Re: [Tutor] What is the augmented assignment operator ^=
On 2/19/07, Dick Moores [EMAIL PROTECTED] wrote: I've tried ^= out a bit: [...] and get that strange alternating behavior. Can someone explain? And while at it, please also explain = and |=. ^ is XOR, is AND, | is OR, all bitwise. You can read more about them here: http://www.somacon.com/p125.php under C bitwise operators. -- - Rikard. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] What is the augmented assignment operator ^=
2007/2/19, Dick Moores [EMAIL PROTECTED]: The docs list it at http://docs.python.org/ref/augassign.html, and send you to http://docs.python.org/ref/primaries.html#primaries, which seems a dead end. I've tried ^= out a bit: n = 5 n ^= 8 n 13 n ^= 8 n 5 n ^= 8 n 13 n ^= 8 n 5 and get that strange alternating behavior. Can someone explain? And while at it, please also explain = and |=. To understand these operators, you will have to think of the numbers as binary numbers. Look at the digits. For two numbers x and y, x^y is the effect of doing an exclusive or on all digits (that is, 0^1 = 1^0 = 1 and 0^0 = 1^1 = 0), of doing an and (11 = 1, 10=01=00=0) and | is an or on all digits (1|1=1|0=0|1 = 1, 0|0 = 0). So 5^8 = 110 ^ 1000 = 0110 ^ 1000 = 1110 = 13 and 13^8 = 1110 ^ 1000 = 0110 = 5 -- Andre Engels, [EMAIL PROTECTED] ICQ: 6260644 -- Skype: a_engels ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] What is the augmented assignment operator ^=
At 02:17 AM 2/19/2007, Andre Engels wrote: To understand these operators, you will have to think of the numbers as binary numbers. Look at the digits. For two numbers x and y, x^y is the effect of doing an exclusive or on all digits (that is, 0^1 = 1^0 = 1 and 0^0 = 1^1 = 0), of doing an and (11 = 1, 10=01=00=0) and | is an or on all digits (1|1=1|0=0|1 = 1, 0|0 = 0). So 5^8 = 110 ^ 1000 = 0110 ^ 1000 = 1110 = 13 and 13^8 = 1110 ^ 1000 = 0110 = 5 Thanks, Andre! I've got it for the three operators, for non-negative integers. But I'm not sure I understand how negative integers work. For example, is 3 -3 = 1 because it is 11 -11 = 01, and that's because one of the first digits of 11 and -11 is not 1, and both of their 2nd digits ARE 1, Q.E.D.? Also, of what practical use are these things? Dick ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] What is the augmented assignment operator ^=
Dick Moores wrote: The docs list it at http://docs.python.org/ref/augassign.html, and send you to http://docs.python.org/ref/primaries.html#primaries, which seems a dead end. a += n is more-or-less a shortcut for a = a + n. There are a few subtleties which the first page you reference talks about, but you can generally think of it as a handy abbreviation. For other operations, the same is true: a op= n is the same as a = a op n I've tried ^= out a bit and get that strange alternating behavior. which is normal operation of ^. Try it with + or * for a simpler example. Kent ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] What is the augmented assignment operator ^=
At 03:32 AM 2/19/2007, you wrote: Dick Moores wrote: The docs list it at http://docs.python.org/ref/augassign.html, and send you to http://docs.python.org/ref/primaries.html#primaries, which seems a dead end. a += n is more-or-less a shortcut for a = a + n. There are a few subtleties which the first page you reference talks about, but you can generally think of it as a handy abbreviation. For other operations, the same is true: a op= n is the same as a = a op n I've tried ^= out a bit and get that strange alternating behavior. which is normal operation of ^. Try it with + or * for a simpler example. Of those listed on http://docs.python.org/ref/augassign.html, I already use and understand += | -= | *= | /= | %= | **= . It's the remaining seven I'm wondering about, or really about , , , ^, and | . Andre's given me a good start, as you may have seen by now. Dick ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] What is the augmented assignment operator ^=
2007/2/19, Dick Moores [EMAIL PROTECTED]: At 02:17 AM 2/19/2007, Andre Engels wrote: To understand these operators, you will have to think of the numbers as binary numbers. Look at the digits. For two numbers x and y, x^y is the effect of doing an exclusive or on all digits (that is, 0^1 = 1^0 = 1 and 0^0 = 1^1 = 0), of doing an and (11 = 1, 10=01=00=0) and | is an or on all digits (1|1=1|0=0|1 = 1, 0|0 = 0). So 5^8 = 110 ^ 1000 = 0110 ^ 1000 = 1110 = 13 and 13^8 = 1110 ^ 1000 = 0110 = 5 Thanks, Andre! I've got it for the three operators, for non-negative integers. But I'm not sure I understand how negative integers work. For example, is 3 -3 = 1 because it is 11 -11 = 01, and that's because one of the first digits of 11 and -11 is not 1, and both of their 2nd digits ARE 1, Q.E.D.? This has to do with the internal representation of negative numbers: -1 is represented as 111111, with one 1 more than maxint. -2 is 111...110 and -3 is 111...101. Thus 3 -3 is 11 111...101 = 1 Also, of what practical use are these things? I guess they have their uses for people accustomed to dealing with hardware. Apart from that, you can get a very space-efficient representation for multiple boolean variables. If you have what looks like an array of booleans, you could also represent it as a single natural number, for example [true, true, false, false, true, false] would then be 1*1 + 1*2 + 0*4 + 0*8 + 1*16 + 0*32 = 19. Using such a representation and the above operators would enable one to write z = x y instead of z = [x[i] and y[i] for i in range(len(x))] when modelling the same using lists. Of course this does come at the price of complicating x[i] = true to x |= 2 ** i which though not really longer does definitely look harder to understand. -- Andre Engels, [EMAIL PROTECTED] ICQ: 6260644 -- Skype: a_engels ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] What is the augmented assignment operator ^=
On 2/19/07, Dick Moores [EMAIL PROTECTED] wrote: It's the remaining seven I'm wondering about, or really about , , , ^, and | . This webpage will tell you - in detail - about all the operators: http://www.lnf.infn.it/Calcolo/doc/aixcxx/html/language/ref/ruclxbin.htm The bitwise operators are hard to understand without proper knowledge about the binary number system. I recommend you to read up about it, if you feel the operators are somewhat difficult to understand. -- - Rikard. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] What is the augmented assignment operator ^=
Dick Moores [EMAIL PROTECTED] wrote
Thanks, Andre! I've got it for the three operators, for non-negative
integers. But I'm not sure I understand how negative integers work.
For example, is 3 -3 = 1
because it is 11 -11 = 01, and that's because one of the first
digits of 11 and -11 is not 1, and both of their 2nd digits ARE 1,
Q.E.D.?
Remember that the digits are 32 bits long so 3 is not 11
but
011
And the representation of negative numbers is more complex
than you might think.
You can see the full hex representations using struct:
import struct
struct.pack('i',-3)
'\xfd\xff\xff\xff'
struct.pack('i',3)
'\x03\x00\x00\x00'
Since most of 3 is zeros the and results will be zro, so
lets look at the significant bits:
0xfd 0x03
1
voila!
Also, of what practical use are these things?
Take a look at my Using the OS topic, there is a side-bar(box)
just short of half way down that describes how these can be
used to test file status. There are many other uses too, but
that's one real-world case.
HTH,
--
Alan Gauld
Author of the Learn to Program web site
http://www.freenetpages.co.uk/hp/alan.gauld
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Re: [Tutor] What is the augmented assignment operator ^=
My sincere thanks to Rikard Bosnjakovic, Andre Engels, and Alan Gauld. I think you've given me a good start toward understanding the operators , , , ^, and | ; 32-bit numbers, and negative binary numbers. Dick Moores ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
