I don't quite understand the Shift Bitwise operators >> and <<

Hi everyone,

I don’t understand how these two shift operators work. I’d be thankful for any help.

shift_right = 0b1100
shift_left = 0b1

shift_right = shift_right >> 2
shift_left = shift_left << 2

print bin(shift_right) #0b11
print bin(shift_left)   #0b100

Answer 52930f9280ff33550c0001c7

25 votes

Permalink

Remember that little trick we all taught in primary school when it comes to multiplying by 10, 100, 1000 etc. We shifted the numbers to the left by 1,2,3 decimal places. Dividing by 10, 100, 1000 etc was a rightward shift by 1,2 or 3.

So a bitwise shift to the left is like multiplying your binary number by 2,4,8,16 etc depending on how many bits you shifted it to the left. A bitwise shift to the right is a like a floor division by 2,4,8,16 etc depending on the number of bits you shifted you number to the right.

shift_left was 1(in binary and decimal) you shifted it two places to the left, multiply by 4, 4 in binary is 100

shift_right was 1100(12 in decimal units) you shifted it two places to the right, division by 4 which gives you 3 in decimal units) 3 in binary form is 11

Submitted by tyrantmonkey

over 11 years

4 comments

Submitted by Undeadbanana

over 11 years

Good comment ; )

Submitted by trishna_g

almost 11 years

Thanks a lot for making me understand :)

Submitted by Schweicen

over 10 years

Great answer, but why mathematically it’s FLOOR division in this case?

Submitted by anonymous

over 10 years

Schweicen, I’m sure someone else will be able to answer this better than I can but I believe it has to do with the fact that it only deals with integers. For example, 14.8 as an integer in Python is read as 14 only. It does not round up, it only takes into consideration the integer in the number. Hope this helps and IF I’m wrong someone please correct me as I’d like to know if I’m right or learn a little from this as well. Good day!

Answer 523eeda5f10c60d1fe003e95

7 votes

Permalink

Note the position of the binary digits before and after the shifts.

0b1100 shifted by two bits to the right moves the 0s off the end and the two 1s to the right-most positions.

0b1 shifted by two bits to the left moves the 1 to the left by two places, and must fill in the two right-most bits with 0s.

Submitted by AppylPye

over 11 years

3 comments

Submitted by kp_569

over 11 years

What I do not understand is what is the whole purpose of this?… Any practical reason that we need to go through all these binary exercises?

Submitted by songofsongs

over 11 years

Same page. I understand how the operators work but not how can this possibly be used practically? Why would we multiply with 2,4,16, etc using these? Is is that much faster than math operations?

Submitted by KuroSetsuna29

over 11 years

As mentioned, bitwise operators are not used in most practical programming applications, however it is nice to at least recognize them when you see them as they are not very hard to understand what they do. There is also a very small niche of programming that uses bitwise operators extensively, most of which deal with hardware and low-level programming.

Answer 52fafb08282ae39f72000c50

-1 votes

Permalink

Oh…thats how the code works! It sure is nice of codeacadamy to provide a decent example for this problem. Silly me, I figured that it would work by something like: slide_right(>>2) or even slide_right = (>>2) but no, once again, the instruction fail completely to define the actual problem. It sure is a good thing that there is such a huge database of help forms.