Bit conversion

goliwala · #1 14-Oct-2009, 00:16

Hi All,

Need help with following program. Trying to input decimal, convert to binary, flip certain bits (IF the value for the bits 11-8 is even, THEN reverse the value for the bits 11-8, ELSE reverse the value for the bits 15-12.) Then convert back to decimal.
Please help, Here is the code I have come up with so far.

Thanks.

JAVA Code:

import java.util.Scanner;

//I have to use short.  Its one of the requirements.
public class testConversion
{
    static Scanner scan = new Scanner (System.in);
    public static void main(String[] args)
    {
        System.out.print("Enter a integer: ");
        short x = (short) scan.nextInt();
        System.out.print(clcValue(x));
    }
    public static short clcValue(short memadd)
    {
        short memval = 0;
        short a,y;
        short num[] = new short[16];
        //short returnVal = 0;
        a = 0; 
        while(memadd!=0) 
        { 
            y = (short) (memadd % 2); 
            memadd = (short) (memadd / 2); 
            num[a]= y; 
            a++; 
        }
        a--;
        for(a = (short) (num.length - 1); a >= 0; a--)
        {
            if (num[8] == 0)
            {
                for(short i = 8; i <= 11; i++)
                {
                    //                    num [i] = (short) ~(num [i]); // no worki
                    if (num [i] == 0)
                    {
                        num [i] = 1;
                    }
                    else
                    {
                        num[i] = 0;
                    }
                }
            }
            else if (num[8] == 1)
            {
                for(short i = 12; i <= 15; i++)
                {
                    //    num [i] = (short) ~(num [i]);  // that did't work
                    if (num [i] == 0)
                    {
                        num [i] = 1;
                    }
                    else
                    {
                        num[i] = 0;
                    }
                }
            }
            System.out.print(num[a]);  // should invert selected bits
        }
        for (int i = 0; i >= num.length; i--)
        {
            memval = (short) (memval + num[i] * Math.pow(2, i));
        }
        System.out.println(memval); // just checking the output
        return memval;
    }
}

TurboPT · #2 14-Oct-2009, 08:51

It's not 100% clear (to me, anyway) exactly what is the goal?

Say I input 15, should I expect to see this:

Code:

[0..3]  [4..7]  [8..11]  [12..15]   //'bit' ranges
 1111    0000    0000     1111

...or is there something else that is supposed to happen? Please provide a sample input/output expectation?

Also, this loop:

JAVA Code:

    for (int i = 0; i >= num.length; i--)

...will NEVER execute, as 0 is not greater than 16.

goliwala · #3 15-Oct-2009, 00:55

Thanks for the reply.

I had realized the problem with for loop and corrected it. Now, I understand that this program can be much simpler then what I have so I am giving up on the original code. Here is what I am trying to do now:

I have an input “memadd” which is 2 bytes (short). I check the eighth bit, if it is 0 then I flip (XOR) bits 8 – 11 but if it is 1 then I flip (XOR bits 12 – 15. I know this can be done like this;
memval = memadd & 0x00000f00 == 0 // AND 8th bit in memadd with 1, I don't know if the code is correct. That is where I need help.
memval = memadd ^ 0x0000f00. //If result is 0 then XOR memadd bits 8 – 1 with 0xf (1111).
Else memval = memadd ^ 0x0000f000 // else XOR bits 12 – 15 with all ones and print out memval (decimal value). Just don’t know how to code it correctly.

Example:
If memadd value is 13088, which has a binary representation of: 0011 0011 0010 0000 so bits 11-8 are 0011, which is ODD, therefore bits 15-12 should be flipped, ie: from 0011 to 1100. the returned value 'memval' is therefore: the original memadd value with the bits 15-12 flipped or 1100 0011 001000 00 which is 49952 in decimal.

goliwala · #4 15-Oct-2009, 01:09

OK, I got it. just took me 5 minutes

what a difference between two codes. I wish I had known about bitwise operators earlier.
Sorry I don't know how to make the code appear proper.

JAVA Code:

import java.util.Scanner;
public class testConversion 
{
	static Scanner scan = new Scanner (System.in);
	public static void main(String[] args)
	{
		System.out.print("Enter a integer: ");
		int x = scan.nextInt();
		System.out.print(clcValue(x));
	}
	public static int clcValue(int memadd)
	{
		int memval = 0;		
		if ((memadd & 0x00000100) == 0)
		{
			memval = (memadd ^ 0x00000f00);
		}
		else
			memval = (memadd ^ 0x0000f000);
		return memval;
	}
}

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