When iterating over a large array with a range expression, should I use Python's built-in range
function, or numpy's arange
to get the best performance?
My reasoning so far:
range
probably resorts to a native implementation and might be faster therefore. On the other hand, arange
returns a full array, which occupies memory, so there might be an overhead. Python 3's range expression is a generator, which does not hold all the values in memory.
NumPy Arrays Are Faster Than Lists.
The main difference between the two is that range is a built-in Python class, while arange() is a function that belongs to a third-party library (NumPy). In addition, their purposes are different! Generally, range is more suitable when you need to iterate using the Python for loop.
pandas provides a bunch of C or Cython optimized functions that can be faster than the NumPy equivalent function (e.g. reading text from text files). If you want to do mathematical operations like a dot product, calculating mean, and some more, pandas DataFrames are generally going to be slower than a NumPy array.
The main difference between range and np. arange is that the range() function returns an iterator instead of a list and np. arange() function gives a numpy array that consists of evenly spaced values within a given interval. The range() function generates a sequence of integer values lying between a certain range.
For large arrays, a vectorised numpy operation is the fastest. If you must loop, prefer xrange
/range
and avoid using np.arange
.
In numpy you should use combinations of vectorized calculations, ufuncs and indexing to solve your problems as it runs at C
speed. Looping over numpy arrays is inefficient compared to this.
(Something like the worst thing you could do would be to iterate over the array with an index created with range
or np.arange
as the first sentence in your question suggests, but I'm not sure if you really mean that.)
import numpy as np import sys sys.version # out: '2.7.3rc2 (default, Mar 22 2012, 04:35:15) \n[GCC 4.6.3]' np.version.version # out: '1.6.2' size = int(1E6) %timeit for x in range(size): x ** 2 # out: 10 loops, best of 3: 136 ms per loop %timeit for x in xrange(size): x ** 2 # out: 10 loops, best of 3: 88.9 ms per loop # avoid this %timeit for x in np.arange(size): x ** 2 #out: 1 loops, best of 3: 1.16 s per loop # use this %timeit np.arange(size) ** 2 #out: 100 loops, best of 3: 19.5 ms per loop
So for this case numpy is 4 times faster than using xrange
if you do it right. Depending on your problem numpy can be much faster than a 4 or 5 times speed up.
The answers to this question explain some more advantages of using numpy arrays instead of python lists for large data sets.
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