On Jan 30, 3:31 am, Steven D'Aprano
<ste...@remove.this.cybersource.com.au> wrote:
> On Thu, 29 Jan 2009 17:50:04 -0800, tony.clarke5 wrote:
> > On Jan 30, 12:29 am, Eric Kang <y...@sfu.ca> wrote:
> >> In python, I set:
>
> >> x=1
> >> y=3
>
> >> z = x
> >> x = y
> >> y = z
>
> >> This gave me 3 1, which are the values of x and y swapped. The
> >> following would have given me the same result: x, y = y, x
>
> >> But could the swapping be done using less extra memory than this? What
> >> is the minimum amount of extra memory required to exchange two 32-bit
> >> quantities? What would be the pseudocode that achieves this minimum?
>
> > How about:
> > def transpose(x, y):
> > print x, y, 'becomes: ',
> > x = x + y
> > y = x - y
> > x = x - y
> > print x, ' ', y
>
> > transpose(1,3)
> > transpose (9,8)
>
> I'm not sure what the point of that function is. It doesn't actually swap
> its arguments:
>
> >>> x = 23
> >>> y = 42
> >>> transpose(x, y)
>
> 23 42 becomes: 42 23>>> x
> 23
> >>> y
>
> 42
>
> And it certainly doesn't save memory, as the original poster asked:
>
> >>> import dis
> >>> swap = compile('x, y = y, x', '', 'single')
> >>> dis.dis(swap)
>
> 1 0 LOAD_NAME 0 (y)
> 3 LOAD_NAME 1 (x)
> 6 ROT_TWO
> 7 STORE_NAME 1 (x)
> 10 STORE_NAME 0 (y)
> 13 LOAD_CONST 0 (None)
> 16 RETURN_VALUE
>
> >>> dis.dis(transpose)
>
> 2 0 LOAD_FAST 0 (x)
> 3 PRINT_ITEM
> 4 LOAD_FAST 1 (y)
> 7 PRINT_ITEM
> 8 LOAD_CONST 1 ('becomes: ')
> 11 PRINT_ITEM
>
> 3 12 LOAD_FAST 0 (x)
> 15 LOAD_FAST 1 (y)
> 18 BINARY_ADD
> 19 STORE_FAST 0 (x)
>
> 4 22 LOAD_FAST 0 (x)
> 25 LOAD_FAST 1 (y)
> 28 BINARY_SUBTRACT
> 29 STORE_FAST 1 (y)
>
> 5 32 LOAD_FAST 0 (x)
> 35 LOAD_FAST 1 (y)
> 38 BINARY_SUBTRACT
> 39 STORE_FAST 0 (x)
>
> 6 42 LOAD_FAST 0 (x)
> 45 PRINT_ITEM
> 46 LOAD_CONST 2 (' ')
> 49 PRINT_ITEM
> 50 LOAD_FAST 1 (y)
> 53 PRINT_ITEM
> 54 PRINT_NEWLINE
> 55 LOAD_CONST 0 (None)
> 58 RETURN_VALUE
>
> The compiled code of the transpose function *alone* (not including all
> the other associated parts) takes 59 bytes, or 472 bits.
>
> >>> len(transpose.func_code.co_code)
>
> 59
>
> Even if it worked, that's hardly using less memory than a direct swap.
>
> --
> Steven
Should have been more explicit about that: the values are swapped
within the namespace of the function, the function is just for
demonstration of the process. WIthout the function, this is the
result:
Python 2.5.1 (r251:54869, Apr 18 2007, 22:08:04)
[GCC 4.0.1 (Apple Computer, Inc. build 5367)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> x = -10
>>> y = 4
>>> x = x + y
>>> y = x - y
>>> x = x - y
>>> x
4
>>>
>>> y
-10
>>>
Need to think about swapping floats though.
Tony
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