Math.abs

[xX_MayDay_Xx]

ok....i know that Math.abs gives the absolute value of a number..ex(the absolute value of -12...is..12) put i need to know how to do this without using the Math.abs application....i think it has something to do with multipling the number by it's self then dividing by 2?

[Andy]

how about give us the definition of absolute value, once you understand what it means, this question becomes intuitive

[xX_MayDay_Xx]

yeah.....i was an idiot......basic rule of math....anything *1 equals its self...that combined with the rules of integers -*+=-....or -*-=+...thats all had to do....i was an idiot...i will no freely except harassment for the uber n()()b...

code:

public class AbsoluteValue {         public static void main(String[]args)         {                 int num;                 System.out.println("please enter a number");                 num=In.getInt ();                 if (num>=0)                 {                 System.out.println("the absolute  value of that number is..."                                    + num);             }             else             {                         System.out.println("the absolute value of "+num+" is..."                                           +num*-1);                                                   }         } }

[Andy]

there ya go

[xX_MayDay_Xx]

ummm....whats a switch application?

[Krabjuice]

Something that uses the switch statement, i'd assume. Switch is a "in the case of" type of control sequence. People generally use if's, but switch has it's uses.

[Dan]

I think the most effsent way of doing this whould be to set the singed bit to 0 or what ever means postive on your cpu.

Tho this whould probly be a bit complicated for most peoleop and is not real high level progaming but rather low level asm type stuff. I guse this is what Math.abs is for =p.

[md]

I dunno dan; getting a single bit isn't easy; I think a strait compare might be faster

n is $2
$3 is temporary storage

compare:

mips:

bgtz $2, ret addi $3, $0, -1 mult $2, $3 mflo $2 ret:

bit-wise

mips:

addi $3, $3, 0 sra $3, $3, 31 bne $3, $0, ret addi $3, $0, -1 mult $2, $3 mflo $2 ret:

[Dan]

I am prity shure that in asm at least seting one byte to 0 with a mov instruction is faster.

[md]

Wha? I'm using words... and I don't set any to zero..

setting the sign bit to 0 instead of 1 would not change the sign of the number at all, it would radically change the number though.

1110 + 0010 = 0000. 0010 is the positive number, 1110 is it's negative. 0110 + 0010 = 1000, clearly not zero

[rizzix]

1101 & 0111 = 0101
^-- sign bit


I believe that's what Dan was aiming for.

[md]

yeah, but just checking it is still slow. It's best case two instructions to compare just hte bit, whereas a simple less then is only one.

[rizzix]

1101 & 0111 = 0101
^-- sign bit / resut is a positive number: 0xxx

0110 & 0111 = 0110
^-- sign bit / result is a positive number 0xxx

It is fast. But as we discussed in #compsci.ca it isin't actually the case. Two's complement is a whole lot trickier than just fixing one bit.