do ... while loop

[skaller]

Joshua Marcus wrote:
> 
> Okay, I'm terribly embarassed.  My example was hasty and poorly chosen.
> I blame the heat of the moment.
> 
> In my initial example, I wanted to assume that one wanted to run the 'process'
> function at least once-- which is why the initial line couldn't be
> "line = fp.readline()".
> 
> But -- the "for line in fp.readlines():" is fine for many situations.
> Let's move away from this example.
> 
> There are, in fact, other situations in which one actually wants a
> "do...while" loop & one is forced to either repeat code or make a meaningless
> initial assignment.  

While this is true, just adding another hacked loop
construct isn't the answer. It will solve some problems,
and leave others. A more general solution is:

	loop: 
		code ..
		if ...: break
		code ..

However, even that isn't general enough, because
many loops have multiple exits. For example,
a search loop: there is no clean way to program
this using C style loops.

So an even more general construction is:

	loop:
		..
		breakto label1
		..
		breakto label2
	label1: ...
	label2: ...


This construction was invented by Knuth, except he put
the handlers at the top of the loop. A python
implementation is:

	try:
		while 1:
			.. raise Label1
			... raise Label1
	except Label1: ..
	except Label2:


-- 
John Skaller, mailto:skaller at maxtal.com.au
1/10 Toxteth Rd Glebe NSW 2037 Australia
homepage: http://www.maxtal.com.au/~skaller
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