Friday, February 5, 2010

signed shift and unsigned shifts

Q1: why is there signed-right-shift and unsigned-right-shift but just a single "left-shift"?
Q2: if i multiply 2 positive int in java, do i always get a positive int?

In java, usually the "thing" to be shifted is an 32-bit int, but can also be a 64-bit long. For both, the left-most leading bit controls the sign -- 0 means non-negative, 1 means negative.

Now consider an example. ...0110 right shift once --> leaves the left-most leading position empty. Either put a 0 (unsigned shift) or (signed) keep the original leading bit there.

By definition
* Signed shift keeps the sign.
* unsigned shift always returns non-negative

Finally we can give A1 ie why there's just a single left-shift. Left shift always shifts some bit INTO the left-most leading position. It never becomes empty. The sign of the result depends on that bit.

* new sign may be same as before
* new sign may be different

A2: 0x55555555<< 1 == 0x55555555*2 < 0 // so a positive int * 2 can be negative.

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