Re: while loop - multiple condition
On Wed, Oct 15, 2014 at 10:04 AM, giacomo boffi wrote:
> Tim Chase writes:
>
>> On 2014-10-12 22:16, Marko Rauhamaa wrote:
>>> is equivalent with
>>>
>>> while ans.lower()[0] != 'y':
>>> ans = input('Do you like python?')
>>
>> And still better improved with
>>
>> while ans[:1].lower() != 'y':
>> ans = input('Do you like python?')
>
> yok is Turkish for an EMPHATIC NO
> (or, at least, that's what I was led to think many years ago)
Corrupted core, are you ready to start the procedure?
What do you think?
Interpreting vague answer as: Yes!
If your program misinterprets a non-English response to an English
question, that's not critical. It just means you haven't implemented
full i18n. :)
ChrisA
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Re: while loop - multiple condition
Tim Chase writes:
> On 2014-10-12 22:16, Marko Rauhamaa wrote:
>> is equivalent with
>>
>> while ans.lower()[0] != 'y':
>> ans = input('Do you like python?')
>
> And still better improved with
>
> while ans[:1].lower() != 'y':
> ans = input('Do you like python?')
yok is Turkish for an EMPHATIC NO
(or, at least, that's what I was led to think many years ago)
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Re: while loop - multiple condition
On 10/13/2014 11:12 AM, Rustom Mody wrote: > On Monday, October 13, 2014 10:13:20 PM UTC+5:30, Rob Gaddi wrote: >> On Mon, 13 Oct 2014 09:26:57 -0700 (PDT) >> Rustom Mody wrote: > >>> On Monday, October 13, 2014 9:43:03 PM UTC+5:30, Rob Gaddi wrote: On Mon, 13 Oct 2014 09:56:02 +1100 Steven D'Aprano wrote: > When you have multiple clauses in the condition, it's easier to reason > about > them if you write the clauses as positive statements rather than negative > statements, that is, "something is true" rather than "something is not > true", and then use `not` to reverse it if you want to loop *until* the > overall condition is true. I was just explaining this concept to a young pup the other day. De Morgan's lets you say that (not (p and q)) == ((not p) or (not q)), but the positive logic flavor is substantially less error-prone. People are fundamentally not as good at thinking about inverted logic. >>> Curious: Which of >>> - (not (p and q)) >>> - ((not p) or (not q)) >>> is more positive (less negative)?? > >> The first is asking you to compare positive conditions (p and q) and >> negate the entire thing (NAND). The second asks you to think about >> the combination of two different "not true" pieces of logic >> (OR of two inverted inputs). The first is pretty straightforward, and >> I usually see people get it right. The second gets screwed up as often >> as not. > >> And of course, any combination of ands and ors should be broken into >> multiple statements with a descriptive variable name in the middle or >> all hope is lost. > > Yeah I guess 2 nots is one more than one! > > However (to my eyes) > while i < N and a[i] != X: > > looks less negative than > > while not (i==N or a[i] == X): > > [Of course i < N is not identical to i != N ] Right it should have been not (i >= N or a[i] == X) to be equivalent. In assembler it's often best to reverse the condition and then use the opposite jump mnemonic. IE, if the test is to see if a number not zero, use the jump if zero command instead. Often it reduces the number of jumps required and eliminates the need to jump over the body of the "if" block. if a != 0 then jump to bigger jump to end bigger: blah blah end: vs if a == 0 then jump to end blah blah end: -- https://mail.python.org/mailman/listinfo/python-list
Re: while loop - multiple condition
On Monday, October 13, 2014 10:13:20 PM UTC+5:30, Rob Gaddi wrote: > On Mon, 13 Oct 2014 09:26:57 -0700 (PDT) > Rustom Mody wrote: > > On Monday, October 13, 2014 9:43:03 PM UTC+5:30, Rob Gaddi wrote: > > > On Mon, 13 Oct 2014 09:56:02 +1100 > > > Steven D'Aprano wrote: > > > > When you have multiple clauses in the condition, it's easier to reason > > > > about > > > > them if you write the clauses as positive statements rather than > > > > negative > > > > statements, that is, "something is true" rather than "something is not > > > > true", and then use `not` to reverse it if you want to loop *until* the > > > > overall condition is true. > > > I was just explaining this concept to a young pup the other day. De > > > Morgan's lets you say that (not (p and q)) == ((not p) or (not q)), but > > > the positive logic flavor is substantially less error-prone. People > > > are fundamentally not as good at thinking about inverted logic. > > Curious: Which of > > - (not (p and q)) > > - ((not p) or (not q)) > > is more positive (less negative)?? > The first is asking you to compare positive conditions (p and q) and > negate the entire thing (NAND). The second asks you to think about > the combination of two different "not true" pieces of logic > (OR of two inverted inputs). The first is pretty straightforward, and > I usually see people get it right. The second gets screwed up as often > as not. > And of course, any combination of ands and ors should be broken into > multiple statements with a descriptive variable name in the middle or > all hope is lost. Yeah I guess 2 nots is one more than one! However (to my eyes) while i < N and a[i] != X: looks less negative than while not (i==N or a[i] == X): [Of course i < N is not identical to i != N ] -- https://mail.python.org/mailman/listinfo/python-list
Re: while loop - multiple condition
On Mon, 13 Oct 2014 09:26:57 -0700 (PDT) Rustom Mody wrote: > On Monday, October 13, 2014 9:43:03 PM UTC+5:30, Rob Gaddi wrote: > > On Mon, 13 Oct 2014 09:56:02 +1100 > > Steven D'Aprano wrote: > > > When you have multiple clauses in the condition, it's easier to reason > > > about > > > them if you write the clauses as positive statements rather than negative > > > statements, that is, "something is true" rather than "something is not > > > true", and then use `not` to reverse it if you want to loop *until* the > > > overall condition is true. > > > I was just explaining this concept to a young pup the other day. De > > Morgan's lets you say that (not (p and q)) == ((not p) or (not q)), but > > the positive logic flavor is substantially less error-prone. People > > are fundamentally not as good at thinking about inverted logic. > > Curious: Which of > > - (not (p and q)) > - ((not p) or (not q)) > > is more positive (less negative)?? The first is asking you to compare positive conditions (p and q) and negate the entire thing (NAND). The second asks you to think about the combination of two different "not true" pieces of logic (OR of two inverted inputs). The first is pretty straightforward, and I usually see people get it right. The second gets screwed up as often as not. And of course, any combination of ands and ors should be broken into multiple statements with a descriptive variable name in the middle or all hope is lost. -- Rob Gaddi, Highland Technology -- www.highlandtechnology.com Email address domain is currently out of order. See above to fix. -- https://mail.python.org/mailman/listinfo/python-list
Re: while loop - multiple condition
On Monday, October 13, 2014 9:43:03 PM UTC+5:30, Rob Gaddi wrote: > On Mon, 13 Oct 2014 09:56:02 +1100 > Steven D'Aprano wrote: > > When you have multiple clauses in the condition, it's easier to reason about > > them if you write the clauses as positive statements rather than negative > > statements, that is, "something is true" rather than "something is not > > true", and then use `not` to reverse it if you want to loop *until* the > > overall condition is true. > I was just explaining this concept to a young pup the other day. De > Morgan's lets you say that (not (p and q)) == ((not p) or (not q)), but > the positive logic flavor is substantially less error-prone. People > are fundamentally not as good at thinking about inverted logic. Curious: Which of - (not (p and q)) - ((not p) or (not q)) is more positive (less negative)?? -- https://mail.python.org/mailman/listinfo/python-list
Re: while loop - multiple condition
On Mon, 13 Oct 2014 09:56:02 +1100 Steven D'Aprano wrote: > > When you have multiple clauses in the condition, it's easier to reason about > them if you write the clauses as positive statements rather than negative > statements, that is, "something is true" rather than "something is not > true", and then use `not` to reverse it if you want to loop *until* the > overall condition is true. > I was just explaining this concept to a young pup the other day. De Morgan's lets you say that (not (p and q)) == ((not p) or (not q)), but the positive logic flavor is substantially less error-prone. People are fundamentally not as good at thinking about inverted logic. -- Rob Gaddi, Highland Technology -- www.highlandtechnology.com Email address domain is currently out of order. See above to fix. -- https://mail.python.org/mailman/listinfo/python-list
Re: while loop - multiple condition
On Mon, Oct 13, 2014 at 11:09 PM, Marko Rauhamaa wrote:
> Chris Angelico :
>
>> Or, even simpler: Use an active condition.
>>
>> while input('Do you like python?') not in ('yes', 'y'): pass
>
> Instead of the traditional "pull" technology, you could take advantage
> of the state-of-the-art "push" approach:
>
>print("You must love python -- everybody does!")
Nay, there is love in excess. I thank heaven there are many pythons in
England; but if thou lovest them all, I withdraw my thanks!
-- Colonel Fairfax
ChrisA
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Re: while loop - multiple condition
On Mon, Oct 13, 2014 at 11:10 PM, Skip Montanaro
wrote:
> On Mon, Oct 13, 2014 at 6:59 AM, Chris Angelico wrote:
>>
>> while input('Do you like python?') not in ('yes', 'y'): pass
>
>
> Unfortunately, you probably have to account for people who SHOUT:
>
> while input('Do you like python?').lower() not in ('yes', 'y'): pass
>
>
Welcome to collaborative editing. I make a change and introduce a bug.
Fortunately, someone else can, just like that, fix that bug. Thanks!
:)
ChrisA
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Re: while loop - multiple condition
On Mon, Oct 13, 2014 at 6:59 AM, Chris Angelico wrote:
> while input('Do you like python?') not in ('yes', 'y'): pass
Unfortunately, you probably have to account for people who SHOUT:
while input('Do you like python?').lower() not in ('yes', 'y'): pass
Skip
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Re: while loop - multiple condition
Chris Angelico :
> Or, even simpler: Use an active condition.
>
> while input('Do you like python?') not in ('yes', 'y'): pass
Instead of the traditional "pull" technology, you could take advantage
of the state-of-the-art "push" approach:
print("You must love python -- everybody does!")
Marko
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Re: while loop - multiple condition
On Mon, Oct 13, 2014 at 7:31 PM, Gelonida N wrote:
> Taking into account the Steven's suggestion about using the 'in' expression
> it could be:
>
>
> while True:
> ans = input('Do you like python?')
> if ans.lower() in ('yes', 'y'):
> break
Or, even simpler: Use an active condition.
while input('Do you like python?') not in ('yes', 'y'): pass
ChrisA
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Re: while loop - multiple condition
On 10/12/2014 07:08 PM, Shiva wrote:
while ans.lower() != 'yes' or ans.lower()[0] != 'y':
ans = input('Do you like python?')
I personally consider double negations less intuitive than following:
while not( ans.lower() == 'yes' and ans.lower()[0] == 'y' ):
Reading this line yoy would have noticed as wellm that what you really
wanted would have been:
while not( ans.lower() == 'yes' or ans.lower()[0] == 'y' ):
I would write the coder differently.
With your code you have to pre-initialze the variable ans.
I personally consider it also more 'intuitive' / easier to understand if
I can see the break conditiion. to many nots / != / negations can be
confusing as you noticed yourself.
Taking into account the Steven's suggestion about using the 'in'
expression it could be:
while True:
ans = input('Do you like python?')
if ans.lower() in ('yes', 'y'):
break
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Re: while loop - multiple condition
On Mon, Oct 13, 2014 at 11:43 AM, Dennis Lee Bieber wrote: > ONE: Python uses short circuit evaluation: for an OR, the second > clause > is only looked at if the first clause is FALSE (for an AND, the first > clause has to be TRUE before the second is evaluated). Short-circuiting doesn't actually matter here, incidentally. It could be eagerly evaluated and nothing would be different. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: while loop - multiple condition
Tim Chase wrote:
> On 2014-10-12 22:16, Marko Rauhamaa wrote:
>> is equivalent with
>>
>> while ans.lower()[0] != 'y':
>> ans = input('Do you like python?')
>
> And still better improved with
>
> while ans[:1].lower() != 'y':
> ans = input('Do you like python?')
The intention is to loop forever until the victim breaks down and accepts
that they like Python, by entering "yes" or "y". I'm pretty sure that
entering "yellow" or "you've got to be kidding" should not break the loop.
When you have multiple clauses in the condition, it's easier to reason about
them if you write the clauses as positive statements rather than negative
statements, that is, "something is true" rather than "something is not
true", and then use `not` to reverse it if you want to loop *until* the
overall condition is true.
When checking for equality against multiple values, instead of:
x == a or x == b or x == c
it is usually easier to use `in`:
x in (a, b, c)
So we have:
# loop so long as the victim answers "yes" or "y"
while ans.lower() in ('yes', 'y'):
ans = input('Do you like python? Well, do you?')
# loop until the victim answers "yes" or "y"
while not (ans.lower() in ('yes', 'y')):
ans = input('Do you like python? Well, do you?')
both of which work fine if the user stays mum by entering the empty string.
You can also write:
while ans.lower() not in ('yes', 'y'):
ans = input('Do you like python? Well, do you?')
if you prefer. In my opinion, that's the most readable version of all.
One last note: if you are using Python 3.3 or better, you can support
international text better by using the casefold() method rather than
lower(). In this case, it won't make a difference, but the technically
correct method for doing case-insensitive comparisons is to casefold both
strings rather than lower- or upper-case them.
--
Steven
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Re: while loop - multiple condition
On 2014-10-12 22:16, Marko Rauhamaa wrote:
> is equivalent with
>
> while ans.lower()[0] != 'y':
> ans = input('Do you like python?')
And still better improved with
while ans[:1].lower() != 'y':
ans = input('Do you like python?')
in the event that len(ans)==0 (a situation which will crash the
various current examples).
It also only expends the effort to lowercase 0-or-1 characters,
rather than lowercasing an entire string just to throw away
everything except the first character.
-tkc
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Re: while loop - multiple condition
On Sun, 12 Oct 2014 17:08:00 +, Shiva wrote: > while ans.lower() != 'yes' or ans.lower()[0] != 'y': while ans.lower() is not equal to "yes" or ans.lower()[0] is not equal to "y" the loop will continue to run Note that if ans.lower() == 'y', then the first clause ( ans.lower() != 'yes' ) is true, so the loop will continue to run, ignoring the result of the second clause ( ans.lower()[0] != 'y' ), This will also be the case if you reverse the order of the clauses. It seems that you need a better understanding of combinatorial logic, perhaps http://www.ee.surrey.ac.uk/Projects/Labview/boolalgebra/ index.html#booleantheorems will help. -- Denis McMahon, denismfmcma...@gmail.com -- https://mail.python.org/mailman/listinfo/python-list
Re: while loop - multiple condition
On Mon, Oct 13, 2014 at 6:16 AM, Marko Rauhamaa wrote:
> The corrected version
>
> while ans.lower() != 'yes' and ans.lower()[0] != 'y':
> ans = input('Do you like python?')
>
> is equivalent with
>
> while ans.lower()[0] != 'y':
It's true that the first part is redundant, but trimming that out
wouldn't have taught the OP about boolean logic :)
ChrisA
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Re: while loop - multiple condition
Chris Angelico :
> On Mon, Oct 13, 2014 at 4:59 AM, Shiva
> wrote:
>> Bit confusing to use in While loop - Should have used the 'and' condition
>> instead of OR- then it works fine.
>> for OR both condition need to be false to produce a false output and break
>> the loop.
>
> Correct, what you're looking for here is indeed an 'and' (also called
> a conjunction, as opposed to the disjunction of 'or'). Both operators
> are important; and you need to understand what they do so you know
> when to use each.
The corrected version
while ans.lower() != 'yes' and ans.lower()[0] != 'y':
ans = input('Do you like python?')
is equivalent with
while ans.lower()[0] != 'y':
ans = input('Do you like python?')
Marko
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Re: while loop - multiple condition
On Mon, Oct 13, 2014 at 4:59 AM, Shiva wrote: > Bit confusing to use in While loop - Should have used the 'and' condition > instead of OR- then it works fine. > for OR both condition need to be false to produce a false output and break > the loop. Correct, what you're looking for here is indeed an 'and' (also called a conjunction, as opposed to the disjunction of 'or'). Both operators are important; and you need to understand what they do so you know when to use each. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: while loop - multiple condition
Bit confusing to use in While loop - Should have used the 'and' condition instead of OR- then it works fine. for OR both condition need to be false to produce a false output and break the loop. More of SET operations. Thanks, Shiva -- https://mail.python.org/mailman/listinfo/python-list
Re: while loop - multiple condition
On Mon, Oct 13, 2014 at 4:21 AM, Shiva
wrote:
>> The loop will continue while either part is true - that's what "or"
>> means. Is that what you intended it to be doing?
>>
>> ChrisA
>>
>
>
> Yes..however, the second part of the or condition doesn't get evaluated.
> So if I enter a 'y' - I expect the second part to evaluate and the loop to
> break - but that doesn't seem to happen.
Break it down into separate parts and have a look at what's happening.
while ans.lower() != 'yes' or ans.lower()[0] != 'y':
ans = input('Do you like python?')
print("ans.lower() = {!r}; first cond = {!r}, second cond {!r},
disjunction {!r}".format(
ans.lower(), (ans.lower() != 'yes'), (ans.lower()[0] != 'y'),
(ans.lower() != 'yes' or ans.lower()[0] != 'y')
))
The while loop will continue so long as the disjunction is True. See
what it's actually doing.
This is fundamental Boolean logic, a basic skill of programming.
You're going to need to master this. Look at the different pieces, and
get your head around what the 'or' operator actually does.
ChrisA
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Re: while loop - multiple condition
In article ,
Shiva wrote:
> Why is the second part of while condition not being checked?
>
> while ans.lower() != 'yes' or ans.lower()[0] != 'y':
> ans = input('Do you like python?')
>
>
> My intention is if either of the conditions are true the loop should break.
> But the condition after 'or' doesn't seem to evaluate.
A general rule of programming (be it Python or any other language) is to
break up complicated code into smaller pieces which can be understood
and tested in isolation. Your 'while' condition is kind of hairy.
There's a couple of complicated expressions, a logical conjuction, and
two different negated comparisons. That's a lot to understand all at
once.
I would factor that out into a little function:
---
def is_yes(answer):
if answer.lower() == 'yes':
return True
if answer.lower()[0] == 'y':
return True
return False
while is_yes(ans):
ans = input('Do you like python?')
---
Now, you can play around with your is_yes() function in an interactive
session and see what it returns for various inputs, in isolation from
the rest of your program:
print is_yes('y')
print is_yes('yes')
print is_yes('no')
print is_yes('you bet your sweet bippy')
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Re: while loop - multiple condition
> The loop will continue while either part is true - that's what "or" > means. Is that what you intended it to be doing? > > ChrisA > Yes..however, the second part of the or condition doesn't get evaluated. So if I enter a 'y' - I expect the second part to evaluate and the loop to break - but that doesn't seem to happen. -- https://mail.python.org/mailman/listinfo/python-list
Re: while loop - multiple condition
On Mon, Oct 13, 2014 at 4:08 AM, Shiva
wrote:
> Why is the second part of while condition not being checked?
>
> while ans.lower() != 'yes' or ans.lower()[0] != 'y':
> ans = input('Do you like python?')
>
>
> My intention is if either of the conditions are true the loop should break.
> But the condition after 'or' doesn't seem to evaluate.
The loop will continue while either part is true - that's what "or"
means. Is that what you intended it to be doing?
ChrisA
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while loop - multiple condition
Why is the second part of while condition not being checked?
while ans.lower() != 'yes' or ans.lower()[0] != 'y':
ans = input('Do you like python?')
My intention is if either of the conditions are true the loop should break.
But the condition after 'or' doesn't seem to evaluate.
Thanks,
Shiva.
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