Packing vars

[Supamagier]

Hey,

I'm experimenting somewhat with the packing of vars, but I ran into a problem.

The code:

Code:

import java.applet.Applet;
 
public class bitwiseOperationsTest
             extends Applet
{
    public void init() {
        new bitwiseOperationsTest();
    }
   
    public bitwiseOperationsTest() {
        test();
    }
   
    private final void test() {
        echo("value 1: "+val1);
        echo("value 2: "+val2);
        echo("value 3: "+val3);
       
        //packed_data = (short)(((packed_data & ~(0x7f << 16) | val1 << 16) & ~(0x7f << 8) | val2 << 8) & ~0x7f | val3);
        /** The above is the below in one line ;) **/
        //packed_data = (short)(packed_data & ~(0x7f << 16) | val1 << 16);
        //packed_data = (short)(packed_data & ~(0x7f << 8) | val2 << 8);
        //packed_data = (short)(packed_data & ~0x7f | val3);
 
        echo("packed: "+packed_data);
        echo("new val 1: "+((packed_data >>> 16) & 0x7f));  // ALWAYS 0
        echo("new val 2: "+((packed_data >>> 8) & 0x7f));
        echo("new val 3: "+(packed_data & 0x7f));
    }
   
   
   
   
    private final void echo(String s) {
        System.out.println(s);
    }
   
    private byte val1 = 45;
    private byte val2 = 95;
    private byte val3 = 102;
 
    private short packed_data = 0;
}

(Don't ask why I made it an Applet, I have my reasons.

The problem is that the 'new' value 1 always is 0. value 2 and 3 are fine, but 1 always is 0. How come? Please help me.

[Norm]

Try debugging your code by breaking multi step statements into single operations and printing out their values using println();

Can you post the output from your program? The actual output is often better than someone's description in text.

What is "packing of vars"?

[Supamagier]

the Packing of vars is putting multiple variables in one. Using bitwise operations you can, for example store a few bytes/shorts in an short/int. This saves space and is more efficient code.

My output:

Quote:

value 1: 45
value 2: 95
value 3: 102

packed: 24422

new val 1: 0
new val 2: 95
new val 3: 102

[Norm]

(packed_data >>> 16)

Do the above with shift values from 1 to 16 and see what you get.

A few comments in you code describing how you are using the bits of the short to hold data might help you and others understand what is happening

BTW I programmed mainframe assembly code for years.

[Supamagier]

Thanks, but I found the problem (with some help).

I used << 16, while a short only has 16 bits, so I would just 'throw away' the var.

[fishtoprecords]

Why (other than as a mental challenge) are you doing this?

Saving a couple of bytes, at the cost of code clarity, is a questionable choice when a terabyte disk drive costs $150

[Supamagier]

:P Mental challenge, as you said and I'm just experimenting with it :P not really for a reason, just wanna know how it works