Re: python 3: sorting with a comparison function
On 10 Okt., 23:04, Terry Reedy <[EMAIL PROTECTED]> wrote: > Kay Schluehr wrote: > > Me too because I don't get this: > > > "key specifies a function of one argument that is used to extract a > > comparison key from each list element: key=str.lower. The default > > value is None." > > I am not sure what you do not get, but it should say 'for example, > key=str.lower." None is the default value of key. See the discussion above. -- http://mail.python.org/mailman/listinfo/python-list
Re: python 3: sorting with a comparison function
Kay Schluehr wrote: Me too because I don't get this: "key specifies a function of one argument that is used to extract a comparison key from each list element: key=str.lower. The default value is None." I am not sure what you do not get, but it should say 'for example, key=str.lower." None is the default value of key. -- http://mail.python.org/mailman/listinfo/python-list
Re: python 3: sorting with a comparison function
[EMAIL PROTECTED] schrieb: > Kay Schluehr: >> Sometimes it helps when people just make clear how they use technical >> terms instead of invoking vague associations. > > And generally Python docs can enjoy growing few thousands examples... Well, that may not be necessary. But I think that a clear example how to use the 'key=' parameter in the sort() and sorted() method/function is badly needed. Thomas -- http://mail.python.org/mailman/listinfo/python-list
Re: python 3: sorting with a comparison function
On 10 Okt., 20:38, [EMAIL PROTECTED] wrote: > Kay Schluehr: > > > Sometimes it helps when people just make clear how they use technical > > terms instead of invoking vague associations. > > And generally Python docs can enjoy growing few thousands examples... Cleaning up and extending documentation is a large community effort that requires an informational PEP for guidelines and management support by the python-dev leads. The official documentation is ad hoc and just about better than nothing. A Python documentation guideline might also have positive impact on 3rd party package authors like us. Generally Python has become a very well managed project. I hope the docs as well as the stdlib will become the major concerns of Python 3.1. -- http://mail.python.org/mailman/listinfo/python-list
Re: python 3: sorting with a comparison function
On Oct 10, 12:22 pm, [EMAIL PROTECTED] wrote: > On Oct 10, 8:35 am, Kay Schluehr <[EMAIL PROTECTED]> wrote: > > > On 9 Okt., 22:36, [EMAIL PROTECTED] wrote: > > > > Yes, that's a wonderful thing, because from the code I see around > > > 99.9% of people see the cmp and just use it, totally ignoring the > > > presence of the 'key' argument, that allows better and shorter > > > solutions of the sorting problem. > > > Me too because I don't get this: > > > "key specifies a function of one argument that is used to extract a > > comparison key from each list element: key=str.lower. The default > > value is None." > > > Kay > > Don't know if further explanation is needed, but here is the deal: > > cmp is a function that receives two values and you return -1, 0 or 1 > depending if the first is smaller, equal or bigger. 99% of the time > you will do some operation on the values that come in and then do a if > statement with ">" or "<" and return -1,0,1. > > key is a function that receives one value and you return the value > that you would normally compare against. > > Let me show an example: > > >>> data=[(4,'v'),(2,'x'),(1,'a')] > >>> sorted(data) > > [(1, 'a'), (2, 'x'), (4, 'v')] > > OK, we sorted the data, but What if we want to sort by the letter > instead of the number? Let's use cmp: > > >>> def comp(x, y): > > key_of_x=x[1] > key_of_y=y[1] > if key_of_x < key_of_y: > return -1 > elif key_of_x > key_of_y: > return 1 > else: > return 0 #key_of_x == key_of_y > > >>> sorted(data,cmp=comp) > > [(1, 'a'), (4, 'v'), (2, 'x')] > > Very well, so how do we do this using key? > > >>> def keyfunc(x): > > key_of_x=x[1] > return key_of_x > > >>> sorted(data,key=keyfunc) > > [(1, 'a'), (4, 'v'), (2, 'x')] > > Same output. Very good. > > (Of course a smart python developer would use the operator module so > he doesn't even have to write keyfunc but this was just an example) > IIRC, the return values are not limited to -1, 0, and 1, but are more like "any value less than 0", 0, and "any value greater than 0". This allows you to implement numeric cmp routines as: def cmp(x,y): return x-y or just: cmp = lambda x,y: x-y -- Paul -- http://mail.python.org/mailman/listinfo/python-list
Re: python 3: sorting with a comparison function
Kay Schluehr: > Sometimes it helps when people just make clear how they use technical > terms instead of invoking vague associations. And generally Python docs can enjoy growing few thousands examples... Bye, bearophile -- http://mail.python.org/mailman/listinfo/python-list
Re: python 3: sorting with a comparison function
On 10 Okt., 19:22, [EMAIL PROTECTED] wrote: > On Oct 10, 8:35 am, Kay Schluehr <[EMAIL PROTECTED]> wrote: > > > On 9 Okt., 22:36, [EMAIL PROTECTED] wrote: > > > > Yes, that's a wonderful thing, because from the code I see around > > > 99.9% of people see the cmp and just use it, totally ignoring the > > > presence of the 'key' argument, that allows better and shorter > > > solutions of the sorting problem. > > > Me too because I don't get this: > > > "key specifies a function of one argument that is used to extract a > > comparison key from each list element: key=str.lower. The default > > value is None." > > > Kay > > Don't know if further explanation is needed, but here is the deal: > > cmp is a function that receives two values and you return -1, 0 or 1 > depending if the first is smaller, equal or bigger. 99% of the time > you will do some operation on the values that come in and then do a if > statement with ">" or "<" and return -1,0,1. > > key is a function that receives one value and you return the value > that you would normally compare against. > > Let me show an example: > > >>> data=[(4,'v'),(2,'x'),(1,'a')] > >>> sorted(data) > > [(1, 'a'), (2, 'x'), (4, 'v')] > > OK, we sorted the data, but What if we want to sort by the letter > instead of the number? Let's use cmp: > > >>> def comp(x, y): > > key_of_x=x[1] > key_of_y=y[1] > if key_of_x < key_of_y: > return -1 > elif key_of_x > key_of_y: > return 1 > else: > return 0 #key_of_x == key_of_y > > >>> sorted(data,cmp=comp) > > [(1, 'a'), (4, 'v'), (2, 'x')] > > Very well, so how do we do this using key? > > >>> def keyfunc(x): > > key_of_x=x[1] > return key_of_x > > >>> sorted(data,key=keyfunc) > > [(1, 'a'), (4, 'v'), (2, 'x')] > > Same output. Very good. > > (Of course a smart python developer would use the operator module so > he doesn't even have to write keyfunc but this was just an example) > > In summary to transform most cmp functions to a key function you just > take the code that calculates the first value to be compared and leave > out the rest of the logic. > > Hope that was helpful. Yes, thanks a lot. In essence the "key" is a function that maps each list element onto a value of a type for which a known order is defined e.g. an integer. Applying sorted() sorts the list elements according to the list of those values. Sometimes it helps when people just make clear how they use technical terms instead of invoking vague associations. -- http://mail.python.org/mailman/listinfo/python-list
Re: python 3: sorting with a comparison function
On Oct 10, 8:35 am, Kay Schluehr <[EMAIL PROTECTED]> wrote: > On 9 Okt., 22:36, [EMAIL PROTECTED] wrote: > > > Yes, that's a wonderful thing, because from the code I see around > > 99.9% of people see the cmp and just use it, totally ignoring the > > presence of the 'key' argument, that allows better and shorter > > solutions of the sorting problem. > > Me too because I don't get this: > > "key specifies a function of one argument that is used to extract a > comparison key from each list element: key=str.lower. The default > value is None." > > Kay Don't know if further explanation is needed, but here is the deal: cmp is a function that receives two values and you return -1, 0 or 1 depending if the first is smaller, equal or bigger. 99% of the time you will do some operation on the values that come in and then do a if statement with ">" or "<" and return -1,0,1. key is a function that receives one value and you return the value that you would normally compare against. Let me show an example: >>> data=[(4,'v'),(2,'x'),(1,'a')] >>> sorted(data) [(1, 'a'), (2, 'x'), (4, 'v')] OK, we sorted the data, but What if we want to sort by the letter instead of the number? Let's use cmp: >>> def comp(x, y): key_of_x=x[1] key_of_y=y[1] if key_of_x < key_of_y: return -1 elif key_of_x > key_of_y: return 1 else: return 0 #key_of_x == key_of_y >>> sorted(data,cmp=comp) [(1, 'a'), (4, 'v'), (2, 'x')] Very well, so how do we do this using key? >>> def keyfunc(x): key_of_x=x[1] return key_of_x >>> sorted(data,key=keyfunc) [(1, 'a'), (4, 'v'), (2, 'x')] Same output. Very good. (Of course a smart python developer would use the operator module so he doesn't even have to write keyfunc but this was just an example) In summary to transform most cmp functions to a key function you just take the code that calculates the first value to be compared and leave out the rest of the logic. Hope that was helpful. -- http://mail.python.org/mailman/listinfo/python-list
Re: python 3: sorting with a comparison function
On 9 Okt., 22:36, [EMAIL PROTECTED] wrote: > Yes, that's a wonderful thing, because from the code I see around > 99.9% of people see the cmp and just use it, totally ignoring the > presence of the 'key' argument, that allows better and shorter > solutions of the sorting problem. Me too because I don't get this: "key specifies a function of one argument that is used to extract a comparison key from each list element: key=str.lower. The default value is None." Kay -- http://mail.python.org/mailman/listinfo/python-list
Re: python 3: sorting with a comparison function
> Thomas Heller wrote: >> Does Python 3 have no way anymore to sort with a comparison function? >> >> Both [].sort() and sorted() seem to accept only 'key' and 'reverse' >> arguments, >> the 'cmp' argument seems to be gone. Can that be? Terry Reedy schrieb: > Yes. When this was discussed, no one could come up with an actual use > case in which the compare function was not based on a key function. > Calling the key function n times has to be faster than calling a compare > function n to O(nlogn) times with 2 keys computed for each call. The > main counter argument would be if there is no room in memory for the > shadow array of key,index pairs. And that can be at least sometimes > handled by putting the original on disk and sorting an overt key,index > array. Or by using a database. > [EMAIL PROTECTED] schrieb: > Yes, that's a wonderful thing, because from the code I see around > 99.9% of people see the cmp and just use it, totally ignoring the > presence of the 'key' argument, that allows better and shorter > solutions of the sorting problem. So removing the cmp is the only way > to rub the nose of programmers on the right solution, and it goes well > with the Python "There should be one-- and preferably only one -- > obvious way to do it.". Thanks, I got it now. Thomas -- http://mail.python.org/mailman/listinfo/python-list
Re: python 3: sorting with a comparison function
Thomas Heller wrote: Does Python 3 have no way anymore to sort with a comparison function? Both [].sort() and sorted() seem to accept only 'key' and 'reverse' arguments, the 'cmp' argument seems to be gone. Can that be? Yes. When this was discussed, no one could come up with an actual use case in which the compare function was not based on a key function. Calling the key function n times has to be faster than calling a compare function n to O(nlogn) times with 2 keys computed for each call. The main counter argument would be if there is no room in memory for the shadow array of key,index pairs. And that can be at least sometimes handled by putting the original on disk and sorting an overt key,index array. Or by using a database. -- http://mail.python.org/mailman/listinfo/python-list
Re: python 3: sorting with a comparison function
Thomas Heller: > the 'cmp' argument seems to be gone. Can that be? Yes, that's a wonderful thing, because from the code I see around 99.9% of people see the cmp and just use it, totally ignoring the presence of the 'key' argument, that allows better and shorter solutions of the sorting problem. So removing the cmp is the only way to rub the nose of programmers on the right solution, and it goes well with the Python "There should be one-- and preferably only one -- obvious way to do it.". For most of very uncommon situations where key isn't the right thing, you can use this code by Hettinger: def cmp2key(mycmp): "Converts a cmp= function into a key= function" class K: def __init__(self, obj, *args): self.obj = obj def __cmp__(self, other): return mycmp(self.obj, other.obj) return K s.sort(key=cmp2key(lambda p, q: cmp(p.lower(), q.lower( That code can't be used in one situation: when the array to be sorted is huge, that situation can be handled by the original true cmp function, but not by that cmp2key(). But I have met such situation so far. When you have an huge amount of data, use an external sort, even Windows has one, even if its usage is a bit tricky (linux sort command is safer). Bye, bearophile -- http://mail.python.org/mailman/listinfo/python-list
python 3: sorting with a comparison function
Does Python 3 have no way anymore to sort with a comparison function? Both [].sort() and sorted() seem to accept only 'key' and 'reverse' arguments, the 'cmp' argument seems to be gone. Can that be? Thomas -- http://mail.python.org/mailman/listinfo/python-list
