Re: Loop until condition is true
Michael Hoffman <[EMAIL PROTECTED]> writes: > Mike Meyer wrote: > >> Making None a constant broke existing code (and I just saw old code >> that assigned to None). Are True and False that much more common as >> variable names than None? > > Yes. In fact, I count at least 4 different modules in the Python 2.4 > standard library that assign to True or False, mainly as a > compatibility measure for the days before they were built-ins. If you > try assigning None, CPython will refuse to compile the module, even if > the code where None is assigned is unreachable. > > If there was ever a good reason to assign to None, I don't know it. For the record, the code I just saw did something like this: (var1, var2, None, None) = make_tuple(args) Pretty clearly, they were just throwing away some unused values. Who knows what it did to None... http://www.mired.org/home/mwm/ Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information. -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Michael Hoffman <[EMAIL PROTECTED]> wrote: > I count at least 4 different modules in the Python 2.4 standard > library that assign to True or False, mainly as a compatibility > measure for the days before they were built-ins. Hopefully, none of them as creative as http://thedailywtf.com/forums/36067/ShowPost.aspx -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Antoon Pardon wrote: > Op 2005-06-22, Michael Hoffman schreef <[EMAIL PROTECTED]>: > >>Remi Villatel wrote: >> >>>Fredrik Lundh wrote: >>> checking if a logical expression is true by comparing it to True is bad style, and comparing values using "is" is also bad style. >>> >>>I wrote it this way because, first, it's perfectly valid Python code and, >>>second and most important, it's also a valid english sentence. >> >>As others have pointed out, it's not perfectly valid code. In fact it is >>frequently incorrect. > > > That the same code would be incorrect in other circumstances doesn't > make it invalid as it was used in a specific situation. > > I have written code my self with an "if var is True" statement and > that is exactly what I wanted. Replacing it with "if var" would > have broken the the program. That point was not lost on me (which is why I said it was frequently incorrect). I think it was lost on the OP though. -- Michael Hoffman -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Mike Meyer wrote: > Making None a constant broke existing code (and I just saw old code > that assigned to None). Are True and False that much more common as > variable names than None? Yes. In fact, I count at least 4 different modules in the Python 2.4 standard library that assign to True or False, mainly as a compatibility measure for the days before they were built-ins. If you try assigning None, CPython will refuse to compile the module, even if the code where None is assigned is unreachable. If there was ever a good reason to assign to None, I don't know it. -- Michael Hoffman -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Mike Meyer wrote: > Making None a constant broke existing code (and I just saw old code > that assigned to None). Are True and False that much more common as > variable names than None? I would think so. I know that my pre-booleans-in-Python code routinely did something like "from booleans import True, False". Of course the fix is easy, but it still must be applied before the code will run. -- Benji York -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Stelios Xanthakis <[EMAIL PROTECTED]> writes: > Michael Hoffman wrote: >> Stelios Xanthakis wrote: >> >>> Magnus Lycka wrote: >> > >> Right. Silly me. Maybe in some future Python version, True and False will be constants, like None is since Python 2.4. >>> >>> >>> Actually, there is support in marshal to write True and False objects so >>> I don't understand why this isn't in 2.4 >> Because it would break existing code. > > Yes. Code that has variables named 'True' and 'False'. Making None a constant broke existing code (and I just saw old code that assigned to None). Are True and False that much more common as variable names than None? http://www.mired.org/home/mwm/ Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information. -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Michael Hoffman wrote: > Stelios Xanthakis wrote: > >> Magnus Lycka wrote: > > > > >>> Right. Silly me. Maybe in some future Python version, True and False >>> will be constants, like None is since Python 2.4. >> >> >> Actually, there is support in marshal to write True and False objects so >> I don't understand why this isn't in 2.4 > > > Because it would break existing code. Yes. Code that has variables named 'True' and 'False'. Stelios -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Op 2005-06-22, Michael Hoffman schreef <[EMAIL PROTECTED]>: > Remi Villatel wrote: >> Fredrik Lundh wrote: >>> checking if a logical expression is true by comparing it to True is bad >>> style, and comparing values using "is" is also bad style. >> >> I wrote it this way because, first, it's perfectly valid Python code and, > > second and most important, it's also a valid english sentence. > > As others have pointed out, it's not perfectly valid code. In fact it is > frequently incorrect. That the same code would be incorrect in other circumstances doesn't make it invalid as it was used in a specific situation. I have written code my self with an "if var is True" statement and that is exactly what I wanted. Replacing it with "if var" would have broken the the program. -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Stelios Xanthakis wrote: > Magnus Lycka wrote: > >> Right. Silly me. Maybe in some future Python version, True and False >> will be constants, like None is since Python 2.4. > > Actually, there is support in marshal to write True and False objects so > I don't understand why this isn't in 2.4 Because it would break existing code. -- Michael Hoffman -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Magnus Lycka wrote: > In some cases, "==" and "is" happens to give the same result. > >>> a = 1 > >>> b = 1 > >>> a == b > 1 > >>> a is b > 1 > > But often not. > > >>> c = 123456789 > >>> d = 123456789 > >>> c == d > 1 > >>> c is d > 0 > ... > First of all, a lot of Python values except 1 (a.k.a. True) > are logically true in a Python expression Actually, True and 1 are interesting examples. >>> a, b = 1, True >>> a == b True >>> a is b False I suspect you simply forgot this fact. --Scott David Daniels [EMAIL PROTECTED] -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
def until(pred): yield None while True: if pred(): break yield None def example(): i = 0 for _ in until(lambda: x==0): x = 10 - i i += 1 print x, i example() pgpeP7iW6mcQm.pgp Description: PGP signature -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Remi Villatel wrote: > Fredrik Lundh wrote: >> checking if a logical expression is true by comparing it to True is bad >> style, and comparing values using "is" is also bad style. > > I wrote it this way because, first, it's perfectly valid Python code and, > second and most important, it's also a valid english sentence. As others have pointed out, it's not perfectly valid code. In fact it is frequently incorrect. > [CENSORED] I keep for myself how stupid I found your post... > Don't try to teach your grandfather how to suck eggs. You apparently don't know how to do it without getting egg on your face. -- Michael Hoffman -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Remi Villatel wrote: > Erm... You totally missed the point. I wrote it this way because, first, > it's perfectly valid Python code and, second and most important, it's > also a valid english sentence. Remi, I think you have failed to understand what Fredrik was telling you. I can understand that, because he didn't actually explain the problem. He probably thought it was obvious, but it seems it wasn't. "if a is b:" is equivalent to "if id(a) == id(b)". It doesn't test whether two values are the equal, it tests whether a and b are the same object. None is a singleton object in Python, but True is not. (At least not in the Python versions I use.) That means that you are relying on Python to use a performance optimization (interning of some common values, in this case the integer 1, which is what True is in practice). In some cases, "==" and "is" happens to give the same result. >>> a = 1 >>> b = 1 >>> a == b 1 >>> a is b 1 But often not. >>> c = 123456789 >>> d = 123456789 >>> c == d 1 >>> c is d 0 Or, for an example of the opposite: >>> class Smaller: ... def __cmp__(self, other): ... return -1 ... >>> s = Smaller() >>> b = a >>> a < b 1 >>> b < a 1 >>> a == b 0 >>> a is b 1 In other words, a test like "if x is None:" makes sense, since None is a singleton, but "if x is True:" certainly seems like buggy code that just happens to work. You could write "if x == True:" without risking a failure in some Python implementation that doesn't intern True (i.e. 1) if you are certain that x is 0 or 1, but it's completely redundant. Why stop there? Why not write "if x == True == True:" or "if x == True == True == True:"? These are all logically equivalent. You could also write "if x and True:", or "if x or not True" :) I understand what your intentions are: You feel that your code is easier to understand written the way you did it, but it stops feeling like that once you have gotten fully into your head that the expressions (a) and (a == True) are logically equivalent if a is either True or False. If you manage to get away from this "x == True" mind-set you are likely to write better Python code. First of all, a lot of Python values except 1 (a.k.a. True) are logically true in a Python expression. It's considered good form to exploit that. E.g. if y: z = x / y if text: print text if l: cols = len(l[0]) In these cases, adding a "==True", would break the code, and adding more "correct" comparisions would just clutter the code, which hides the intent and raises the risk for bugs, and make it more difficult to keep the code as flexible as Python code can be. E.g. if you change the last statement to "if l != []: cols = len(l[0])", you'll get in trouble if it turns out that l = (). The uncluttered code will still work. Secondly, if you ever write logical expressions that are more complex, you will make them much more difficult to read unless you drop that "== True" baggage. I'm convinced that it's easier to understand if past_six or past_three and saturday or sunday: than to understand if (past_six==True or past_three==True and saturday==True or sunday==True): I'm also pretty sure it's easier to write the first without making any mistake. -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
- Original Message - From: "Greg Lindstrom" <[EMAIL PROTECTED]> > A bit off topic, but what does the expression "Don't try to teach your > grandfather how to suck eggs." mean? I've never heard it before and am > curious to the story behind it. A relatively well know phrase, however as quoted it is incorrect, it should have referred to "grandmother" http://www.phrases.org.uk/bulletin_board/18/messages/50.html http://www.worldwidewords.org/qa/qa-tea1.htm :) -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
A bit off topic, but what does the expression "Don't try to teach your grandfather how to suck eggs." mean? I've never heard it before and am curious to the story behind it. Thanks, --greg -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
On Wed, 22 Jun 2005 09:54:44 +0200, Fredrik Lundh wrote: >> [CENSORED] I keep for myself how stupid I found your post. > > so let's see if everyone who understands how embarrassingly stupid > your post is will keep that to themselves... Damn, did I fail some test? -- Steven. -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
> "Stelios" == Stelios Xanthakis <[EMAIL PROTECTED]> writes: Stelios> Anyway, if you can't wait for 2.5 either use 'while 1:', Stelios> or pyc[1] ... and I can't see why people don't want to use 'while 1:' in the first place, given that everyone can identify the idiom immediately. It's 4 keystrokes less. -- Ville Vainio http://tinyurl.com/2prnb -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Remi Villatel wrote: > >>while True: > >> some(code) > >> if final_condition is True: > >> break > >> # > >># > > > checking if a logical expression is true by comparing it to True is bad > > style, and comparing values using "is" is also bad style. > > Erm... You totally missed the point. I wrote it this way because, first, > it's perfectly valid Python code and, second and most important, it's also a > valid english sentence. I suggest looking up "is" in the Python manual instead of looking it up in a dictionary. (hint: it doesn't do what you think it does, and code using your "perfectly valid" pattern doesn't always work.) > [CENSORED] I keep for myself how stupid I found your post. so let's see if everyone who understands how embarrassingly stupid your post is will keep that to themselves... -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Remi Villatel wrote: > Hi there, > > There is always a "nice" way to do things in Python but this time I can't > find one. > > What I'm trying to achieve is a conditionnal loop of which the condition > test would be done at the end so the loop is executed at least once. It's > some way the opposite of "while". > > So far, all I got is: > > while True: > some(code) > if final_condition is True: > break > # > # > > What I don't find so "nice" is to have to build an infinite loop only to > break it. FWIW, my own experience is that the "while True" idiom is actually safer and better than alternatives like do/while. I used to write do/while loops all the time, but I wound up with more than my fair share of unintentionally infinite loops. I put too much trust in the syntax: I expected that since I was using the cleaner construct, I didn't have to worry about infinite loops. Now, I think of "while True" not as an infinite loop, but rather as a sign reminding me to be wary of looping infinitely in a particular spot. I feel like this has resulted in a lot fewer infinite loops in my own code. Now I believe that any loop that can't be represented well with "for" or a conditional "while" has enough inherent complexity to justify a warning sign, and "while True" has bright yellow flashing lights all over it. Thus I'm quite in favor of the status quo. Shane -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
On Wed, 22 Jun 2005 02:01:14 +0200, Remi Villatel wrote: > Fredrik Lundh wrote: > >>>while True: >>> some(code) >>> if final_condition is True: >>> break >>> # >>># > >> checking if a logical expression is true by comparing it to True is bad >> style, and comparing values using "is" is also bad style. > > Erm... You totally missed the point. I wrote it this way because, first, > it's perfectly valid Python code and, second and most important, it's also a > valid english sentence. Why is it more important for Python code to be a valid English sentence than for it to be valid Python code? Does that mean that instead of writing: squares = [x**2 for x in range(10)] you instead write the valid English sentence: the list of squares is given by the set of x squared for x between 0 and 9 inclusive ? The point being, I would have thought that most programmers would have put "being a valid English sentence" right down at the bottom of their list of optional conditions, and not as the "most important" requirement ahead of even being valid code. I also question your understanding of valid English. I don't know about you, but I usually say "if the apple is on the bench..." and hardly ever "if the apple is on the bench is true...". > [CENSORED] I keep for myself how stupid I found your post. Whatever. >> the correct way to write that if-statement is: > > Don't try to teach your grandfather how to suck eggs. It must be nice to have learnt everything that there is to learn about Python. I hope I get there someday. > There is no "correct > way" to code and "superfluous" isn't a synonym of "incorrect". "if final_condition" will always work. "if final_condition is True" will break if final_condition is not a bool. Depending on where final_condition gets its result from, that may or may not be a high risk. It also relies on an implementation detail, that True and False are singletons (doubletons?), and will break if that changes. And finally, since True is just an object, and not a constant like None, it will break if True gets set to some other value. In other words, it is not only superfluous but also fragile. -- Steven. -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Fredrik Lundh wrote: >>while True: >> some(code) >> if final_condition is True: >> break >> # >># > checking if a logical expression is true by comparing it to True is bad > style, and comparing values using "is" is also bad style. Erm... You totally missed the point. I wrote it this way because, first, it's perfectly valid Python code and, second and most important, it's also a valid english sentence. [CENSORED] I keep for myself how stupid I found your post. > the correct way to write that if-statement is: Don't try to teach your grandfather how to suck eggs. There is no "correct way" to code and "superfluous" isn't a synonym of "incorrect". > and yes, the "infinite" while loop is a standard Python pattern. get > used to it. Grandfathers and eggs. Now, excuse me but I have a group of savage AI written in bad style Python to tame. -- == Remi Villatel [EMAIL PROTECTED] == -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Magnus Lycka wrote: > Konstantin Veretennicov wrote: > >> On 6/21/05, Magnus Lycka <[EMAIL PROTECTED]> wrote: >> >>> I don't know anything about the Python compiler internals, >>> but it doesn't seem very hard to identify simple literals following >>> while and if, and to skip the runtime test. (Perhaps it's done >>> already?) >> >> >> >> True doesn't seem to be a literal, it is looked up by name and can be >> rebound to, say, 0 anytime: > > > Right. Silly me. Maybe in some future Python version, True and False > will be constants, like None is since Python 2.4. Actually, there is support in marshal to write True and False objects so I don't understand why this isn't in 2.4 Anyway, if you can't wait for 2.5 either use 'while 1:', or pyc[1] Stelios [1] http://students.ceid.upatras.gr/~sxanth/pyc/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Konstantin Veretennicov wrote: > On 6/21/05, Magnus Lycka <[EMAIL PROTECTED]> wrote: > >>I don't know anything about the Python compiler internals, >>but it doesn't seem very hard to identify simple literals following >>while and if, and to skip the runtime test. (Perhaps it's done >>already?) > > > True doesn't seem to be a literal, it is looked up by name and can be > rebound to, say, 0 anytime: Right. Silly me. Maybe in some future Python version, True and False will be constants, like None is since Python 2.4. -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
On 6/21/05, Magnus Lycka <[EMAIL PROTECTED]> wrote:
> I don't know anything about the Python compiler internals,
> but it doesn't seem very hard to identify simple literals following
> while and if, and to skip the runtime test. (Perhaps it's done
> already?)
True doesn't seem to be a literal, it is looked up by name and can be
rebound to, say, 0 anytime:
>>> import dis
>>> o = compile('while True: pass', '', 'exec')
>>> dis.dis(o)
1 0 SETUP_LOOP 12 (to 15)
>>3 LOAD_NAME0 (True)
6 JUMP_IF_FALSE4 (to 13)
9 POP_TOP
10 JUMP_ABSOLUTE3
...
OTOH,
>>> p = compile('while 1: pass', '', 'exec')
>>> dis.dis(p)
1 0 SETUP_LOOP 5 (to 8)
>>3 JUMP_ABSOLUTE3
...
- kv
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Re: Loop until condition is true
Benji York wrote: > If by "economy" you mean "optimization", then I would suggest that the > difference would be unnoticeable. If there is a measurable performance gain in skipping the runtime test in "while True", then this is a compiler issue, not a language issue. I don't know anything about the Python compiler internals, but it doesn't seem very hard to identify simple literals following while and if, and to skip the runtime test. (Perhaps it's done already?) I hope the time is over when language designers build ugly quirks into programming language syntaxes to make life a little easier for the compilers. -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Remi Villatel wrote: > There is always a "nice" way to do things in Python but this time I can't > find one. > > What I'm trying to achieve is a conditionnal loop of which the condition > test would be done at the end so the loop is executed at least once. It's > some way the opposite of "while". > > So far, all I got is: > > while True: > some(code) > if final_condition is True: > break > # > # > > What I don't find so "nice" is to have to build an infinite loop only to > break it. checking if a logical expression is true by comparing it to True is bad style, and comparing values using "is" is also bad style. the correct way to write that if-statement is: if final_condition: break and yes, the "infinite" while loop is a standard Python pattern. get used to it. -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
On Tue, 21 Jun 2005 12:05:25 +0200, Magnus Lycka <[EMAIL PROTECTED]> wrote: > Remi Villatel wrote: >> while True: >> some(code) >> if final_condition is True: >> break >> # >> # >> >> What I don't find so "nice" is to have to build an infinite loop only to >> break it. > > This is a common Python idiom. I think you will get used to it. > > >> Is there a better recipe? > > final_condition = False > while not final_condition: > some(code) To the OP, don't get hung up on the syntax we use to implement a loop. I took my first programming class long enough ago that we had to do things like this (roughly translating the FORTRAN IV I remember into p-code) To do: while TrueCondition (loop body) we'd have to write: TopOfLoop: if not TrueConditional goto loopEnd (loop body) goto TopOfLoop loopEnd: We were even required to write our source twice: The first pass was "structured" and had to be proven correct. The second pass was translated into something our compiler supported. We still called it a "while loop" even though the syntax was icky. When C came out with its built in looping, it was an unbelievable luxury. -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Cyril BAZIN wrote: > Another question could be: why is there not a statement "whileTrue" or > "loop"? I don't think the saving of a single space to transform "while True:" into "WhileTrue:" is really worth it. The same goes for "loop", the added complexity to the language (as little as it is) isn't rewarded by a large enough improvement to make it worth it. > It could be an economy of one unuseful test by loop. If by "economy" you mean "optimization", then I would suggest that the difference would be unnoticeable. -- Benji York -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Another question could be: why is there not a statement "whileTrue" or "loop"? For exemple: whileTrue: statement 1 if condition: break statement 2 It could be an economy of one unuseful test by loop.On 6/21/05, Magnus Lycka <[EMAIL PROTECTED]> wrote: Remi Villatel wrote:> while True:> some(code)> if final_condition is True: > break> #> #>> What I don't find so "nice" is to have to build an infinite loop only to> break it.This is a common Python idiom. I think you will get used to it. > Is there a better recipe?final_condition = Falsewhile not final_condition: some(code)--http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Remi Villatel wrote: > while True: > some(code) > if final_condition is True: > break > # > # > > What I don't find so "nice" is to have to build an infinite loop only to > break it. This is a common Python idiom. I think you will get used to it. > Is there a better recipe? final_condition = False while not final_condition: some(code) -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Joseph Garvin wrote:
> Peter Otten wrote:
>
>> I found 136 occurrences of "do {" versus 754 of "while (" and 1224 of
>> "for
>> (" in the Python 2.4 source, so using these rough estimates do-while
>> still
>> qualifies as "rarely used".
>>
>> Peter
>>
>>
>>
> That's 136 times you'd have to use an ugly hack instead. I definitely
> wouldn't mind an until or do/while.
Yeah a do while loop was proposed over 2 years ago, but nothing ever
came of it:
http://www.python.org/peps/pep-0315.html
It's been discussed for many many years, and again recently:
http://mail.python.org/pipermail/python-dev/2005-June/054167.html
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See Pep 315. was: Re: Loop until condition is true
See Pep 315, which is still open, and targeted at 2.5. It survived the recent spate of PEP closings and rejections. http://www.python.org/peps/pep-0315.html John Roth "Remi Villatel" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] > Hi there, > > There is always a "nice" way to do things in Python but this time I can't > find one. > > What I'm trying to achieve is a conditionnal loop of which the condition > test would be done at the end so the loop is executed at least once. It's > some way the opposite of "while". > > So far, all I got is: > > while True: > some(code) > if final_condition is True: > break > # > # > > What I don't find so "nice" is to have to build an infinite loop only to > break it. > > Is there a better recipe? > > -- > == > Remi Villatel > [EMAIL PROTECTED] > == -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
Joseph Garvin wrote:
> Peter Otten wrote:
>
>> I found 136 occurrences of "do {" versus 754 of "while (" and 1224 of
>> "for
>> (" in the Python 2.4 source, so using these rough estimates do-while
>> still
>> qualifies as "rarely used".
>>
>> Peter
>>
>>
>>
> That's 136 times you'd have to use an ugly hack instead. I definitely
> wouldn't mind an until or do/while.
I would happy with just having while: without a 1 or True to indicate a
continuous loop. Having a if-break at the end doesn't bother me at all.
while:
if : break
Being able to move the break point to where it's needed or have more
than one is a feature and not a problem. IMHO of course.
It also has the benefit that you have the option to do an extra bit of
cleanup between the if and the break if you need to. The until or
do-while doesn't give you that option.
I suppose if an until : , could be made to be more
efficient and faster than an if : ; break, then I'd
be for that.
while:
until [: suite] # optional suite or block
But I doubt it would be significantly faster than an if statement with a
break. So the only benefit I see is you don't have to use the break
keyword, and the exit conditions will stand out in blocks with a lot of
if statements in them.
Regards, Ron
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Re: Loop until condition is true
Peter Otten wrote:
>I found 136 occurrences of "do {" versus 754 of "while (" and 1224 of "for
>(" in the Python 2.4 source, so using these rough estimates do-while still
>qualifies as "rarely used".
>
>Peter
>
>
>
That's 136 times you'd have to use an ugly hack instead. I definitely
wouldn't mind an until or do/while.
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Re: Loop until condition is true
In article <[EMAIL PROTECTED]>,
Andrea Griffini <[EMAIL PROTECTED]> wrote:
> On Sat, 18 Jun 2005 13:35:16 -, Grant Edwards <[EMAIL PROTECTED]>
> wrote:
>
> >AFAICT, the main use for do/while in C is when you want to
> >define a block of code with local variables as a macro:
>
> When my job was squeezing most out of the CPU (videogame
> industry) I remember that the asm code generated by
>
>while (sz-- > 0)
>{
> /* do some stuff */
>}
>
> was indeed worse than
>
>do
>{
> /* do some stuff */
>} while (--sz);
>
> because of the initial "empty-loop" test (conditional jumps
> were bad, and forward conditional jumps were worse).
> So where at least one iteration was guaranteed the do-while
> loop was a better choice.
Hmm. I don't know what compiler you were using, but in my experience
it's fairly typical to compile while( ...) { ... } as
j test
body:
...
test:
...
je body ;; or whatever your condition is
To turn this into a do { ... } while( ...), you need only
remove the initial "j test" instruction. Even if forward conditional
jumps are bad for your particular architecture, this method avoids the
problem neatly.
Personally, I'd be pretty suspicious of the quality of a compiler that
produced radically different code for these two constructs without some
damned good reason.
-M
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Michael J. Fromberger | Lecturer, Dept. of Computer Science
http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA
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Re: Loop until condition is true
On Sat, 18 Jun 2005 13:35:16 -, Grant Edwards <[EMAIL PROTECTED]>
wrote:
>AFAICT, the main use for do/while in C is when you want to
>define a block of code with local variables as a macro:
When my job was squeezing most out of the CPU (videogame
industry) I remember that the asm code generated by
while (sz-- > 0)
{
/* do some stuff */
}
was indeed worse than
do
{
/* do some stuff */
} while (--sz);
because of the initial "empty-loop" test (conditional jumps
were bad, and forward conditional jumps were worse).
So where at least one iteration was guaranteed the do-while
loop was a better choice.
Also I've been told there were compilers that if using
for or while loops the generated code was
L1:
je L2
jmp L1
L2:
Instead the do-while loop would have been
L1:
jne L1
I.e. the code was better *for each iteration* (one conditional
jump instead of one conditional jump and one inconditional jump).
I think compiler got better since then, even if I don't think
they already so smart to be able to infer the "one interation
guaranteed" property to avoid the initial test that often.
Andrea
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Re: Loop until condition is true
Quoth Peter Otten <[EMAIL PROTECTED]>: ... | 'until' in C is actually | | do | statement | while (expression); Oops. Well, QED - I sure don't need it often. Donn Cave, [EMAIL PROTECTED] -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
- Original Message - From: "Remi Villatel" <[EMAIL PROTECTED]> > There is always a "nice" way to do things in Python but this time I can't > find one. > So far, all I got is: > > while True: > some(code) > if final_condition is True: > break > # > # > > What I don't find so "nice" is to have to build an infinite loop only to > break it. If your final_condition is already defined outside the "while" then how about while not final_condition: some(code) Simplistically, this equates to >>> x = 0 >>> while not x == 5: ... x += 1 ... >>> print x 5 :) -- http://mail.python.org/mailman/listinfo/python-list
Re: Loop until condition is true
On 2005-06-18, Peter Otten <[EMAIL PROTECTED]> wrote:
>> If you look at C code, at least in my experience the
>> "until" loop is quite rarely used. (I don't see it once in the source
>> to Python 2.4, for example.)
>
> Long time no C?
>
> 'until' in C is actually
>
> do
> statement
> while (expression);
>
> I found 136 occurrences of "do {" versus 754 of "while (" and 1224 of "for
> (" in the Python 2.4 source, so using these rough estimates do-while still
> qualifies as "rarely used".
AFAICT, the main use for do/while in C is when you want to
define a block of code with local variables as a macro:
#define DoSomething(foo) \
do \
{ \
int i; \
/* do something with foo and i */ \
} while (0)
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Grant Edwards grante Yow! I HIJACKED a 747 to
at get here!! I hope those
visi.comfabulous CONEHEADS are
at HOME!!
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Re: Loop until condition is true
Donn Cave wrote:
> If you look at C code, at least in my experience the
> "until" loop is quite rarely used. (I don't see it once in the source
> to Python 2.4, for example.)
Long time no C?
'until' in C is actually
do
statement
while (expression);
I found 136 occurrences of "do {" versus 754 of "while (" and 1224 of "for
(" in the Python 2.4 source, so using these rough estimates do-while still
qualifies as "rarely used".
Peter
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Re: Loop until condition is true
Quoth Remi Villatel <[EMAIL PROTECTED]>: | What I'm trying to achieve is a conditionnal loop of which the condition | test would be done at the end so the loop is executed at least once. It's | some way the opposite of "while". | | So far, all I got is: | | while True: | some(code) | if final_condition is True: | break | # | # | | What I don't find so "nice" is to have to build an infinite loop only to | break it. | | Is there a better recipe? It depends, but that isn't too bad. The alternative would be flag variable, like "looping = 1; while looping: ..." The construct you're looking for is "until" in C, and there have been plenty of proposals to add this or other improvements to Python's repertoire. As far as I know, it hasn't happened because it isn't really needed. If you look at C code, at least in my experience the "until" loop is quite rarely used. (I don't see it once in the source to Python 2.4, for example.) Meanwhile, the "while True" (or "while 1") idiom is very familiar to Python programmers (just as the Python source has dozens of "for (;;)"), so it's preferred for legibility. It's nice, don't worry. Donn Cave, [EMAIL PROTECTED] -- http://mail.python.org/mailman/listinfo/python-list
