Getting a specific group of binary numbers in an Int?

Question

If I am trying to get a specific range of bits (in decimal) at a specific range, is the best way to go about this leftshifting to the end index, and then right shifting back to the start index?

#pesudo-code:
    def group(aNumber,start, end):
        n = aNumber << 32 - n
        a = n >> end - start
        print(a)

"trying to get a specific range of bits (in decimal) at a specific range" - what ??? — Nir Alfasi
– Nir Alfasi, Commented Aug 29, 2015 at 22:49
Sorry for the confusion. If I have an int, I would like to extract the group of binary numbers that represent a certain index of that int. e.g. 5000, if end = 3 and start = 2, the binary number output should be 50. e.g. 0110 0010 — Shan
– Shan, Commented Aug 29, 2015 at 22:51
That won't work. Because Python will automatically turn the int into a long (which is essentially unlimited in size, and only constrained by your memory), so you can never get rid of bits on the left. Eg, 5 << 64 = 92233720368547758080L. Keep in mind that integers in Python don't work like those in C. — texasflood
– texasflood, Commented Aug 29, 2015 at 22:53

spectras · Accepted Answer · 2015-08-29 23:07:14Z

1

You can do that in python, you're just not using the correct method. When you want to retrieve part of a number, the operation you're trying to accomplish is called masking.

And you do that with a mask: number & 0xF0 will retreive bits 4 to 7 (without moving them). So (number & 0xF0) >> 4 will retrieve bits 4 to 7, and shift them all the way to the right.

As for the mask, ((1 << end) - 1) - ((1 << start) - 1) should build a correct mask. So...

mask = ((1 << end) - 1) - ((1 << start) - 1)
result = (number & mask) >> start

Actually, since you shift to the right right after, you don't even have to mask out the bits below those you want, so this will work just as well:

mask = ((1 << end) - 1)
result = (number & mask) >> start

edited Aug 29, 2015 at 23:07

answered Aug 29, 2015 at 22:58

spectras

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texasflood Over a year ago

Does this answer the question? I think the OP wanted a mask, but they wanted one in decimal, not in binary, like you've done. E.g. group(5000,2,3) = 50, which is not what your code does.

rodrigo · Accepted Answer · 2015-08-29 23:04:51Z

1

If you want to get from bit b0 a total of bn bits:

def getbits(x, b0, bn):
    return (x >> b0) & ((1 << bn) - 1)

answered Aug 29, 2015 at 23:04

rodrigo

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