CODEWITHSUNDEEP
pythonbit-shiftnegative-number
aaron-prindle
aaron-prindle

Reputation: 3317

Python Bit Shifting Negative vs positive values

I was recently going over bit shifting and was wondering why in the below iPython output, shifting the value -1 vs shifting the value 4294967295 yielded different results?

In [30]: val = -1

In [31]: print "hex x %d 0x%08X" % (val, val & 0xffffffff)

hex x -1 0xFFFFFFFF

In [32]: val = val >>22

In [33]: print "hex x %d 0x%08X" % (val, val & 0xffffffff)

hex x -1 0xFFFFFFFF

In [34]: val = 4294967295

In [35]: print "hex x %d 0x%08X" % (val, val & 0xffffffff)

hex x 4294967295 0xFFFFFFFF

In [36]: val = val >>22

In [37]: print "hex x %d 0x%08X" % (val, val & 0xffffffff)

hex x 1023 0x000003FF

I would really appreciate any clarification, thanks.

Upvotes: 3

Views: 3955

Answers (3)

Karl Knechtel
Karl Knechtel

Reputation: 61643

Short version: Because the operation is defined that way.

Long version: per the documentation, "The rules for integer representation are intended to give the most meaningful interpretation of shift and mask operations involving negative integers." In a system where numbers can be arbitrarily sized but have 2s complement representation, it follows that - at least conceptually - the sign extension carries arbitrarily far as well. So this is really the only interpretation that makes sense.

This preserves the invariant that x >> n <=> int(x / (2 ** n)) across the negative numbers as well.

Upvotes: 2

Mark Ransom
Mark Ransom

Reputation: 308530

When a negative number is used in an expression, by the rules of two's complement arithmetic the sign bit must be extended all the way to the left bit of the expression. A right shift is preserving those left-most 1 bits. A negative number shifted right by a sufficient number of bits will always equal -1, just as a positive number shifted right by a sufficient number of bits will always equal 0.

Upvotes: 1

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 799390

4294967295 is 0x0...0FFFFFFFF. -1 is 0xF...FFFFFFFF. Forever.

Upvotes: 2

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