I would like to operate on lists element by element without using numpy, for example, i want add([1,2,3], [2,3,4]) = [3,5,7]
and mult([1,1,1],[9,9,9]) = [9,9,9]
, but i'm not sure which way of doing is it considered 'correct' style.
The two solutions i came up with were
def add(list1,list2):
list3 = []
for x in xrange(0,len(list1)):
list3.append(list1[x]+list2[x])
return list3
def mult(list1, list2):
list3 = []
for x in xrange(0,len(list1)):
list3.append(list1[x]*list2[x])
return list3
def div(list1, list2):
list3 = []
for x in xrange(0,len(list1)):
list3.append(list1[x]/list2[x])
return list3
def sub(list1, list2):
list3 = []
for x in xrange(0,len(list1)):
list3.append(list1[x]-list2[x])
return list3
where each operator is given a separate function
and
def add(a,b)
return a+b
def mult(a,b)
return a*b
def div(a,b)
return a/b
def sub(a,b)
return a-b
def elementwiseoperation(list1, list2, function):
list3 = []
for x in xrange(0,len(list1)):
list3.append(function(list1[x],list2[x]))
return list3
where all the basic functions are defined, and I have a separate function to use them on each element. I skimmed through PEP8, but didn't find anything directly relevant. Which way is better?
The normal way to do this would be to use map
or itertools.imap
:
import operator
multiadd = lambda a,b: map(operator.add, a,b)
print multiadd([1,2,3], [2,3,4]) #=> [3, 5, 7]
Ideone: http://ideone.com/yRLHxW
map
is a c-implemented version of your elementwiseoperation
, with the advantage of having the standard name, working with any iterable type and being faster (on some versions; see @nathan's answer for some profiling).
Alternatively, you could use partial
and map
for a pleasingly pointfree style:
import operator
import functools
multiadd = functools.partial(map, operator.add)
print multiadd([1,2,3], [2,3,4]) #=> [3, 5, 7]
Ideone: http://ideone.com/BUhRCW
Anyway, you've taken the first steps in functional programming yourself. I suggest you read around the topic.
As a general matter of style, iterating by index using range
is generally considered the wrong thing, if you want to visit every item. The usual way of doing this is simply to iterate the structure directly. Use zip
or itertools.izip
to iterate in parallel:
for x in l:
print l
for a,b in zip(l,k):
print a+b
And the usual way to iterate to create a list is not to use append
, but a list comprehension:
[a+b for a,b in itertools.izip(l,k)]
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