Shift operators

[Shweta R Dhaneshwar]

For any valid int value i, the following will always be true:

System.out.println(i << -1 == i << 31); 

o/p:true.....how?

[Henry Wong]

When shifting an integer, only the lowest 5 bits, of the amount to shift, is used. Hence...

-1 & 0x1f = 0x1f = 31

Henry

[amit taneja]

didn't get it henry....

pls explain in details if possible...

regards,

[Henry Wong]

Originally posted by amit taneja:
didn't get it henry....

pls explain in details if possible...

regards,

I don't know how much more detail I can give... but I'll try...

    -1 = 1111 1111 1111 1111 1111 1111 1111 1111 (bit pattern)
 
       = 0000 0000 0000 0000 0000 0000 0001 1111 (lowest 5 bits)
       = 0x1F
       = 31

Since only the lowest 5 bits are used, shifting by -1 is the same as shifting by 31.

Henry

[Chitra AP]

Henry,

I am kind of understood but not completely. Why should we take only the lowest 5 bits?

[amit taneja]

i also didn't get fully...
what i get is that ..we cant shift -1 so we get its binary value and take 5 lower bits which is 31 and we can say that -1=31 with respect to shifting...

but why 31 is that is because we can shift maximum 31 digits so we take lower 5 bits which amounts to 31 but...didnt get
satisfied with the whole logic...

[Henry Wong]

Originally posted by Chitra AP:
I am kind of understood but not completely. Why should we take only the lowest 5 bits?

It is defined that way in the specification. When shifting ints, only the lowest 5 bits of the amount to shift is used. And when shifting longs, only the lowest 6 bits of the amount to shift is used.

Henry

[Geethakrishna Srihari]

The following code:
a << b
Where a is a int,
shifts a by b%32 places left.

So -1%32 -> as -1 is negative, add 32 to it
so 31....reasoned it logically...

shifting an integer by ........-33 -1 31 63.......all these should
give same result logically

Similarily
given x, shifting x by
x+32,x+64,.....
x-32,x-64..... should give same result

[Geethakrishna Srihari]

An int can be shifted by 0-31 places
An long can be shifted by 0-63 places...

So 31 -> last 5 digits is used (11111 = 31)
and 63 -> last 6 digits is used (111111 = 64)