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:

data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
result = []
for d in data: .... for w in d:
.... result.append(w ) print result

['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]

Will Stuyvesant wrote:

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

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]

Will Stuyvesant wrote:

data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
result = []
for d in data:
... for w in d:
... result.append(w )
print result
['foo', 'bar', 'baz', 'my', 'your', 'holy', 'grail']

Take advantage of the fact that you can have more than one 'for' in a
list comprehension:

data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
[item for item_list in data for item in item_list]

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

Steve

[Peter Otten]

Will Stuyvesant wrote:

data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
result = []
for d in data: ... for w in d:
... result.append(w ) print result ['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?

data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
[w for d in data for w in d]

['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]

>>> data = [['foo','bar','ba z'],['my','your'],['holy','grail']]

[e for l in data for e in l]

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

Diez B. Roggisch

[Fredrik Lundh]

Max M wrote:

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

well, syntactically correct or not, it doesn't do what he want...
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

how about (slightly evil):

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

</F>

[Max M]

Fredrik Lundh wrote:

Max M wrote:

I tried funnies like [[w for w in L] for L in data],

That is absolutely correct. It's not a funnie at all.

well, syntactically correct or not, it doesn't do what he want...

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]

I guess the simplest to do it is like this:

data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
result=[w for d in data for w in d]
result ['foo', 'bar', 'baz', 'my', 'your', 'holy', 'grail']

[Luis M. Gonzalez]

I guess the simplest way to do it is like this:

data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
result=[w for d in data for w in d]
result ['foo', 'bar', 'baz', 'my', 'your', 'holy', 'grail']

[James Stroud]

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

unroll([[1, 2, 3], ('fred', 'barney', ['wilma', 'betty']), 'dino'])

[1, 2, 3, 'fred', 'barney', 'wilma', 'betty', 'dino']


On Monday 13 December 2004 12:51 pm, Will Stuyvesant wrote:

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: data = [['foo','bar','ba z'],['my','your'],['holy','grail']]
result = []
for d in data:
... for w in d:
... result.append(w )
print result

['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.

--
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/