difference between JavaScript bit-wise operator code and Python bit-wise operator code

Question

I have converted JavaScript code which uses bit-wise operators in that code to Python code, but there is one problem when i do this in JavaScript and Python

412287 << 10

then I get this 422181888 same results in both languages. but when i do this in both

424970184 << 10

then i get different results in both of the languages 1377771520 in JavaScript and 435169468416 in Python

can anybody help me with this? any help would be appreciated.

Taufiq Rahman · Accepted Answer · 2021-01-12 13:14:13Z

6

If you want the JavaScript equivalent value then what you can do is :

import ctypes

print(ctypes.c_int(424970184 << 10 ^ 0).value)

Output:

1377771520

edited Jan 12, 2021 at 13:14

answered Jan 12, 2017 at 10:09

Taufiq Rahman

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5 Comments

VASIM SETA Over a year ago

Hi @Achilles thank you for your answer, I have tried this and it is working, will it work for >> operator as well?

Taufiq Rahman Over a year ago

Yes,this would get the same overflow behavior as Javascript.

VASIM SETA Over a year ago

Thanks @Achilles, last question will it work for all bit-wise operators? like ^ operator?

Taufiq Rahman Over a year ago

Yes it should work without a problem as it's only limiting the unlimited integer precision of Python.

VASIM SETA Over a year ago

Thank you @Achilles for your all replies, it helps me a lot.

Community · Accepted Answer · 2017-05-23 12:13:58Z

4

As stated in this SO answer, in javascript the bitwise operators and shift operators operate on 32-bit ints, and your second example overflows the 32 bit capacity, so the python equivalent would be:

(424970184 << 10) & 0x7FFFFFFF

(you get a "modulo"/"masked" value with the signed 32 bit integer mask, not the actual value)

In Python there's no limit in capacity for integers, so you get the actual value.

edited May 23, 2017 at 12:13

CommunityBot

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answered Jan 12, 2017 at 10:05

Jean-François Fabre♦

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6 Comments

willeM_ Van Onsem Over a year ago

Afaik, JavaScript uses 54 bits to represent numbers.

Jean-François Fabre Over a year ago

but (424970184 << 10) & (2**53-1) yields 435169468416

willeM_ Van Onsem Over a year ago

Ah but the bitwise operators operate on 32 bits. But an int itself has storage for 54 bits.

Jean-François Fabre Over a year ago

just checked my self. Thanks for pointing this out, my answer would have been incorrect without that info.

RemcoGerlich Over a year ago

@WillemVanOnsem: JavaScript numbers are actually double-precision (8 byte) floats, not ints. Doubles can represent integers up to 2^53 exactly, just like a 54-bit int could, but that's not what they are.

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