Selecting elements from a list

[Martin Christensen]

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Howdy!

Suppose I have a list A containing elements that I want to put into
list B if they satisfy some property. The obvious way of doing it
would be,

B = []
for i in A:
if A.property():
B.append(i)

Now, list comprehensions, map() etc. can make a lot of list operations
easier, but I haven't found a shorter, more elegant way of doing this.
Have I missed something, or will I have to stick to my explicit loops?

Martin

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GPG public key: http://www.cs.auc.dk/~factotum/gpgkey.txt
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[Jeremy Jones]

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Howdy!

Suppose I have a list A containing elements that I want to put into
list B if they satisfy some property. The obvious way of doing it
would be,

B = []
for i in A:
if A.property():
B.append(i)

Now, list comprehensions, map() etc. can make a lot of list operations
easier, but I haven't found a shorter, more elegant way of doing this.
Have I missed something, or will I have to stick to my explicit loops?

Martin

Is this the sort of thing you mean?

A = [0, 1, 2, 'four', 'five', 6.0]
[x for x in A if isinstance(x, str)] ['four', 'five']

filter() works well as well:

foo [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
filter(lambda x: x > 4, foo)

[5, 6, 7, 8, 9]

Jeremy Jones

 

[Mike C. Fletcher]

B = []
for i in A:
if A.property():
B.append(i)

B = [ i for i in A if i.property() ]

Seems elegant, but then I'd like dictionary-comps if they came along,
Mike

_______________________________________
Mike C. Fletcher
Designer, VR Plumber, Coder
http://members.rogers.com/mcfletch/

 

[David C. Fox]

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Howdy!

Suppose I have a list A containing elements that I want to put into
list B if they satisfy some property. The obvious way of doing it
would be,

B = []
for i in A:
if A.property():
B.append(i)

Now, list comprehensions, map() etc. can make a lot of list operations
easier, but I haven't found a shorter, more elegant way of doing this.
Have I missed something, or will I have to stick to my explicit loops?

Martin

B = filter(lambda x: x.property(), A)

David

 

[Martin Christensen]

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David> B = filter(lambda x: x.property(), A)

Now, if I'd paid better attention to the library documentation...

Martin

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Homepage: http://www.cs.auc.dk/~factotum/
GPG public key: http://www.cs.auc.dk/~factotum/gpgkey.txt
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[Michael Peuser]

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Howdy!

Suppose I have a list A containing elements that I want to put into
list B if they satisfy some property. The obvious way of doing it
would be,

B = []
for i in A:
if A.property():
B.append(i)

Now, list comprehensions, map() etc. can make a lot of list operations
easier, but I haven't found a shorter, more elegant way of doing this.
Have I missed something, or will I have to stick to my explicit loops?

from random import random
rowA1 =[random()]*100000; rowA2=[random()]*100000
rowB1 = rowA1[:]; rowB2 =rowA2[:]
rowC1 = rowA1[:]; rowC2 =rowA2[:]

def A(row1,row2):
for x in row2:
if x<0.5:
row1.append(x)

def B(row1, row2):
row1 += filter(lambda x: x<0.5,row2)

def C(row1, row2):
row1 += [x for x in row2 if x<0.5]


def main():
A(rowA1,rowA2)
B(rowB1,rowB2)
C(rowC1,rowC2)

profile.run("main()")

 

[Duncan Smith]

Martin,
Computer Science at Aalborg I see. A Bayes net man perchance? I
hope Uffe, Kristian et al are well.

Duncan

 

[Martin Christensen]

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Duncan> Computer Science at Aalborg I see. A Bayes net man perchance?
Duncan> I hope Uffe, Kristian et al are well.

Haha! If you're talking about the guys I'm thinking of, I just came
home from a lovely Friday afternoon beer with them. And yes, I
happen to be doing a project involving Bayesian networks, but I'm
originally a database guy. To keep it relevant, I did my masters
thesis implementation stuff in Python and am very happy I did.

Martin

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Homepage: http://www.cs.auc.dk/~factotum/
GPG public key: http://www.cs.auc.dk/~factotum/gpgkey.txt
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[Duncan Smith]

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Duncan> Computer Science at Aalborg I see. A Bayes net man perchance?
Duncan> I hope Uffe, Kristian et al are well.

Haha! If you're talking about the guys I'm thinking of, I just came
home from a lovely Friday afternoon beer with them. And yes, I
happen to be doing a project involving Bayesian networks, but I'm
originally a database guy. To keep it relevant, I did my masters
thesis implementation stuff in Python and am very happy I did.

Martin

Yes, messrs. Kjaerulff and Olesen. All my Bayes net stuff is in Python. I
must tidy it up (and rewrite my Digraph class) so I can make it available.

Duncan

 

[Martin Christensen]

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Duncan> Yes, messrs. Kjaerulff and Olesen.

Then I'm afraid that we were talking about different people, and these
people are probably less likely to run about getting wet on random
Fridays. They're swell people, though, or at least Kristian is, in
my experience (only know Uffe by sight, hardly even voice). Oh well,
it was still amusing despite the Fates not playing _that_ much with
us.

Martin

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