Possibly better loop construct, also labels+goto important and on the fly compiler idea.

[Steven D'Aprano]

On Tue, 29 Oct 2013 12:37:36 +0100, Skybuck Flying wrote:

> To put the exit condition at the bottom is logical.
> 
> The exit condition glues the loop to the code that will be executed next
> which is also at the bottom.

Skybuck, please excuse my question, but have you ever done any 
programming at all? You don't seem to have any experience with actual 
programming languages.

In a while loop, such as in Python, the test is at the top of the loop 
because the test is performed at the start of the loop, not the end:

while x > 0:
    do_this()
    do_that()


It would be inappropriate (as well as ugly!) to put the test at the end 
of the loop, like this:

x = 0
while:
    do_this()
    do_that()
    many more lines go here
    possibly even pages of code
    until finally, at long last
    you get to the end where you find the test...
x > 0

... and discover whether or not the while loop was actually entered or 
not. Similarly with for-loops, the loop condition is at the beginning 
because it runs at the beginning. This would be silly:

for i:
    body of loop goes here
    could be many many lines of code
    even pages of code
    but eventually, at long last
    we get to the end, and find out
    what the first value for i will be
in range(100, 110)


There is one sort of loop where it makes sense to have the loop condition 
at the end. Python doesn't have such a loop, but Pascal does: the repeat 
until loop. Unlike while, repeat until is always executed at least once, 
so the loop condition isn't tested until the end of the loop:

repeat
    do this
    do that
    do something else
until x > 0



[...]
> LoopBegin( Step = 10 )
> 
>     if ButtonExists then
>     begin
>         ClickButton()
>     end;
> 
> LoopEnd( ButtonClicked )
> 
> Execute next code...
> 
> This loop waits for the button to appear, once it's found it is clicked
> and then the loop exits to continue the next code.

What if the button has already appeared before the loop starts?

I think that is better written as:


# Check the loop condition at the start
while the button does not exist:
    wait a bit
# Outside the loop
click the button

(Although such a loop is called a "busy wait" loop, since it keeps the 
computer being busy without doing anything useful. There are better ways 
to do this than a loop.)

Even in Pascal, I would use a while loop rather than repeat, but if you 
insist on using repeat, clicking the button still should go on the 
outside of the loop:

# this is wasteful, since even if the button exists, the loop still 
# waits a bit, for no good reason
repeat
     wait a bit
until the button exists
click the button



> Putting this exit condition on the top makes no sense.

Wait until you actually start programming before deciding what makes 
sense or doesn't.



-- 
Steven