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# Bit Shifting Hell

Greenhorn
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I am trying to come up with a good approach to the problem: I need feedback.
how to calculate
x [operator] y
where operator is >>> , >> , <<

stated that:

a, x , y are ints
a = (int)Math.pow(2,y)
x>0
y>0 and y < 32
if ( y > 32 )
subtract 32 until y < 32 and proceed
if ( y == 32 )
the result is x (with sign unchanged)
because any shift by 32
gives back x by definition.

-------------------------

x>>y or x >>>y

RESULT: x / a
-x>>y

if x <= a
RESULT: -1
if ( x > a && (x % a != 0 ))
RESULT: (-x / a) -1
if ( x > a && (x % a == 0))
RESULT: -x / a

-x>>>y

RESULT: A POSITIVE NUMBER -->

convert to binary* and shift to
the right of Y places filling in with
zeros from the left
* to get a negative number in binary:

1) forget the sign and convert to bin
2) invert the bits
3) add 1 to the result
(if x is small expect a very big result)
x << y

RESULT : x * a
-x << y

RESULT : -x * a
-------------------------------

Author and all-around good cowpoke
Posts: 13078
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I find it much easier just to think in terms of the bit pattern rather than the result of multplying or dividing.
You can use the static Integer method toBinaryString to see the bit pattern for any int.
Bill

andrea fare
Greenhorn
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I agree with you, the only problem is that
I need a rule that I can use WHILE I am taking the exam.

Andrea

Author & Gold Digger
Posts: 7617
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andrea,
during the exam you'll have a piece of paper and a pen on which you can write your numbers as bit sequences and do the shifting accordingly. I think it is much more easier that way than trying to learn a set of rules to apply in each different case.
Just learn how to convert numbers to a binary sequence (Campfire Story: Cat and Mouse Games with Bits with a nice bit shifting applet at the bottom of the page) and then apply the shifting to them, you'll see it will be easier to move bits around than trying to remember your rules during the exam...
[ February 13, 2002: Message edited by: Valentin Crettaz ]

andrea fare
Greenhorn
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Good I didn't know you could use paper.
Thanks.
Andrea

 Don't get me started about those stupid light bulbs.