Re: [Pythonmac-SIG] If/else vs or
I've been trying to use more list comprehensions recently. I was just fleshing something out which brought on my post. On Fri, 29 Oct 2010 09:15:47 -0400, Henry Olders wrote: When dealing with lists, list comprehensions are shorter and easier to work with: l=['red','green','orange','blue','red','white'] [x for x in l if x in ['red','blue']] Henry On 2010-10-28, at 10:21 , Dan Ross wrote: I don't think this is Mac specific, but I wonder if someone could explain why these two groups of code behave differently: [code] colors = ['red', 'green', 'blue', 'orange', 'fuscia', 'black', 'white'] list_of_matches = [] for x in colors: if x == 'red' or 'green' or 'blue': list_of_matches.append(x) print list_of_matches list_of_matches2 = [] for x in colors: if x == 'red': list_of_matches2.append(x) elif x == 'green': list_of_matches2.append(x) elif x == 'blue': list_of_matches2.append(x) else: pass print list_of_matches2 [/code] list_of_matches contains every item in colors. list_of_matches2 only contains the matches, as I would expect. I don't get it.. Thanks, Dan ___ Pythonmac-SIG maillist - Pythonmac-SIG@python.org http://mail.python.org/mailman/listinfo/pythonmac-sig unsubscribe: http://mail.python.org/mailman/options/Pythonmac-SIG
Re: [Pythonmac-SIG] If/else vs or
On 10/29/10 7:56 AM, Dan Ross wrote: I've been trying to use more list comprehensions recently. ahh -- then you want something like: In [15]: colors = ['red','green','blue','orange','fuchsia','black','white'] In [16]: subset = ['red','green','blue','purple'] In [17]: [c for c in colors if c in subset] Out[17]: ['red', 'green', 'blue'] (so much for one obvious way to do it!) -Chris -- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/ORR(206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception chris.bar...@noaa.gov ___ Pythonmac-SIG maillist - Pythonmac-SIG@python.org http://mail.python.org/mailman/listinfo/pythonmac-sig unsubscribe: http://mail.python.org/mailman/options/Pythonmac-SIG
Re: [Pythonmac-SIG] If/else vs or
Indeed. That's awfully nice and concise. On Fri, 29 Oct 2010 09:14:06 -0700, Christopher Barker chris.bar...@noaa.gov wrote: On 10/29/10 7:56 AM, Dan Ross wrote: I've been trying to use more list comprehensions recently. ahh -- then you want something like: In [15]: colors = ['red','green','blue','orange','fuchsia','black','white'] In [16]: subset = ['red','green','blue','purple'] In [17]: [c for c in colors if c in subset] Out[17]: ['red', 'green', 'blue'] (so much for one obvious way to do it!) -Chris ___ Pythonmac-SIG maillist - Pythonmac-SIG@python.org http://mail.python.org/mailman/listinfo/pythonmac-sig unsubscribe: http://mail.python.org/mailman/options/Pythonmac-SIG
Re: [Pythonmac-SIG] If/else vs or
Or you could use actual sets: colors = set(['red', 'green', 'blue', 'orange', 'fuscia', 'black', 'white']) subset = set(['red', 'green', 'blue', 'purple']) subset.intersection(colors) set(['blue', 'green', 'red']) Of course, this loses your ordering, but it's otherwise far easier to read than a list comprehension. -Chris On Fri, Oct 29, 2010 at 9:19 AM, Dan Ross d...@rosspixelworks.com wrote: Indeed. That's awfully nice and concise. On Fri, 29 Oct 2010 09:14:06 -0700, Christopher Barker chris.bar...@noaa.gov wrote: On 10/29/10 7:56 AM, Dan Ross wrote: I've been trying to use more list comprehensions recently. ahh -- then you want something like: In [15]: colors = ['red','green','blue','orange','fuchsia','black','white'] In [16]: subset = ['red','green','blue','purple'] In [17]: [c for c in colors if c in subset] Out[17]: ['red', 'green', 'blue'] (so much for one obvious way to do it!) -Chris ___ Pythonmac-SIG maillist - Pythonmac-SIG@python.org http://mail.python.org/mailman/listinfo/pythonmac-sig unsubscribe: http://mail.python.org/mailman/options/Pythonmac-SIG ___ Pythonmac-SIG maillist - Pythonmac-SIG@python.org http://mail.python.org/mailman/listinfo/pythonmac-sig unsubscribe: http://mail.python.org/mailman/options/Pythonmac-SIG
[Pythonmac-SIG] If/else vs or
I don't think this is Mac specific, but I wonder if someone could explain why these two groups of code behave differently: [code] colors = ['red', 'green', 'blue', 'orange', 'fuscia', 'black', 'white'] list_of_matches = [] for x in colors: if x == 'red' or 'green' or 'blue': list_of_matches.append(x) print list_of_matches list_of_matches2 = [] for x in colors: if x == 'red': list_of_matches2.append(x) elif x == 'green': list_of_matches2.append(x) elif x == 'blue': list_of_matches2.append(x) else: pass print list_of_matches2 [/code] list_of_matches contains every item in colors. list_of_matches2 only contains the matches, as I would expect. I don't get it.. Thanks, Dan___ Pythonmac-SIG maillist - Pythonmac-SIG@python.org http://mail.python.org/mailman/listinfo/pythonmac-sig unsubscribe: http://mail.python.org/mailman/options/Pythonmac-SIG
Re: [Pythonmac-SIG] If/else vs or
On 28 Oct, 2010, at 16:21, Dan Ross wrote:
I don't think this is Mac specific, but I wonder if someone could explain why
these two groups of code behave differently:
[code]
colors = ['red', 'green', 'blue', 'orange', 'fuscia', 'black', 'white']
list_of_matches = []
for x in colors:
if x == 'red' or 'green' or 'blue':
This parses as:
if x == ('red' or 'green' or blue'):
Ronald
smime.p7s
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Re: [Pythonmac-SIG] If/else vs or
On Oct 28, 2010, at 11:08 AM, Ronald Oussoren wrote:
On 28 Oct, 2010, at 16:21, Dan Ross wrote:
I don't think this is Mac specific, but I wonder if someone could
explain why these two groups of code behave differently:
[code]
colors = ['red', 'green', 'blue', 'orange', 'fuscia', 'black',
'white']
list_of_matches = []
for x in colors:
if x == 'red' or 'green' or 'blue':
This parses as:
if x == ('red' or 'green' or blue'):
This would always lead to the if-test failing: ('red' or 'green' or
'blue') evaluates to True, and x != True. What's observed is the if-
test always passing... As the equality operator is higher-precedence
than boolean operators, and equal precedence operators group left-to-
right, the above parses as:
if (((x == 'red') or 'green') or 'blue'):
noting that non-empty strings (like 'green') evaluate as True in an if-
test, this will test if x == 'red', and if not, it will go on to
testing if 'green' evaluates to True (which it does), and so forth.
Dan, you could fix your code as:
if x == 'red' or x == 'green' or x == 'blue':
But this is better:
if x in ('red', 'green', 'blue'):
and this scales best:
good_colors = set(['red', 'green', 'blue'])
if x in good_colors:
Zach
Ronald
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Re: [Pythonmac-SIG] If/else vs or
I have attached a copy of the originally-posted Python code and also have
attached an IDLE session based on that code, which seems instructive.
(Copying and pasting the IDLE session into the email message seems to mess
up the indentation.)
But Zachary's suggestions for rewriting the original Python to attain the
intended result are very helpful.
On Thu, Oct 28, 2010 at 8:20 AM, Zachary Pincus zachary.pin...@yale.eduwrote:
On Oct 28, 2010, at 11:08 AM, Ronald Oussoren wrote:
On 28 Oct, 2010, at 16:21, Dan Ross wrote:
I don't think this is Mac specific, but I wonder if someone could explain
why these two groups of code behave differently:
[code]
colors = ['red', 'green', 'blue', 'orange', 'fuscia', 'black', 'white']
list_of_matches = []
for x in colors:
if x == 'red' or 'green' or 'blue':
This parses as:
if x == ('red' or 'green' or blue'):
This would always lead to the if-test failing: ('red' or 'green' or 'blue')
evaluates to True, and x != True. What's observed is the if-test always
passing... As the equality operator is higher-precedence than boolean
operators, and equal precedence operators group left-to-right, the above
parses as:
if (((x == 'red') or 'green') or 'blue'):
noting that non-empty strings (like 'green') evaluate as True in an
if-test, this will test if x == 'red', and if not, it will go on to testing
if 'green' evaluates to True (which it does), and so forth.
Dan, you could fix your code as:
if x == 'red' or x == 'green' or x == 'blue':
But this is better:
if x in ('red', 'green', 'blue'):
and this scales best:
good_colors = set(['red', 'green', 'blue'])
if x in good_colors:
Zach
Ronald
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Oct28.py
Description: Binary data
IdleSessionOct28
Description: Binary data
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Re: [Pythonmac-SIG] If/else vs or
Thank you for your help guys. Zach, I appreciate the explanation. That's what I was looking for. Dan Part 3 Description: boundary/apple-mail-7-733662729 ___ Pythonmac-SIG maillist - Pythonmac-SIG@python.org http://mail.python.org/mailman/listinfo/pythonmac-sig unsubscribe: http://mail.python.org/mailman/options/Pythonmac-SIG
Re: [Pythonmac-SIG] If/else vs or
On 28 Oct 2010, at 15:21, Dan Ross wrote:
if x == 'red' or 'green' or 'blue':
if x == 'red' or 'green' or 'blue':
I think your logic might need straightening here, you're saying
if (x == 'red')
or
if 'green'
or
if 'blue'
but I think you mean
if (x == 'red')
or
if (x == 'green')
or
if (x == 'blue')
so try this
if (x == 'red') or (x == 'green') or (x == 'blue'):
or maybe even
if x in ('red', 'green', 'blue'):
*oops* I see Ronald has got his answer in first!
HTH
Dan
Daniel O'Donovan
dan.odono...@gmail.com
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