On Mon, Aug 1, 2011 at 7:51 PM, Noufal Ibrahim <nou...@gmail.com> wrote:
> Anand Balachandran Pillai <abpil...@gmail.com> writes:
>
>> On Mon, Aug 1, 2011 at 6:08 AM, Anand Chitipothu <anandol...@gmail.com>wrote:
>
> [...]
>
>> It is more subtler than that.
>>
>> List comprehensions are faster than map functions when
>> the latter needs to invoke a user-defined function call or a lambda.
>>
>> Maps score over list comprehensions in most cases where the function
>> is a Python built-in and when no lambda is used.
>>
>> Example of former:
>>
>>>>> def f1(): map(sqr, range(1, 100))
>> ...
>>>>> def f2(): [sqr(x) for x in range(1, 100)]
>> ...
>>>>> mytimeit.Timeit(f1)
>> '37.91 usec/pass'
>>>>> mytimeit.Timeit(f2)
>> '37.50 usec/pass'
>>
>> Example of latter:
>>
>>>>> def f1(): map(hex, range(1, 100))
>> ...
>>>>> def f2(): [hex(x) for x in range(1, 100)]
>> ...
>>>>> mytimeit.Timeit(f1)
>> '49.41 usec/pass'
>>>>> mytimeit.Timeit(f2)
>> '55.29 usec/pass'
>
> This is confusing. Why is
>
> map(sqr, range(1, 100))
>
> faster than
>
> map(hex, range(1, 100))
>
> Assuming sqr is implemented in python, it should be slower than hex
> which is implemented in C.
Here's what I get (note: sqrt is faster than hex - not sqr)
Program
=======
from math import sqrt
from timeit import Timer
def sqr(x) : x * x
print "Simple sqr", Timer("sqr(50)","from __main__ import sqr").timeit()
print "Simple sqrt", Timer("sqrt(50)","from math import sqrt").timeit()
print "Simple hex", Timer("hex(50)").timeit()
print "Map sqr", Timer("map(sqr,range(1,100))","from __main__
import sqr").timeit()
print "Map sqrt", Timer("map(sqrt,range(1,100))","from math import
sqrt").timeit()
print "Map hex", Timer("map(hex,range(1,100))").timeit()
Output
======
Simple sqr 0.185955047607
Simple sqrt 0.108409881592
Simple hex 0.143438816071
Map sqr 21.4051530361
Map sqrt 12.3786129951
Map hex 13.8608310223
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