Question

I solved a problem on Project Euler but it took about 4 minutes to run, which is above the recommended time, so I was looking through the different solutions in the forum. One of them included the symbol << in a list comprehension. This is what it looked like

blist.extend([(i << 1) + 3 for i in range(num) if alist.get(i)])  

I can’t find anywhere what exactly this << symbol does. Can someone help me?

Answer 1

It’s a bit shift operator (Python docs), and is common among many programming languages, such as C, Java, PHP, etc. According to the Python docs:

They shift the first argument to the
left or right by the number of bits
given by the second argument.

A right shift by n bits is defined as
division by pow(2, n). A left shift by
n bits is defined as multiplication
with pow(2, n). Negative shift counts
raise a ValueError exception.

So in your specific case, i << 1 means left shift by 1 bit, which is equivalent to multiplying by 2^1, or just 2.