[Python] Don't use "<<" to double a number

As we know, we can use "a << b" to represent "a*(2^b)." But this method is much slower than just using addition, performing "a+=a" for b times. In other words, "<<" operation is much more expensive than "+" operation.

I have performed some simple experiments. The results are shown below.

	import datetime
	"""test << operation"""
	# Test A
	x = 3
	print datetime.datetime.now() #2015-05-28 10:00:34.886600
	x <<=1
	print datetime.datetime.now() #2015-05-28 10:00:34.887079
	# Test B
	x = 3
	print datetime.datetime.now() #2015-05-28 10:02:51.338504
	x <<=10
	print datetime.datetime.now() #2015-05-28 10:02:51.339138
	"""test + operation"""
	#Test C
	x = 3
	print datetime.datetime.now() #2015-05-28 10:00:34.887085
	x += x
	print datetime.datetime.now() #2015-05-28 10:00:34.887089
	# Test D
	x = 3
	print datetime.datetime.now() #2015-05-28 10:02:51.339146
	x += x
	x += x
	x += x
	x += x
	x += x
	x += x
	x += x
	x += x
	x += x
	x += x
	print datetime.datetime.now() #2015-05-28 10:02:51.339152

view raw CostofDouble.py hosted with ❤ by GitHub

As we can see, the running time for "<<" operations are much slower than that of "+" operation. It takes more than several hundred times for "<<" to produce the same result as "+" does. Therefore, using "+" operation instead of using "<<" for the same purpose. (Here I am under the restriction of not using multiplication.)