Re: semantics of ** (unexpected/inconsistent?)
In article <87eingrbh9@benfinney.id.au>, Ben Finney wrote: >Lie Ryan writes: > >> I generally do not expect operator precedence to be reliable at all > >Have another read of the thread. The OP's confusion was not over >operator precedence, but over how names resolve to values in >expressions. Operator precedence comes naturally into this matter. For example, in algol 68 -3**2 is parsed as (-3)**2 because of the simple rule that all unary operators have precedence over all binary operators. (It is a good rule, and this is about the only way to get a somewhat surprising result. Unary operators -- as long as they are always put up front -- need not have a precedence among themselves, so with this rule they don't need a precedence full stop. ) >Ben Finney Groetjes Albert -- -- Albert van der Horst, UTRECHT,THE NETHERLANDS Economic growth -- being exponential -- ultimately falters. alb...@spe&ar&c.xs4all.nl &=n http://home.hccnet.nl/a.w.m.van.der.horst -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
On Mon, Nov 30, 2009 at 2:05 PM, Lie Ryan wrote: > On 12/1/2009 5:58 AM, inhahe wrote: >> >> i wasn't suggesting it as a feature for python, just pointing out why >> it might seem counterintuitive. > > I'm interested, what do YOU (inhahe) think the result should be? Should both > become -9 or both become 9. What was your expectation when you wrote that > post? > -- > http://mail.python.org/mailman/listinfo/python-list > i think the way it works currently is the sane way..pays respect to the standard order of operations, doesn't have to do anything special with x = -3, just treats it as an integer, and corresponds with normal algebra. -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
On Sun, Nov 29, 2009 at 5:58 PM, Esmail wrote: > Brian J Mingus wrote: > >> >> >> >> I think you answered your own question. 3**2 comes first in the order of >> operations, followed by the negation. >> > > No, that's not the problem, I'm ok with the operator precedence of - vs ** > > My problem is why I don't get the same result if I use the literal -3 or > a variable that contains -3 (x in my example) Yes, that is the problem. Setting x=-3 is the same as writing (-3)**2 vs. -(3**2). -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
On 12/1/2009 5:58 AM, inhahe wrote: i wasn't suggesting it as a feature for python, just pointing out why it might seem counterintuitive. I'm interested, what do YOU (inhahe) think the result should be? Should both become -9 or both become 9. What was your expectation when you wrote that post? -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
On Mon, Nov 30, 2009 at 1:53 PM, Chris Rebert wrote: >> On Mon, Nov 30, 2009 at 3:46 AM, Gregory Ewing >> wrote: >>> Esmail wrote: >>> Wow .. never heard of Concatenative_languages languages before or the distinction you make. Your distinction explains the behavior, but I find it somewhat counter-intuitive. >>> >>> You shouldn't find it any more surprising than the fact that >>> >>> a = 2 + 3 >>> print a * 5 >>> >>> gives a different result from >>> >>> print 2 + 3 * 5 > > On Mon, Nov 30, 2009 at 3:41 AM, inhahe wrote: >> one point of confusion could be the use of ** instead of superscript. >> it might make things a little bit more counterintuitive-looking than >> with superscripts, since the issue with > > Well, since source code is almost universally just plain ASCII and not > in some file format with typesetting, superscripts aren't going to > happen any time soon. > (Also, avoid top-posting in the future.) > i wasn't suggesting it as a feature for python, just pointing out why it might seem counterintuitive. -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
> On Mon, Nov 30, 2009 at 3:46 AM, Gregory Ewing > wrote: >> Esmail wrote: >> >>> Wow .. never heard of Concatenative_languages languages before or the >>> distinction you make. Your distinction explains the behavior, but I >>> find it somewhat counter-intuitive. >> >> You shouldn't find it any more surprising than the fact that >> >> a = 2 + 3 >> print a * 5 >> >> gives a different result from >> >> print 2 + 3 * 5 On Mon, Nov 30, 2009 at 3:41 AM, inhahe wrote: > one point of confusion could be the use of ** instead of superscript. > it might make things a little bit more counterintuitive-looking than > with superscripts, since the issue with Well, since source code is almost universally just plain ASCII and not in some file format with typesetting, superscripts aren't going to happen any time soon. (Also, avoid top-posting in the future.) Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
Lie Ryan wrote: On 11/30/2009 12:38 PM, Esmail wrote: Thanks all!! I get it now :-) It helped to have a number of different explanations, thanks for taking the time to post. Much appreciated. I generally do not expect operator precedence to be reliable at all except for: + - (binary ops, not the unary) * / ** for other operators I would have explicit parens. It's too much work to remember the rest of the precedence sheet. Most programming languages don't differentiate in text between the number "negative 3" and the expression "negated 3". APL does. The former is written as "¯3" (3 preceded by the overscore character) and the latter as "-3" (3 preceded by the minus sign). -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
one point of confusion could be the use of ** instead of superscript. it might make things a little bit more counterintuitive-looking than with superscripts, since the issue with would only apply to exponents, as -5*4 and a = -5 a*4 return the same answer, and superscripts make it a little easier to associate the exponent value with the base more than with the - before it. On Mon, Nov 30, 2009 at 3:46 AM, Gregory Ewing wrote: > Esmail wrote: > >> Wow .. never heard of Concatenative_languages languages before or the >> distinction you make. Your distinction explains the behavior, but I >> find it somewhat counter-intuitive. > > You shouldn't find it any more surprising than the fact that > > a = 2 + 3 > print a * 5 > > gives a different result from > > print 2 + 3 * 5 > > -- > Greg > -- > http://mail.python.org/mailman/listinfo/python-list > -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
Esmail wrote: Wow .. never heard of Concatenative_languages languages before or the distinction you make. Your distinction explains the behavior, but I find it somewhat counter-intuitive. You shouldn't find it any more surprising than the fact that a = 2 + 3 print a * 5 gives a different result from print 2 + 3 * 5 -- Greg -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
On Sun, Nov 29, 2009 at 8:04 PM, Esmail wrote: > Chris Rebert wrote: > Wow .. never heard of Concatenative_languages languages before or the > distinction you make. Your distinction explains the behavior, but I > find it somewhat counter-intuitive. (I use the Python interpreter frequently > for small calculations - otherwise I'd never have realized this) Well I think of it like this -3**2, because of the order of operations as you know, groups as -(3**2) now if you set x to -3, (-3) is now automatically, inseparably grouped. so in a sense by setting x to that you're grouping it. x == (-3) x**2 == (-3)**2 if it still seems counterintuitive, what about the fact that it's exactly the same way in Algebra? basic algebra: -3(superscript)2 is -9 but if x is -3, then x(superscript)2 is 9 -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
Lie Ryan writes: > I generally do not expect operator precedence to be reliable at all Have another read of the thread. The OP's confusion was not over operator precedence, but over how names resolve to values in expressions. -- \ “Life does not cease to be funny when people die any more than | `\ it ceases to be serious when people laugh.” —George Bernard Shaw | _o__) | Ben Finney -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
On 11/30/2009 12:38 PM, Esmail wrote: Thanks all!! I get it now :-) It helped to have a number of different explanations, thanks for taking the time to post. Much appreciated. I generally do not expect operator precedence to be reliable at all except for: + - (binary ops, not the unary) * / ** for other operators I would have explicit parens. It's too much work to remember the rest of the precedence sheet. -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
Thanks all!! I get it now :-) It helped to have a number of different explanations, thanks for taking the time to post. Much appreciated. Cheers, Esmail -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
Esmail wrote: > Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41) > [GCC 4.3.3] on linux2 > Type "help", "copyright", "credits" or "license" for more information. > > > >>> -3**2 > -9 > > >>> x = -3 > > >>> x**2 > 9 > >>> > > I would have expected the same result in both cases. > > Initially I would have expected -3**2 to yield 9, but I can accept > that ** binds tighter than the unary -, but shouldn't the results > be consistent regardless if I use a literal or a variable? When you say ** binds tighter than unary -, you're also saying that -3 isn't a literal: it's an expression. Try y=3 -y**2 Mel. -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
* Esmail: Ok, this is somewhat unexpected: Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41) [GCC 4.3.3] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> -3**2 -9 >>> x = -3 >>> x**2 9 >>> I would have expected the same result in both cases. Initially I would have expected -3**2 to yield 9, but I can accept that ** binds tighter than the unary -, but shouldn't the results be consistent regardless if I use a literal or a variable? It is. >>> -3**2 -9 >>> x = 3 >>> -x**2 -9 >>> :-) I guess you expect your expression "x**2" to somehow be evaluated as "-3**2". But x doesn't contain text, it contains an integer value that presumably (I don't know) is represented in the binary number system, so it's evaluated as "(-3)**2". If x contained text and was evaluated as such, pure text replacement, then you should be able to write 2 x and have that evaluated as "2 -x"... Cheers & hth., - Alf -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
Esmail writes: > Brian J Mingus wrote: > > > > > > > > I think you answered your own question. 3**2 comes first in the > > order of operations, followed by the negation. > > No, that's not the problem, I'm ok with the operator precedence of - vs ** > > My problem is why I don't get the same result if I use the literal -3 > or a variable that contains -3 (x in my example). You seem to be confusing the values, which (in this case) are numbers, with their incidental representation in Python code, which is literal expressions in text. Perhaps the following dissection will help: >>> -3**2 The expression ‘-3**2’ is evaluated, and a single value is the result. In that expression, there are two operators: the unary-minus operator and the exponential operator. There are two values as parameters in the expression: the natural number three, and the natural number two. >>> x = -3 The expression ‘-3’ is evaluated, and a single value is the result. That value has an internal representation of “the integer that is three less than zero”. It is *not* the sequence of characters that you see in the Python code; it is a number. The name ‘x’ is then bound to that value. >>> x**2 The expression ‘x**2’ is evaluated, and a single value is the result. In that expression, there is *one* operator: the exponential operator. There is no unary-minus operator in the expression. There are two values as parameters in the expression: the value referenced by the name ‘x’ which is the integer negative-three, and the natural number two. I hope that explains the difference in behaviour. -- \ “I don't like country music, but I don't mean to denigrate | `\ those who do. And for the people who like country music, | _o__)denigrate means ‘put down’.” —Bob Newhart | Ben Finney -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
It's just like in algebra. You evaluate exponents before the - which, after all, is just another way to write -1, or times-negative-one. However, a variable with a negative value is not the same as a value that is being multiplied by a negative. -3 ** 2 = (-1)(3)^(2) in algebraic terms. Exponents first, then multiplication. However, x ** 2 = (x)^(2) = (-3)^(2) regardless of the value of x which, in this case, is -3. When you multiply a negative by itself, you get a positive. In short, the ** operator appears to have a higher precedence than the - operator, based on your results. On Sun, Nov 29, 2009 at 6:39 PM, Esmail wrote: > Ok, this is somewhat unexpected: > > Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41) > [GCC 4.3.3] on linux2 > Type "help", "copyright", "credits" or "license" for more information. > > > >>> -3**2 > -9 > > >>> x = -3 > > >>> x**2 > 9 > >>> > > I would have expected the same result in both cases. > > Initially I would have expected -3**2 to yield 9, but I can accept > that ** binds tighter than the unary -, but shouldn't the results > be consistent regardless if I use a literal or a variable? > > > > -- > http://mail.python.org/mailman/listinfo/python-list > -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
>> I think you answered your own question. 3**2 comes first in the order >> of operations, followed by the negation. > > No, that's not the problem, I'm ok with the operator precedence of - vs ** > > My problem is why I don't get the same result if I use the literal -3 or > a variable that contains -3 (x in my example). There is no literal -3 in Python, only a literal (+)3, see http://docs.python.org/reference/lexical_analysis.html#integer-and-long-integer-literals So -3**2 means -(3**2) == -9. Regards, Martin -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
On 29-Nov-09 19:50 PM, Chris Rebert wrote: On Sun, Nov 29, 2009 at 4:39 PM, Esmail wrote: Ok, this is somewhat unexpected: Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41) [GCC 4.3.3] on linux2 Type "help", "copyright", "credits" or "license" for more information. -3**2 -9 x = -3 x**2 9 I would have expected the same result in both cases. Initially I would have expected -3**2 to yield 9, but I can accept that ** binds tighter than the unary -, but shouldn't the results be consistent regardless if I use a literal or a variable? _No_, because using the variable evaluates "-3" as a unit separately by itself, before the exponentiation ever occurs; it's the same as the difference between (-3)**2 and -3**2. Python is not a concatenative programming language[*]; you can't directly textually replace a variable with its value and expect to get the same result from an expression. For instance, in this case, you need to add the parentheses. Cheers, Chris -- http://blog.rebertia.com [*] http://en.wikipedia.org/wiki/Concatenative_language See the Operator Order of Precedence: http://docs.python.org/reference/expressions.html#summary Parentheses permit the user to vary the precedence. Colin W. -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
Chris Rebert wrote: _No_, because using the variable evaluates "-3" as a unit separately by itself, before the exponentiation ever occurs; it's the same as the difference between (-3)**2 and -3**2. Python is not a concatenative programming language[*]; you can't directly textually replace a variable with its value and expect to get the same result from an expression. For instance, in this case, you need to add the parentheses. Cheers, Chris -- http://blog.rebertia.com [*] http://en.wikipedia.org/wiki/Concatenative_language Wow .. never heard of Concatenative_languages languages before or the distinction you make. Your distinction explains the behavior, but I find it somewhat counter-intuitive. (I use the Python interpreter frequently for small calculations - otherwise I'd never have realized this) Thanks, Esmail -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
Brian J Mingus wrote: I think you answered your own question. 3**2 comes first in the order of operations, followed by the negation. No, that's not the problem, I'm ok with the operator precedence of - vs ** My problem is why I don't get the same result if I use the literal -3 or a variable that contains -3 (x in my example). -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
On Sun, Nov 29, 2009 at 4:39 PM, Esmail wrote: > Ok, this is somewhat unexpected: > > Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41) > [GCC 4.3.3] on linux2 > Type "help", "copyright", "credits" or "license" for more information. > > -3**2 > -9 > x = -3 > x**2 > 9 > > I would have expected the same result in both cases. > > Initially I would have expected -3**2 to yield 9, but I can accept > that ** binds tighter than the unary -, but shouldn't the results > be consistent regardless if I use a literal or a variable? _No_, because using the variable evaluates "-3" as a unit separately by itself, before the exponentiation ever occurs; it's the same as the difference between (-3)**2 and -3**2. Python is not a concatenative programming language[*]; you can't directly textually replace a variable with its value and expect to get the same result from an expression. For instance, in this case, you need to add the parentheses. Cheers, Chris -- http://blog.rebertia.com [*] http://en.wikipedia.org/wiki/Concatenative_language -- http://mail.python.org/mailman/listinfo/python-list
Re: semantics of ** (unexpected/inconsistent?)
On Sun, Nov 29, 2009 at 5:39 PM, Esmail wrote: > Ok, this is somewhat unexpected: > > Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41) > [GCC 4.3.3] on linux2 > Type "help", "copyright", "credits" or "license" for more information. > > > >>> -3**2 > -9 > > >>> x = -3 > > >>> x**2 > 9 > >>> > > I would have expected the same result in both cases. > > Initially I would have expected -3**2 to yield 9, but I can accept > that ** binds tighter than the unary -, but shouldn't the results > be consistent regardless if I use a literal or a variable? > I think you answered your own question. 3**2 comes first in the order of operations, followed by the negation. >>> (-3)**2 9 >>> 3**2 9 >>> -3**2 -9 -- http://mail.python.org/mailman/listinfo/python-list
