On Jan 30, 8:31 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
Is it possible to swap two floats without a variable? -- http://mail.python.org/mailman/listinfo/python-list