != secret and tries < allowed:   //user gets to try again
>     print("Nope, try again")
>     guess = int(input("Guess? "))
>     tries += 1
>
> if tries <= allowed:
>    print("You guessed it")
> else:
>    print("You've maxed out.")
>

This is where the while statement's option else suite proves useful,
if you want to execute a block precisely because the while condition
has flipped.

I tend to find this useful in conjunction with 'break' though,
exploding my earlier hypothesis that while loops should have only one
entrance and one exit.

while guess != secret:  # front door
    if tries > allowed:
        print("Bummer")
        break  # back door
    tries += 1
    guess = input ("Guess? ")
else:
    print("Congratulations!")

Most public spaces have multiple exits, including a fire escape for
emergencies.  We haven't even touched on try: / except: as a way to
escape a while loop.

Mostly I just want these considerations to rise to the surface and
ruffle the surface awareness of the beginner mind programmer.

This is being a good thread for that.

Kirby