do you master list comprehensions?

[Will Stuyvesant]

Here is a question about list comprehensions [lc]. The
question is dumb because I can do without [lc]; but I am
posing the question because I am curious.

This:
[color=blue][color=green][color=darkred]
>>> data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
>>> result = []
>>> for d in data:[/color][/color][/color]
.... for w in d:
.... result.append(w )[color=blue][color=green][color=darkred]
>>> print result[/color][/color][/color]
['foo', 'bar', 'baz', 'my', 'your', 'holy', 'grail']

puts all the words in a list, like I want.

How to do this with [lc] instead of for-loops?

I tried funnies like [[w for w in L] for L in data],
that is correct syntax, but you'd never guess.

I know, silly! No need for [lc]! So there's my
question. I am sure a one-liner using [lc] will be very
enlightening. Like studying LISP.


--
I wish there was a knob on the TV to turn up the intelligence.
There's a knob called `brightness', but it doesn't work.
-- Gallagher

[Max M]

Re: do you master list comprehensions?

Will Stuyvesant wrote:
[color=blue]
> I tried funnies like [[w for w in L] for L in data],
> that is correct syntax, but you'd never guess.[/color]

That is absolutely correct. It's not a funnie at all. If you find it odd
it's only because you are not used to list comprehensiones .

In that case you might be more comfortable with:

data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
result = []
for l in data:
result += l


--

hilsen/regards Max M, Denmark

http://www.mxm.dk/

IT's Mad Science

[Steven Bethard]

Re: do you master list comprehensions?

Will Stuyvesant wrote:[color=blue][color=green][color=darkred]
>>>>data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
>>>>result = []
>>>>for d in data:[/color][/color]
>
> ... for w in d:
> ... result.append(w )
>[color=green][color=darkred]
>>>>print result[/color][/color]
>
> ['foo', 'bar', 'baz', 'my', 'your', 'holy', 'grail']
>[/color]

Take advantage of the fact that you can have more than one 'for' in a
list comprehension:
[color=blue][color=green][color=darkred]
>>> data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
>>> [item for item_list in data for item in item_list][/color][/color][/color]
['foo', 'bar', 'baz', 'my', 'your', 'holy', 'grail']

Steve

[Peter Otten]

Re: do you master list comprehensions?

Will Stuyvesant wrote:
[color=blue][color=green][color=darkred]
>>>> data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
>>>> result = []
>>>> for d in data:[/color][/color]
> ... for w in d:
> ... result.append(w )[color=green][color=darkred]
>>>> print result[/color][/color]
> ['foo', 'bar', 'baz', 'my', 'your', 'holy', 'grail']
>
> puts all the words in a list, like I want.
>
> How to do this with [lc] instead of for-loops?[/color]
[color=blue][color=green][color=darkred]
>>> data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
>>> [w for d in data for w in d][/color][/color][/color]
['foo', 'bar', 'baz', 'my', 'your', 'holy', 'grail']

See how the for expressions in the list comprehension exactly match your
nested for loops? That's all there is to it.

Peter

[Diez B. Roggisch]

Re: do you master list comprehensions?

>>> data = [['foo','bar','ba z'],['my','your'],['holy','grail']][color=blue][color=green][color=darkred]
>>> [e for l in data for e in l][/color][/color][/color]
['foo', 'bar', 'baz', 'my', 'your', 'holy', 'grail']
--
Regards,

Diez B. Roggisch

[Fredrik Lundh]

Re: do you master list comprehensions?

Max M wrote:
[color=blue][color=green]
>> I tried funnies like [[w for w in L] for L in data],
>> that is correct syntax, but you'd never guess.[/color]
>
> That is absolutely correct. It's not a funnie at all. If you find it odd it's only because you are
> not used to list comprehensiones .[/color]

well, syntactically correct or not, it doesn't do what he want...
[color=blue]
> In that case you might be more comfortable with:
>
> data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
> result = []
> for l in data:
> result += l[/color]

how about (slightly evil):

result = []; map(result.exte nd, data)

</F>

[Max M]

Re: do you master list comprehensions?

Fredrik Lundh wrote:[color=blue]
> Max M wrote:
>
>[color=green][color=darkred]
>>>I tried funnies like [[w for w in L] for L in data],[/color]
>>
>>That is absolutely correct. It's not a funnie at all.[/color]
>
> well, syntactically correct or not, it doesn't do what he want...[/color]

Doh! *I* might not be used to list comprehensions then... You are right.

That example could have been expressed more clearly as:

result = data

;-)

--

hilsen/regards Max M, Denmark

http://www.mxm.dk/

IT's Mad Science

[Luis M. Gonzalez]

Re: do you master list comprehensions?

I guess the simplest to do it is like this:
[color=blue][color=green][color=darkred]
>>> data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
>>> result=[w for d in data for w in d]
>>> result[/color][/color][/color]
['foo', 'bar', 'baz', 'my', 'your', 'holy', 'grail'][color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

[Luis M. Gonzalez]

Re: do you master list comprehensions?

I guess the simplest way to do it is like this:
[color=blue][color=green][color=darkred]
>>> data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
>>> result=[w for d in data for w in d]
>>> result[/color][/color][/color]
['foo', 'bar', 'baz', 'my', 'your', 'holy', 'grail'][color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

[James Stroud]

Re: do you master list comprehensions?

Here is one for arbitrary depth:

def unroll(ary):
unrolled = []
for item in ary:
# add test for your favorite sequence type
if ( type(item) == types.ListType or \
type(item) == types.TupleType \
):
unrolled.extend (unroll(item))
else:
unrolled.append (item)
return unrolled


[color=blue][color=green][color=darkred]
>>> unroll([[1, 2, 3], ('fred', 'barney', ['wilma', 'betty']), 'dino'])[/color][/color][/color]
[1, 2, 3, 'fred', 'barney', 'wilma', 'betty', 'dino']




On Monday 13 December 2004 12:51 pm, Will Stuyvesant wrote:[color=blue]
> Here is a question about list comprehensions [lc]. The
> question is dumb because I can do without [lc]; but I am
> posing the question because I am curious.
>
> This:[color=green][color=darkred]
> >>> data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
> >>> result = []
> >>> for d in data:[/color][/color]
>
> ... for w in d:
> ... result.append(w )
>[color=green][color=darkred]
> >>> print result[/color][/color]
>
> ['foo', 'bar', 'baz', 'my', 'your', 'holy', 'grail']
>
> puts all the words in a list, like I want.
>
> How to do this with [lc] instead of for-loops?
>
> I tried funnies like [[w for w in L] for L in data],
> that is correct syntax, but you'd never guess.
>
> I know, silly! No need for [lc]! So there's my
> question. I am sure a one-liner using [lc] will be very
> enlightening. Like studying LISP.[/color]

--
James Stroud, Ph.D.
UCLA-DOE Institute for Genomics and Proteomics
611 Charles E. Young Dr. S.
MBI 205, UCLA 951570
Los Angeles CA 90095-1570

http://www.jamesstroud.com/

[Jeremy Bowers]

Re: do you master list comprehensions?

On Tue, 14 Dec 2004 00:41:36 +0100, Max M wrote:
[color=blue]
> Fredrik Lundh wrote:[color=green]
>> Max M wrote:
>>
>>[color=darkred]
>>>>I tried funnies like [[w for w in L] for L in data],
>>>
>>>That is absolutely correct. It's not a funnie at all.[/color]
>>
>> well, syntactically correct or not, it doesn't do what he want...[/color]
>
> Doh! *I* might not be used to list comprehensions then... You are right.
>
> That example could have been expressed more clearly as:
>
> result = data[/color]

result = data[:]

:-)

[Fredrik Lundh]

Re: do you master list comprehensions?

James Stroud wrote:
[color=blue]
> Here is one for arbitrary depth:
>
> def unroll(ary):
> unrolled = []
> for item in ary:
> # add test for your favorite sequence type
> if ( type(item) == types.ListType or \
> type(item) == types.TupleType \
> ):
> unrolled.extend (unroll(item))
> else:
> unrolled.append (item)
> return unrolled
>[/color]
[color=blue][color=green][color=darkred]
>>>> unroll([[1, 2, 3], ('fred', 'barney', ['wilma', 'betty']), 'dino'])[/color][/color]
> [1, 2, 3, 'fred', 'barney', 'wilma', 'betty', 'dino'][/color]

or, shorter:
[color=blue][color=green][color=darkred]
>>> from Tkinter import _flatten as unroll
>>> (1, 2, 3, 'fred', 'wilma', 'betty', 'dino')[/color][/color][/color]

(alright, it returns a tuple, but that can be easily fixed, if necessary)

</F>

[Timothy Babytch]

Re: do you master list comprehensions?

Will Stuyvesant wrote:
[color=blue][color=green][color=darkred]
>>>>data = [['foo','bar','ba z'],['my','your'],['holy','grail']][/color][/color][/color]

sum(data, [])

['foo', 'bar', 'baz', 'my', 'your', 'holy', 'grail']

The second parameter passed to sum is just to overrride default
initial value "zero".

--
Timothy Babytch

[Will Stuyvesant]

Re: do you master list comprehensions?

Okay that was fun. Enlightening as I hoped. unroll() in Python, for
arbitrary depth, _flatten in Tkinter (what else is in Tkinter!), sum()
abuse.

The sum(data,[]) was funniest, it works like ((['foo','bar'] + []) +
['my','your']) + ['holy','grail']. Before I think of such things I
have already coded an algorithm in imperative style. Guess I have not
been exposed to functional programming enough.

[Steven Bethard]

Re: do you master list comprehensions?

Timothy Babytch wrote:[color=blue]
> Will Stuyvesant wrote:
>[color=green][color=darkred]
>>>>> data = [['foo','bar','ba z'],['my','your'],['holy','grail']][/color][/color]
>
>
> sum(data, [])
>
> ['foo', 'bar', 'baz', 'my', 'your', 'holy', 'grail']
>
> The second parameter passed to sum is just to overrride default
> initial value "zero".
>[/color]

It's worth keeping in mind that this solution has the same efficiency
problems that a loop that =+ strings does:
[color=blue]
> python -m timeit -s "data = [range(10) for _ in range(100)]"[/color]
"sum(data, [])"
1000 loops, best of 3: 530 usec per loop
[color=blue]
> python -m timeit -s "data = [range(10) for _ in range(100)]" "[w for[/color]
d in data for w in d]"
10000 loops, best of 3: 151 usec per loop
[color=blue]
> python -m timeit -s "data = [range(10) for _ in range(1000)]"[/color]
"sum(data, [])"
10 loops, best of 3: 54.2 msec per loop
[color=blue]
> python -m timeit -s "data = [range(10) for _ in range(1000)]" "[w for[/color]
d in data for w in d]"
100 loops, best of 3: 1.75 msec per loop

The sum function used in this way (or a loop with a +=) is O(N**2) while
the LC is O(N).

Steve