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best Pythonic way to do this sort: Python newb

P: n/a
Hello all

I have build a list that contains data in the form below
-- simplified for question --
myList = [[value1, value2, value3],[value1, value2, value3], ...]

I have a function which takes value3 from the lists above and returns
another value. I want to use this returned value to sort the lists.

So, my resultant list would be ordered by the return value of the
function with value3 as its argument.

From a relative Python newb, what is the best way to do this?

Thanks for any help offered.
Oct 10 '05 #1
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6 Replies


P: n/a
"Sean Berry" <se**@buildingonline.com> writes:
myList = [[value1, value2, value3],[value1, value2, value3], ...]

I have a function which takes value3 from the lists above and returns
another value. I want to use this returned value to sort the lists.

So, my resultant list would be ordered by the return value of the
function with value3 as its argument.

From a relative Python newb, what is the best way to do this?


def get_key(x): return x[2]
sorted_list = sorted(myList, key=get_key)
Oct 10 '05 #2

P: n/a

"Paul Rubin" <http://ph****@NOSPAM.invalid> wrote in message
news:7x************@ruckus.brouhaha.com...
"Sean Berry" <se**@buildingonline.com> writes:
myList = [[value1, value2, value3],[value1, value2, value3], ...]

I have a function which takes value3 from the lists above and returns
another value. I want to use this returned value to sort the lists.

So, my resultant list would be ordered by the return value of the
function with value3 as its argument.

From a relative Python newb, what is the best way to do this?


def get_key(x): return x[2]
sorted_list = sorted(myList, key=get_key)


Sorry if I am missing something. But. what is sorted here?

My simplified function looks like this

def myFunction( myNumber ):
"do some math calculations to myNumber"
return "result of calculations"

So, I want to sort myList by the return of myFunction( value3 )

I tried doing the following... with no luck so far
myList.sort(lambda x, y: cmp(myFunction(x[2]), myFunction(y[2]))

Thanks for any help.
Oct 10 '05 #3

P: n/a
"Sean Berry" <se**@buildingonline.com> writes:
def get_key(x): return x[2]
sorted_list = sorted(myList, key=get_key)
Sorry if I am missing something. But. what is sorted here?


sorted is a built-in function that sorts the thing that you pass it.
It just appeared in Python 2.4, I think. With older versions, yeah,
you have to use the .sort method that sorts in place.
I tried doing the following... with no luck so far
myList.sort(lambda x, y: cmp(myFunction(x[2]), myFunction(y[2]))


That looks ok to me.
x = [(i,i*i,1 + 17*i**2 - i**3) for i in range(20)]
x [(0, 0, 1), (1, 1, 17), (2, 4, 61), (3, 9, 127), (4, 16, 209), (5, 25,
301), (6, 36, 397), (7, 49, 491), (8, 64, 577), (9, 81, 649), (10,
100, 701), (11, 121, 727), (12, 144, 721), (13, 169, 677), (14, 196,
589), (15, 225, 451), (16, 256, 257), (17, 289, 1), (18, 324, -323),
(19, 361, -721)] x.sort(lambda a,b:cmp(a[2],b[2]))
x [(19, 361, -721), (18, 324, -323), (0, 0, 1), (17, 289, 1), (1, 1,
17), (2, 4, 61), (3, 9, 127), (4, 16, 209), (16, 256, 257), (5, 25,
301), (6, 36, 397), (15, 225, 451), (7, 49, 491), (8, 64, 577), (14,
196, 589), (9, 81, 649), (13, 169, 677), (10, 100, 701), (12, 144,
721), (11, 121, 727)]

Oct 10 '05 #4

P: n/a
(sorted is a built-in function in 2.4)

def myFunction( data ):
""" Take one of your set of 3, grab [2] (the 3rd) and do calcs,
return value """
"do some math calculations to data[2]"
return "result of calculations"

sorted_list = sorted(myList, key=myFunction)

List is sorted in the order of the 'key' values, key being a value
returned from myFunction which operates on [2].

Oct 10 '05 #5

P: n/a

----
Satchidanand Haridas (sharidas at zeomega dot com)

ZeOmega (www.zeomega.com)
Open Minds' Open Solutions

Sean Berry wrote:
"Paul Rubin" <http://ph****@NOSPAM.invalid> wrote in message
news:7x************@ruckus.brouhaha.com...

"Sean Berry" <se**@buildingonline.com> writes:

myList = [[value1, value2, value3],[value1, value2, value3], ...]

I have a function which takes value3 from the lists above and returns
another value. I want to use this returned value to sort the lists.

So, my resultant list would be ordered by the return value of the
function with value3 as its argument.

From a relative Python newb, what is the best way to do this?

def get_key(x): return x[2]
sorted_list = sorted(myList, key=get_key)


Sorry if I am missing something. But. what is sorted here?

My simplified function looks like this

def myFunction( myNumber ):
"do some math calculations to myNumber"
return "result of calculations"

So, I want to sort myList by the return of myFunction( value3 )

I tried doing the following... with no luck so far
myList.sort(lambda x, y: cmp(myFunction(x[2]), myFunction(y[2]))

I think the above statement should be as follows:

myList.sort(lambda x, y: cmp(myFunction(x[2]) - myFunction(y[2]))

hope that helps.

regards,
Satchit
Oct 10 '05 #6

P: n/a
"Satchidanand Haridas" <sh******@zeomega.com> wrote:
So, I want to sort myList by the return of myFunction( value3 )

I tried doing the following... with no luck so far
myList.sort(lambda x, y: cmp(myFunction(x[2]), myFunction(y[2]))

I think the above statement should be as follows:

myList.sort(lambda x, y: cmp(myFunction(x[2]) - myFunction(y[2]))

hope that helps.


It would help more if you tested it before you posted. cmp takes two arguments (let alone that
subtraction may not be defined for the list elements), so the original version is correct.

George
Oct 10 '05 #7

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