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richieboi22
Posts:5
Registered: 4/2/09

How do you Invert signed numbers?
Aug 26, 2009 9:19 AM

Hi there.

A simple question: How can I invert the sign of an integer? ie) convert -12 to +12? No doubt there is a really simple way to do this, but I'm stumped, I cannot find anything and I've been working on this program all day.

I'll be very grateful to anyone who can let me know if Java provides a way to do this, or point me in the right direction if it takes some tweaking of binary numbers using the Math package or something.

Cheers!

paul.miner
Posts:2,841
Registered: 10/8/07

Re: How do you Invert signed numbers?
Aug 26, 2009 9:22 AM (reply 1 of 32) (In reply to original post )

n = -n

kevinaworkman
Posts:1,579
Registered: 30/03/09

Re: How do you Invert signed numbers?
Aug 26, 2009 9:27 AM (reply 2 of 32) (In reply to original post )

Is this a joke? If not, try this:

int x = numberToInvertSign;
boolean pos = x > 0;
for(int i = 0; i < 2*Math.abs(x); i++){
   if(pos){
      numberToInvertSign--;
   }
   else{
      numberToInvertSign++;
   }
}

richieboi22
Posts:5
Registered: 4/2/09

Re: How do you Invert signed numbers?
Aug 26, 2009 9:27 AM (reply 3 of 32) (In reply to #1 )

I knew it would be simple, but not that simple! I feel like a right ******* now.

Thanks very much paul.miner, you saved the day.

richieboi22
Posts:5
Registered: 4/2/09

Re: How do you Invert signed numbers?
Aug 26, 2009 9:27 AM (reply 4 of 32) (In reply to #2 )

No joke, I just tried it and it worked.

sabre150
Posts:22,045
Registered: 10/24/97

Re: How do you Invert signed numbers?
Aug 26, 2009 9:31 AM (reply 5 of 32) (In reply to #1 )

paul.miner wrote:

n = -n

Not obscure enough! I prefer

int n = ....;
 n = (0xffffffff ^ n) + 1;

paul.miner
Posts:2,841
Registered: 10/8/07

Re: How do you Invert signed numbers?
Aug 26, 2009 9:45 AM (reply 6 of 32) (In reply to #5 )

sabre150 wrote:

paul.miner wrote:

n = -n

Not obscure enough! I prefer

int n = ....;
 n = (0xffffffff ^ n) + 1;

Meh:

n = ~n + 1;

paul.miner
Posts:2,841
Registered: 10/8/07

Re: How do you Invert signed numbers?
Aug 26, 2009 9:49 AM (reply 7 of 32) (In reply to #6 )

Or better:

n = ~--n;

sabre150
Posts:22,045
Registered: 10/24/97

Re: How do you Invert signed numbers?
Aug 26, 2009 9:49 AM (reply 8 of 32) (In reply to #6 )

paul.miner wrote:
Meh:

n = ~n + 1;

:-)

JosAH
Posts:13,022
Registered: 4/6/04

Re: How do you Invert signed numbers?
Aug 26, 2009 9:58 AM (reply 9 of 32) (In reply to #5 )

sabre150 wrote:

paul.miner wrote:

n = -n

Not obscure enough! I prefer

int n = ....;
 n = (0xffffffff ^ n) + 1;

Bah, that +1 operation is way too clear; better make that:

n^= 0xffffffff;
int m;
for (m= 1; m != 0 && ((n&m) != 0); m<<= 1);
n|= m;
if (m == 0) n= m;
else for (m >>= 1; m != 0; n^= m, m>>=1);

kind regards,

Jos ;-)

paul.miner
Posts:2,841
Registered: 10/8/07

Re: How do you Invert signed numbers?
Aug 26, 2009 10:11 AM (reply 10 of 32) (In reply to #9 )

JosAH wrote:

else for (m >>= 1; m != 0; n^= m, m>>=1);

Shouldn't this be an unsigned shift, at least for the initial shift?

else for (m >>>= 1; m != 0; n^= m, m>>=1);

JosAH
Posts:13,022
Registered: 4/6/04

Re: How do you Invert signed numbers?
Aug 26, 2009 10:22 AM (reply 11 of 32) (In reply to #10 )

paul.miner wrote:
Shouldn't this be an unsigned shift, at least for the initial shift?

else for (m >>>= 1; m != 0; n^= m, m>>=1);

Yep, you are correct; I now go spank myself thank you ;-)

kind regards,

Jos

jschell
Posts:38,148
Registered: 11/3/97

Re: How do you Invert signed numbers?
Aug 26, 2009 11:46 AM (reply 12 of 32) (In reply to #10 )

paul.miner wrote:
Shouldn't this be an unsigned shift, at least for the initial shift?
else for (m >>>= 1; m != 0; n^= m, m>>=1);

Thank goodness we now have the correct form to use in all our code.

paul.miner
Posts:2,841
Registered: 10/8/07

Re: How do you Invert signed numbers?
Aug 26, 2009 11:53 AM (reply 13 of 32) (In reply to #12 )

jschell wrote:

paul.miner wrote:
Shouldn't this be an unsigned shift, at least for the initial shift?
else for (m >>>= 1; m != 0; n^= m, m>>=1);

Thank goodness we now have the correct form to use in all our code.

Especially since it only affects negating Integer.MIN_VALUE, the odd man out :)

JEisen
Posts:299
Registered: 07/05/07

Re: How do you Invert signed numbers?
Aug 26, 2009 12:20 PM (reply 14 of 32) (In reply to original post )

You guys are all trying way too hard. Clearly, you can do it just with regular addition and subtraction:

public int invert(int i) {
  return i - (i + i);
}

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