swapping numeric items in a list

[Jiang Nutao]

Hi,

I simplify my problem like below

To convert list
aa = [0x12, 0x34, 0x56, 0x78]
into
[0x34, 0x12, 0x78, 0x56]

How to do it fast? My real list is huge.

Thanks a lot.
Jason

[faulkner]

for i in xrange(0, len(your_list), 2):
your_list[i], your_list[i + 1] = your_list[i + 1], your_list[i]
Jiang Nutao wrote:

Hi,

I simplify my problem like below

To convert list
aa = [0x12, 0x34, 0x56, 0x78]
into
[0x34, 0x12, 0x78, 0x56]

How to do it fast? My real list is huge.

Thanks a lot.
Jason

[Simon Forman]

Jiang Nutao wrote:

Hi,

I simplify my problem like below

To convert list
aa = [0x12, 0x34, 0x56, 0x78]
into
[0x34, 0x12, 0x78, 0x56]

How to do it fast? My real list is huge.

Thanks a lot.
Jason

Here's simple and probably fast enough way (but it won't work right on
odd length lists):

def rev(n):
i = iter(n)
while True:
a = i.next()
yield i.next()
yield a
Example of use:

r = range(24)

print list(rev(r))

If your list comes from binary (str) data, and you're dealing with
endianness, I smell a faster way using struct.

Peace,
~Simon

[bearophileHUGS]

Jiang Nutao:

To convert list
aa = [0x12, 0x34, 0x56, 0x78]
into
[0x34, 0x12, 0x78, 0x56]
How to do it fast? My real list is huge.

Note that:

>>a = range(6)
a

[0, 1, 2, 3, 4, 5]

>>a[::2]

[0, 2, 4]

>>a[1::2]

[1, 3, 5]

So you can do:

>>a[::2], a[1::2] = a[1::2], a[::2]
a

[1, 0, 3, 2, 5, 4]

Bye,
bearophile

[Marc 'BlackJack' Rintsch]

In <ma***************************************@python. org>, Jiang Nutao
wrote:

To convert list
aa = [0x12, 0x34, 0x56, 0x78]
into
[0x34, 0x12, 0x78, 0x56]

How to do it fast? My real list is huge.

Use the `array` module and the `array.byteswap()` method.

Ciao,
Marc 'BlackJack' Rintsch

[Boris Borcic]

Jiang Nutao wrote:

Hi,

I simplify my problem like below

To convert list
aa = [0x12, 0x34, 0x56, 0x78]
into
[0x34, 0x12, 0x78, 0x56]

How to do it fast? My real list is huge.

Mark Rintsch's suggestion appears best if applicable, but just to cite yet other
ways to do it :

list(aa[j+(-1)**j] for j in range(len(aa)))

or

sum(reversed(zip(*[reversed(aa)]*2)),())

or (py 2.5)

from itertools import izip
ab = iter(aa)
list((yield y) or x for x,y in izip(ab,ab))

[Jiang Nutao]

Thank you all guys for the help. Guess I'm gonna pick bearophile's way. It's
fast, neat, and easy to read.

array.byteswap() won't work for me easily. I tried this before my 1st post.
I defined

aa = array('H', [0x12, 0x34, 0x56, 0x78])

Then did byteswap aa.byteswap(). The result was

array('H', [0x1200, 0x3400, 0x5600, 0x7800])

You can see it byteswapped within each item.

Jason

"Marc 'BlackJack' Rintsch" <bj****@gmx.netwrote in message
news:pa****************************@gmx.net...

In <ma***************************************@python. org>, Jiang Nutao
wrote:

>To convert list
aa = [0x12, 0x34, 0x56, 0x78]
into
[0x34, 0x12, 0x78, 0x56]

How to do it fast? My real list is huge.

Use the `array` module and the `array.byteswap()` method.

Ciao,
Marc 'BlackJack' Rintsch
--
http://mail.python.org/mailman/listinfo/python-list

[Gabriel Genellina]

At Wednesday 23/8/2006 14:44, Jiang Nutao wrote:

>array.byteswap() won't work for me easily. I tried this before my 1st post.
I defined

aa = array('H', [0x12, 0x34, 0x56, 0x78])

Then did byteswap aa.byteswap(). The result was

array('H', [0x1200, 0x3400, 0x5600, 0x7800])

You can see it byteswapped within each item.

Use array('b') or 'B'. 'H' are two-byes integers.

Gabriel Genellina
Softlab SRL

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[Jiang Nutao]

This is what I got in the debugger:

(Pdb) aa=array('b', [126, 55, 71, 112])
(Pdb) aa
array('b', [126, 55, 71, 112])
(Pdb) aa.byteswap()
(Pdb) aa
array('b', [126, 55, 71, 112])

----- Original Message -----
From: "Gabriel Genellina" <ga******@yahoo.com.ar>
To: "Jiang Nutao" <ji********@gmail.com>
Cc: <py*********@python.org>
Sent: Wednesday, August 23, 2006 11:28 AM
Subject: Re: swapping numeric items in a list

At Wednesday 23/8/2006 14:44, Jiang Nutao wrote:

>>array.byteswap() won't work for me easily. I tried this before my 1st
post.
I defined

aa = array('H', [0x12, 0x34, 0x56, 0x78])

Then did byteswap aa.byteswap(). The result was

array('H', [0x1200, 0x3400, 0x5600, 0x7800])

You can see it byteswapped within each item.

Use array('b') or 'B'. 'H' are two-byes integers.

Gabriel Genellina
Softlab SRL


__________________________________________________
Preguntá. Respondé. Descubrí.
Todo lo que querías saber, y lo que ni imaginabas,
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[Fredrik Lundh]

Jiang Nutao wrote:

array.byteswap() won't work for me easily. I tried this before my 1st post.
I defined

aa = array('H', [0x12, 0x34, 0x56, 0x78])

Then did byteswap aa.byteswap(). The result was

array('H', [0x1200, 0x3400, 0x5600, 0x7800])

You can see it byteswapped within each item.

you need to do things in the right order; first convert to bytes, then
build a 16-bit array, and then byteswap that array.

from array import array

# convert data array to 8-bit byte buffer
buf = array("B", [0x12, 0x34, 0x56, 0x78]).tostring()

# treat byte buffer as list of 16-bit integers
buf = array("H", buf)
buf.byteswap()

buf = buf.tostring() # convert back to bytes

[hex(ord(c)) for c in buf]
-['0x34', '0x12', '0x78', '0x56']

</F>

[Gabriel Genellina]

At Wednesday 23/8/2006 15:32, Jiang Nutao wrote:

>This is what I got in the debugger:

(Pdb) aa=array('b', [126, 55, 71, 112])
(Pdb) aa
array('b', [126, 55, 71, 112])
(Pdb) aa.byteswap()
(Pdb) aa
array('b', [126, 55, 71, 112])

Oh, sorry, to swap by two bytes "H" was the right typecode. But your
input array should be:

>> aa = array('H', [0x1234, 0x5678])

which should give:
array('H', [0x3412, 0x7856])

Gabriel Genellina
Softlab SRL

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[Jiang Nutao]

Thanks. But the thing I need to swap is [0x12, 0x34, 0x56, 0x78], not
[0x1234, 0x5678].

----- Original Message -----
From: "Gabriel Genellina" <ga******@yahoo.com.ar>
To: "Jiang Nutao" <ji********@gmail.com>
Cc: "Gabriel Genellina" <ga******@yahoo.com.ar>; <py*********@python.org>
Sent: Wednesday, August 23, 2006 12:19 PM
Subject: Re: swapping numeric items in a list

At Wednesday 23/8/2006 15:32, Jiang Nutao wrote:

>>This is what I got in the debugger:

(Pdb) aa=array('b', [126, 55, 71, 112])
(Pdb) aa
array('b', [126, 55, 71, 112])
(Pdb) aa.byteswap()
(Pdb) aa
array('b', [126, 55, 71, 112])

Oh, sorry, to swap by two bytes "H" was the right typecode. But your input
array should be:

>>> aa = array('H', [0x1234, 0x5678])

which should give:
array('H', [0x3412, 0x7856])

Gabriel Genellina
Softlab SRL


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