break/continue - newbe

[Jeff Shannon]

Ann wrote:

>I have trouble sometimes figuring out where
>break and continue go to. Is there some easy
>way to figure it out, or a tool?
>  
>

Break and continue always operate on the most-nested loop that's 
currently executing.  To show an example, let's add some line numbers to 
some code...

1) while spam:
2)     if foo(spam):
3)         continue
4)     for n in range(spam):
5)         if bar(n):
6)             break
7)         results.append(baz(spam, n))
8)     spam = spam - 1

Now, when this loop runs, when foo(spam) evaluates as True we execute 
the continue at line 3.  At this time, we're running code that's at the 
loop-nesting level just inside 'while spam' (line 1), so line 1 is the 
loop statement that's affected by the continue on line 3.  If line 3 is 
triggered, then we skip lines 4-8 and go back to line 1.

If line 3 is *not* triggered, then we enter a for loop at line 4.  
There's a break at line 6; if this gets triggered, then we look 
backwards to find the most-current (i.e. most nested) loop, which is now 
that for loop on line 4.  So we break out of that for loop (which 
comprises lines 4-7), and drop down to line 8.

So, in general, the way to determine how break and continue will affect 
program flow is to look backwards (up) for the most recent loop 
statement; the break/continue will be inside that statement's dependent 
body.  Break will drop you down to the next line after the loop body, 
and continue will bring you back up to the top of the loop body and the 
start of the next loop iteration.

Jeff Shannon
Technician/Programmer
Credit International