Swapping values of two variables
Steven D'Aprano
steven at REMOVE.THIS.cybersource.com.au
Fri Jan 30 04:31:03 CET 2009
On Thu, 29 Jan 2009 17:50:04 -0800, tony.clarke5 wrote:
> On Jan 30, 12:29 am, Eric Kang <y... at 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
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