python:loop vs comprehension [duplicate]

Question

This question already has an answer here:

• Are list-comprehensions and functional functions faster than “for loops”? 6 answers

I was trying to do some simple procedures using lists. From the book learning python I saw the method of using a comprehension. Well I also knew that a loop could replace it. Now I really want to know that which is faster, loop or comprehension. These are my programs.

a = []
for x in range(1, 101):
    a.append(x)

This would set a as [1, 2, 3, ......, 99, 100]

Now this is what I have done with the comprehension.

[x ** 2 for x in a]

This is what I did with the loop.

c = []
for x in a:
    b=[x**2]
    c+=b

Could any one say a way to find which of the above is faster.Please also try to explain that how comprehensions differ from loops. Any help is appreciated.

Answer 1

You can use the timeit library, or just use time.time() to time it yourself:

>>> from time import time
>>> def first():
...     ftime = time()
...     _foo = [x ** 2 for x in range(1, 101)]
...     print "First", time()-ftime
... 
>>> def second():
...     ftime = time()
...     _foo = []
...     for x in range(1, 101):
...             _b=[x**2]
...             _foo+=_b
...     print "Second", time()-ftime
... 
>>> first()
First 5.60283660889e-05
>>> second()
Second 8.79764556885e-05
>>> first()
First 4.88758087158e-05
>>> second()
Second 8.39233398438e-05
>>> first()
First 2.8133392334e-05
>>> second()
Second 7.29560852051e-05
>>> 

Evidently, the list comprehension runs faster, by a factor of around 2 to 3.