Breaking out of nested loops
bokr at oz.net
Sun Jan 13 06:53:31 CET 2002
On Sat, 12 Jan 2002 20:18:19 -0500 (EST), Steven Majewski <sdm7g at Virginia.EDU> wrote:
>However, after trying out an example with multilevel nesting,
>I see that, although it works fine, it does appear a bit awkward
>class Break1(Exception): pass
>class Break2(Exception): pass
>class Break3(Exception): pass
>from random import randint
>tab = ' '
> while 1:
> print 'While1'
> try: ##
> while 2:
> print tab,'While2'
> while 3:
> print tab*2,'While3'
> for i in range(30):
> r = randint(0,6)
> print tab*3,'r=',r
> if r == 1: raise Break1
> elif r == 2: raise Break2
> elif r == 3: raise Break3
> except Break3:
> print tab*2,Break3
> except Break2:
> print tab,Break2
> except Break1:
> print Break1
>for test in (1,2,3):
> print 'test',test
>I admit, it's not simple to follow the control thru all of the
>extra try/excepts and indentation. However, since the exceptions
>do work across function boundaries, if you break it up into
>nested functions (that then still raise exceptions) it's easier
>But then, I'm not sure that adding something like named blocks would
>be any more readable. You would still need an extra level of nesting,
>but the lack of an 'except' would make it more difficult to spot
>the point it breaks out to. And having to count levels for a syntax
>like "break 3" would be even worse.
>( Although, I find named blocks in Lisp OK, but you're more free
> to use the indentation to make the flow more clear. )
>That conclusion tends to reinforce my initial reaction to these
>proposals: that the 'problem' is really that deeply nested loops
>in a single procedure is the real 'design error' , not any particular
>syntax. Sometimes it can't be avoided, but I haven't seen a proposal
>for new syntax that is any easier to read than the example above.
How about using a dedent-level marker on the next line after the break,
e.g., a lone ':' ? Then the eye can just drop down to where it's going.
for i in range(30):
r = randint(0,6)
if r == 1: break
elif r == 2: break
elif r == 3: break
for test in (1,2,3):
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