Re: Converting a list of strings into a list of integers?
On Jul 23, 7:27 pm, Grant Edwards wrote: > That said, "map" seems to be frowned upon by the Python community for > reasons I've never really understood,... Maybe the analogy: comprehension : map:: relational calculus : relational algebra In particular map, filter correspond to project and select in algebra. In principle the two are equivalent (Codd's theorem) however in practice, the calculus is found to be more declarative whereas the algebra is more suitable for specifying execution plans. -- http://mail.python.org/mailman/listinfo/python-list
Re: Converting a list of strings into a list of integers?
On 2012-07-22, Jan Riechers wrote: > I am not sure why everyone is using the for-iterator option over a > "map", but I would do it like that: > > MODUS_LIST= map(int, options.modus_list) > > "map" works on a list and does commandX (here "int" conversion, use > "str" for string.. et cetera) on sequenceY, returning a sequence. More > in the help file. "map" is what comes to mind first for me, but that's probably because 1) Before I learned Python, I learned other more functional languages where map was the definitive answer. 2) When I first learned Python it didn't have list comprehensions. That said, "map" seems to be frowned upon by the Python community for reasons I've never really understood, and most people are going to prefer reading a list comprehension. "What most people are going to prefer reading" does matter... -- Grant Edwards grant.b.edwardsYow! ... the MYSTERIANS are at in here with my CORDUROY gmail.comSOAP DISH!! -- http://mail.python.org/mailman/listinfo/python-list
Re: Converting a list of strings into a list of integers?
On 07/22/2012 11:29 AM, Tony the Tiger wrote: > Hi, > Is there such a thing in the language, or do I have to invent it myself? > > I came up with the following: > > # options.modus_list contains, e.g., "[2,3,4]" > # (a string from the command line) > > > > So which is it, a list of strings, or a string? Your subject line does not agree with the comment. -- DaveA -- http://mail.python.org/mailman/listinfo/python-list
Re: Converting a list of strings into a list of integers?
On 22.07.2012 20:01, Steven D'Aprano wrote: [SNIP] map is faster than an ordinary for-loop if the function you are applying is a builtin like int, str, etc. But if you have to write your own pure- Python function, the overhead of calling a function negates the advantage of map, which is no faster than a for-loop. For example: results = map(int, sequence) # calls builtin `int` hoists the call to int into the fast C layer, instead of the slow Python layer, and should be faster than results = [] for x in sequence: results.append(int(x)) which runs at the speed of Python. But: results = map(lambda x: x+1, sequence) # calls pure Python function if no faster than a for-loop: results = [] for x in sequence: results.append(x+1) Note: this has*nothing* to do with the use of lambda. Writing the "+1" function above using def instead of lambda would give the same results. [SNAP] Hi Steven, besides that I testdrive Pypy (and still am impressed, other topic) - your answer was what I was picking for ;) Especially this part of you: > map is faster than an ordinary for-loop if the function you are applying > is a builtin like int, str, etc. [underlaying c-layer] But if you have to write your own pure- > Python function, the overhead of calling a function negates the advantage > of map [...] I did not know that the speed gain is up foremost present when using built-ins, but that's for sure something to keep in mind when writing code. Thanks for your explanation, clarifies a lot! Jan -- http://mail.python.org/mailman/listinfo/python-list
Re: Converting a list of strings into a list of integers?
On Sun, Jul 22, 2012 at 10:20 AM, Jan Riechers wrote: > Hi, > > I am not sure why everyone is using the for-iterator option over a "map", > but I would do it like that: > > MODUS_LIST= map(int, options.modus_list) > > "map" works on a list and does commandX (here "int" conversion, use "str" > for string.. et cetera) on sequenceY, returning a sequence. More in the help > file. > > And if I'm not completely mistaken, it's also the quicker way to do > performance wise. But I can't completely recall the exact reason. My recollection is that map has the edge if you can pass it a built-in or a C extension function, like int, or a complicated Python function that you would end up calling anyway in the list comprehension. The situation changes though if you can write the comprehension to remove the overhead of the Python function call. For example: map(lambda x: x+1, my_list) [x+1 for x in my_list] By performing the addition inline instead of calling a function to do it, the list comprehension wins performance-wise in this scenario. So as a simple rule of thumb I will typically choose between map or a comprehension based on whether I need to call a function or not (and also based on how pretty or ugly the resulting code is). Anything further would just be premature optimization. Also keep in mind that in Python 3 map returns an iterator instead of a list, so for a fair comparison you would have to compose the map with a list() call. Cheers, Ian -- http://mail.python.org/mailman/listinfo/python-list
Re: Converting a list of strings into a list of integers?
On 22.07.2012 20:03, David Robinow wrote: On Sun, Jul 22, 2012 at 12:20 PM, Jan Riechers wrote: On 22.07.2012 18:39, Alister wrote: looks like a classic list comprehension to me and can be achieved in a single line MODUS_LIST=[int(x) for x in options.modus_list] Hi, I am not sure why everyone is using the for-iterator option over a "map", but I would do it like that: MODUS_LIST= map(int, options.modus_list) "map" works on a list and does commandX (here "int" conversion, use "str" for string.. et cetera) on sequenceY, returning a sequence. More in the help file. And if I'm not completely mistaken, it's also the quicker way to do performance wise. But I can't completely recall the exact reason. Because if you don't have a functional background 'map' is unfamiliar. Although I've been aware of it for years I still can't remember if it's map(int, list) or map(list,int) and although with a small effort I could force it into my brain, I know that many of the people reading my code are as ignorant as I am. The list comprehension seems clearer to me. Hello, no offense by that, I just was wondering why everyone uses the list comprehension instead the built-in map in this case - I'm still using Python 2.7.3 so perhaps things might have changed a little. :) So far Jan -- http://mail.python.org/mailman/listinfo/python-list
Re: Converting a list of strings into a list of integers?
On Sun, 22 Jul 2012 19:20:18 +0300, Jan Riechers wrote: > "map" works on a list and does commandX (here "int" conversion, use > "str" for string.. et cetera) on sequenceY, returning a sequence. More > in the help file. > > And if I'm not completely mistaken, it's also the quicker way to do > performance wise. But I can't completely recall the exact reason. The following only applies the standard CPython implementation. Other implementations may be different. In particular, PyPy turns everything you know about optimizing Python code on its head, and can often approach the speed of optimized C code in pure Python. map is faster than an ordinary for-loop if the function you are applying is a builtin like int, str, etc. But if you have to write your own pure- Python function, the overhead of calling a function negates the advantage of map, which is no faster than a for-loop. For example: results = map(int, sequence) # calls builtin `int` hoists the call to int into the fast C layer, instead of the slow Python layer, and should be faster than results = [] for x in sequence: results.append(int(x)) which runs at the speed of Python. But: results = map(lambda x: x+1, sequence) # calls pure Python function if no faster than a for-loop: results = [] for x in sequence: results.append(x+1) Note: this has *nothing* to do with the use of lambda. Writing the "+1" function above using def instead of lambda would give the same results. List comprehensions are at least as fast as map, since they too hoist the calculation into the fast C layer. They have the added advantage that they can calculate arbitrarily complex Python expressions in the C layer without needing an intermediate function. So: map(lambda x: x**2 - 3, sequence) runs more-or-less at the speed of an ordinary for-loop, but the list comprehension version: [x**2 - 3 for x in sequence] should be faster and doesn't rely on an intermediate function. So in general, a list comprehension will be no slower than map, and may be faster; both will be no slower than a for-loop, and may be faster. Or at least, this was the case last time I checked. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Converting a list of strings into a list of integers?
Tony the Tiger writes: > # options.modus_list contains, e.g., "[2,3,4]" Try this: import ast MODUS_LIST = ast.literal_eval(options.modus_list) literal_eval is like eval except it can only evaluate literals rather than calling functions and the like. The idea is you can use it on untrusted data. -- http://mail.python.org/mailman/listinfo/python-list
Re: Converting a list of strings into a list of integers?
On Sun, Jul 22, 2012 at 12:20 PM, Jan Riechers wrote: > On 22.07.2012 18:39, Alister wrote: >> looks like a classic list comprehension to me and can be achieved in a >> single line >> MODUS_LIST=[int(x) for x in options.modus_list] > Hi, > > I am not sure why everyone is using the for-iterator option over a "map", > but I would do it like that: > MODUS_LIST= map(int, options.modus_list) > > "map" works on a list and does commandX (here "int" conversion, use "str" > for string.. et cetera) on sequenceY, returning a sequence. More in the help > file. > > And if I'm not completely mistaken, it's also the quicker way to do > performance wise. But I can't completely recall the exact reason. Because if you don't have a functional background 'map' is unfamiliar. Although I've been aware of it for years I still can't remember if it's map(int, list) or map(list,int) and although with a small effort I could force it into my brain, I know that many of the people reading my code are as ignorant as I am. The list comprehension seems clearer to me. -- http://mail.python.org/mailman/listinfo/python-list
Re: Converting a list of strings into a list of integers?
Tony the Tiger wrote:
> On Sun, 22 Jul 2012 11:39:30 -0400, Roy Smith wrote:
>
>> To answer the question you asked, to convert a list of strings to a list
>> of ints, you want to do something like:
>>
>> MODUS_LIST = [int(i) for i in options.modus_list]
>
> Thanks. I'll look into that. I now remember reading about the technique
> (in Mark Lutz' "Learning Python"), but it seems I'm getting old as I tend
> to forget about it from time to time. ;)
>
>> But, to answer the question you didn't ask, if you're trying to parse
>> command-line arguments, you really want to use the argparse module. It's
>> a little complicated to learn, but it's well worth the effort.
>
> Your suggestions about the argparse. Well, it seems it does pretty much
> the same as OptionParser which I use now. Perhaps it has more features
> (that I probably won't need in my 30 line script), I only need to keep
> track of maybe one or two options. Maybe one of these days, when I have
> little else to do, or when the OptionParser stops working, I'll give it a
> try. Thanks. :)
Here's an argparse example:
$ cat argparse_list.py
import argparse
parser = argparse.ArgumentParser()
parser.add_argument("-m", "--modus", type=int, nargs="*")
print parser.parse_args().modus
$ python argparse_list.py
None
$ python argparse_list.py -m
[]
$ python argparse_list.py -m 1
[1]
$ python argparse_list.py -m 1 2 3
[1, 2, 3]
--
http://mail.python.org/mailman/listinfo/python-list
Re: Converting a list of strings into a list of integers?
On 22.07.2012 18:39, Alister wrote: On Sun, 22 Jul 2012 10:29:44 -0500, Tony the Tiger wrote: I came up with the following: # options.modus_list contains, e.g., "[2,3,4]" # (a string from the command line) # MODUS_LIST contains, e.g., [2,4,8,16] # (i.e., a list of integers) if options.modus_list: intTmp = [] modTmp = options.modus_list[1:-1] for itm in modTmp: intTmp.append(int(itm)) MODUS_LIST = intTmp TIA /Grrr looks like a classic list comprehension to me and can be achieved in a single line MODUS_LIST=[int(x) for x in options.modus_list] Hi, I am not sure why everyone is using the for-iterator option over a "map", but I would do it like that: MODUS_LIST= map(int, options.modus_list) "map" works on a list and does commandX (here "int" conversion, use "str" for string.. et cetera) on sequenceY, returning a sequence. More in the help file. And if I'm not completely mistaken, it's also the quicker way to do performance wise. But I can't completely recall the exact reason. Jan -- http://mail.python.org/mailman/listinfo/python-list
Re: Converting a list of strings into a list of integers?
On Sun, 22 Jul 2012 10:29:44 -0500, Tony the Tiger wrote: > Hi, > Is there such a thing in the language, or do I have to invent it myself? > > I came up with the following: > > # options.modus_list contains, e.g., "[2,3,4]" > # (a string from the command line) > # MODUS_LIST contains, e.g., [2,4,8,16] > # (i.e., a list of integers) > > if options.modus_list: > intTmp = [] > modTmp = options.modus_list[1:-1] > for itm in modTmp: > intTmp.append(int(itm)) > MODUS_LIST = intTmp > > There are probably never more than maybe between one to four items in > the options.modus_list, and its contents as integers should always > replace all of the original MODUS_LIST, because it is up to the user to > decide what should be used for calculating the result. > > The above works (unless I have introduced some bug when I copied into my > editor here), but I would like to know if there already is such a thing, > or something better than the above. I'd hate to re-invent the wheel. > > TIA > > > /Grrr looks like a classic list comprehension to me and can be achieved in a single line MODUS_LIST=[int(x) for x in options.modus_list] -- NOTICE: -- THE ELEVATORS WILL BE OUT OF ORDER TODAY -- (The nearest working elevator is in the building across the street.) -- http://mail.python.org/mailman/listinfo/python-list
Re: Converting a list of strings into a list of integers?
In article <3rcdnuciwpp1gzhnnz2dnuvz7vqaa...@giganews.com>, Tony the Tiger wrote: > Hi, > Is there such a thing in the language, or do I have to invent it myself? > > I came up with the following: > > # options.modus_list contains, e.g., "[2,3,4]" > # (a string from the command line) > # MODUS_LIST contains, e.g., [2,4,8,16] > # (i.e., a list of integers) > > if options.modus_list: > intTmp = [] > modTmp = options.modus_list[1:-1] > for itm in modTmp: > intTmp.append(int(itm)) > MODUS_LIST = intTmp To answer the question you asked, to convert a list of strings to a list of ints, you want to do something like: MODUS_LIST = [int(i) for i in options.modus_list] But, to answer the question you didn't ask, if you're trying to parse command-line arguments, you really want to use the argparse module. It's a little complicated to learn, but it's well worth the effort. -- http://mail.python.org/mailman/listinfo/python-list
