i would like to ask what is the best way to make simple iteration. suppose i want to repeat certain task 1000 times, which one of the following is the best? or is there a better way?
for i in range(1000):
do something with no reference to i
i = 0
while i < 1000:
do something with no reference to i
i += 1
thanks very much
The first is considered idiomatic. In Python 2.x, use xrange instead of range.
xrange, which has almost the exact same performance characteristics (at least in the CPython implementation).
range() creates the list of numbers ahead of time, iterates through it, and then throws it away. xrange() (range() in 3.x) creates a special "generator" object that returns the numbers on demand, which is more efficient. In 3.x, if you actually want the list, you must ask for it: e.g. list(xrange(10)) will use the list constructor to make a list out of the values returned from the generator.
The for loop is more concise and more readable. while loops are rarely used in Python (with the exception of while True).
A bit of idiomatic Python: if you're trying to do something a set number of times with a range (with no need to use the counter), it's good practice to name the counter _. Example:
for _ in range(1000):
# do something 1000 times
_ for a loop but always i, j, k. In the interactive console, _ is the previous result, and in programs _ will often be used for i18n (much less than in other languages as _ is seen more as a throwaway variable in Python).
_('string')? It's a common usage pattern with gettext.In Python 2, use
for i in xrange(1000):
pass
In Python 3, use
for i in range(1000):
pass
Performance figures for Python 2.6:
$ python -s -m timeit '' 'i = 0
> while i < 1000:
> i += 1'
10000 loops, best of 3: 71.1 usec per loop
$ python -s -m timeit '' 'for i in range(1000): pass'
10000 loops, best of 3: 28.8 usec per loop
$ python -s -m timeit '' 'for i in xrange(1000): pass'
10000 loops, best of 3: 21.9 usec per loop
xrange is preferable to range in this case because it produces a generator rather than the whole list [0, 1, 2, ..., 998, 999]. It'll use less memory, too. If you needed the actual list to work with all at once, that's when you use range. Normally you want xrange: that's why in Python 3, xrange(...) becomes range(...) and range(...) becomes list(range(...)).
range continues to perform more or less linearly up to about 130,000 elements or so, and then takes a big jump in time/loop/iteration and continues to get worse. xrange is unstoppable up to at least a hundred million elements, and I would only expect it to start showing any kind of slowdown at all somewhere around the 2-4 billion mark. ;)first. because the integer is done in the internal layer rather than interpretor. Also one less global variable.
