Bitwise differences in Java/Perl

Question

I was messing around and noticed differences that I don't understand between Java and Perl when I bit-shifted -1.

I thought integers are stored in two's complement binary, so if there are 32 bits, -1 is 11...11 (32 times).
I'd expect -1 >> 1 to give $2^31 - 1$ = 011...11 = 2147483647.
I'd expect -1 << 1 to give = 11...110 = -2.

What is the reason for these different behaviors and where are these standards stated in different languages?

Code and printouts for Perl & Java below:

In Perl:

print (-1 >> 1, "\n");
print (-1 << 1, "\n");

2147483647

4294967294

In Java:

public class Tempy {
    public static void main(String[] args){
        System.out.println( -1 >> 1); 
        System.out.println( -1 << 1);
    }
}

-1

-2

twain249 twain249 · Accepted Answer · 2015-07-31T13:51:23

Perl's bit shift is inherently unsigned so -1 is treated as 2^32 -1 and it automatically fills with 0 so -1 >> 1 is 2^31-1 and -1 << 1 is 2^32-2.

[Edit] Thanks @Powerlord using integer will force perl to use signed values.

Java's bit shift sign extends (if using >>) so -1 << 1 is still -1 and -1 >> 1 is -2. If you don't want to sign extend you have to use the logical version >>>.

Bitwise differences in Java/Perl

1 Answers