Statement (un)equality

[Adam Przybyla]

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.[color=blue][color=green][color=darkred]
>>> [[y,x] for x,y in [1,2],[3,4]][/color][/color][/color]
[[2, 1], [4, 3]][color=blue][color=green][color=darkred]
>>> map(lambda x,y: [y,x], [1,2],[3,4])[/color][/color][/color]
[[3, 1], [4, 2]][color=blue][color=green][color=darkred]
>>>[/color][/color][/color]
Why there is a difference? How to make the same thing like in map statement?
Regards
Adam Przybyla

[Erik Max Francis]

Re: Statement (un)equality

Adam Przybyla wrote:
[color=blue][color=green][color=darkred]
> >>> [[y,x] for x,y in [1,2],[3,4]][/color][/color]
> [[2, 1], [4, 3]][color=green][color=darkred]
> >>> map(lambda x,y: [y,x], [1,2],[3,4])[/color][/color]
> [[3, 1], [4, 2]][color=green][color=darkred]
> >>>[/color][/color]
> Why there is a difference? How to make the same thing like in map
> statement?[/color]

Because the same things aren't happening here, despite their outward
similarity. For the second case to be equivalent to the first, you
really meant:
[color=blue][color=green][color=darkred]
>>> map(lambda x: [x[1], x[0]], [[1, 2], [3, 4]])[/color][/color][/color]
[[2, 1], [4, 3]]

You can pass multiple sequences to map, and that interleaves the
results:
[color=blue][color=green][color=darkred]
>>> map(lambda x, y, z: (x, y, z), [1, 2], [3, 4], [5, 6])[/color][/color][/color]
[(1, 3, 5), (2, 4, 6)]

--
__ Erik Max Francis && max@alcyone.com && http://www.alcyone.com/max/
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\__/ Come not between the dragon and his wrath.
-- King Lear (Act I, Scene I)

[Jonas Galvez]

Re: Statement (un)equality

> [Adam Przybyla][color=blue][color=green][color=darkred]
> >>> [[y,x] for x,y in [1,2],[3,4]][/color][/color]
> [[2, 1], [4, 3]][color=green][color=darkred]
> >>> map(lambda x,y: [y,x], [1,2],[3,4])[/color][/color]
> [[3, 1], [4, 2]][/color]
[color=blue][color=green][color=darkred]
>>> def test(a,b):[/color][/color][/color]
print "a:", a, "b:", b
return [b,a][color=blue][color=green][color=darkred]
>>> map(test, [1,2], [3,4])[/color][/color][/color]
a: 1 b: 3
a: 2 b: 4
[[3, 1], [4, 2]][color=blue][color=green][color=darkred]
>>> [test(a, b) for a, b in [1,2], [3,4]][/color][/color][/color]
a: 1 b: 2
a: 3 b: 4
[[2, 1], [4, 3]][color=blue][color=green][color=darkred]
>>> [test(a, b) for a, b in zip([1,2], [3,4])][/color][/color][/color]
a: 1 b: 3
a: 2 b: 4
[[3, 1], [4, 2]]



Jonas

[James Henderson]

Re: Statement (un)equality

On Friday 20 February 2004 10:31 am, Adam Przybyla wrote:[color=blue]
> 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.
>[color=green][color=darkred]
> >>> [[y,x] for x,y in [1,2],[3,4]][/color][/color]
>
> [[2, 1], [4, 3]]
>[color=green][color=darkred]
> >>> map(lambda x,y: [y,x], [1,2],[3,4])[/color][/color]
>
> [[3, 1], [4, 2]]
>
> Why there is a difference?[/color]

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.
[color=blue]
> How to make the same thing like in map
> statement?[/color]

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.)
[color=blue][color=green][color=darkred]
>>> zip([1,2],[3,4])[/color][/color][/color]
[(1, 3), (2, 4)]

so:

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

will match the behaviour of the map statement.
[color=blue]
> Regards
> Adam Przybyla[/color]

James
--
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