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shift operators

Abhishek Reddy
Ranch Hand

Joined: Mar 28, 2006
Posts: 259
class A
{
public static void main(String[] args)
{
test(1<<32, "1<<32");
test(1<<31, "1<<31");
test(1<<30, "1<<30");
test(1, "1");
test(0, "0");
test(-1, "-1");
}
public static void test(int i, String exp)
{
if ((i>>1)!=(i>>>1))
{
System.out.println(exp);
}
}
}




can any one help me out with the output of this program..and kindly explain me......


Abhishek
Swapna Omprakash
Greenhorn

Joined: Aug 22, 2006
Posts: 1
This program prints 1 << 31
-1

The range of int primitive is -2^31 to (2^31) - 1.

and so (1 << 31) does not fit in an int and results in a negative number.

where as due to the masking of bits,
when the value to be shifted is an int,
for the shift values of >=32, the actual size of the shift is the value of the right-hand operand masked by 31.
ie.the shift distance is always between 0 and 31.
if shift value is > 32 shift is value%32

So here, 1 << 32 = 1.
Ex:

16 << 35 => 16 << (35 % 32) => 16 << 3 = 128
16 << 32 => 16 << (32 % 32) => 16 << 0 = 16

16 >> 35 => 16 >> (35 % 32) = 2
16 >> 32 => 16 >> (32 % 32) = 16


similarly for longs
for the shift values of >=64, the actual size of the shift is the value of the right-hand operand masked by 63.
ie.the shift distance is always between 0 and 63.
if shift value is >= 64 shift is value%64

And this question tests one more concept,

in unsigned right shift operation,
if the left-hand operand is negative, the result is equivalent to the left-hand operand right-shifted by the number indicated by the right-hand operand plus two left-shifted by the inverted value of the right-hand operand.
So here -1 >>> 1 = (-1 >> 1) + (2 << ~1) = 2147483647.

and since -1 >> 1 = -1 they both are not equal.

Ex: -16 >>> 2 = (-16 >> 2 ) + ( 2 << ~2 )
---------------------------------------------
Swapna.
Andy Morris
Ranch Hand

Joined: May 30, 2004
Posts: 78
Shift operators are not on the 1.5 exam in case you didn't know.
[ August 23, 2006: Message edited by: Andy Morris ]
 
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subject: shift operators