Statement (un)equality

James Henderson james at logicalprogression.net
Fri Feb 20 12:12:47 CET 2004


On Friday 20 February 2004 10:31 am, Adam Przybyla wrote:
> castle:/home/adam>python
> Python 2.3 (#3, Aug  4 2003, 16:43:33)
> [GCC 2.96 20000731 (Red Hat Linux 7.1 2.96-98)] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
>
> >>> [[y,x] for x,y in [1,2],[3,4]]
>
> [[2, 1], [4, 3]]
>
> >>> map(lambda x,y: [y,x], [1,2],[3,4])
>
> [[3, 1], [4, 2]]
>
> Why there is a difference?

Hi,

    [[y,x] for x,y in [1,2],[3,4]]

is just the same as:

    [[y,x] for (x,y) in ([1,2],[3,4])]

The list comprehension regards "[1,2],[3,4]" as a single argument (a tuple) 
and "x,y" is also a tuple.  "x,y" is assigned first [1,2] and then [3,4].

In the map statement "[1,2],[3,4]" are two different arguments.  map's 
signature allows for any number of iterables to be passed after the first 
argument.

> How to make the same thing like in map
> statement?

zip() gives you the sequence of pairs you want to pass:  (zip() is very 
similar to map with None as the first arguemnt.  The difference is how they 
handle sequences of unequal length.)

>>> zip([1,2],[3,4])
[(1, 3), (2, 4)]
 
so:

    [[y,x] for x,y in zip([1,2],[3,4])]

will match the behaviour of the map statement.

> Regards
> 								Adam Przybyla

James
-- 
James Henderson, Logical Progression Ltd.
http://www.logicalprogression.net/
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