Java math function to convert positive int to negative and negative to positive?

Question

Is there a Java function to convert a positive int to a negative one and a negative int to a positive one?

I'm looking for a reverse function to perform this conversion:

-5  ->  5
 5  -> -5

Related: http://stackoverflow.com/questions/1348080/convert-a-positive-number-to-negative-in-c-sharp and http://stackoverflow.com/questions/5213338/fastest-way-to-flip-the-sign-of-a-double-float-in-c and http://stackoverflow.com/questions/4866187/how-to-flip-the-sign-of-an-integer-value-using-some-constant-and-operators-wit — Shog9, Oct 24 '11 at 17:12

score 281 · Accepted Answer · answered Oct 23 '11 at 23:07

281

What about x *= -1; ? Do you really want a library function for this?

answered Oct 23 '11 at 23:07

FailedDev

26,680
9
53
73

3

@pst No it won't. At least not with JDK 1.6.0_11 ;D – Gandalf Oct 24 '11 at 12:31
14

For me the wrong answer has been accepted. It is a valid answer, but given that the OP is asking to change sign of an int, so there is no concern about negative zero or other concepts, I would suggest x = -x is more expressive (see EJP's answer below)... – drrob Aug 17 '15 at 12:29
1

Addendum to previous comment: see also Bohemian's answer as it explains the unary operator in more detail – drrob Aug 17 '15 at 14:24
@drrob You do realize that any decent compiler will anyway use x = -x right? – FailedDev Aug 17 '15 at 22:56
1

@FailedDev sure, but I said it is more expressive (not better for performance) – drrob Aug 18 '15 at 10:33
3

this answer is not correct. int has the minimum value of -2^31 and a maximum value of 2^31-1 which means that in the particular case when you need to convert Integer.MIN_VALUE this will NOT work and also will not throw an exception (what I think it is really bad) – vinicius.hisao Mar 23 '20 at 12:19

user207421 · Answer 2 · 2014-02-23T01:12:52.257

115

x = -x;

This is probably the most trivial question I have ever seen anywhere.

... and why you would call this trivial function 'reverse()' is another mystery.

edited Feb 23 '14 at 01:12

answered Oct 23 '11 at 23:09

user207421

305,947
44
307
483

8

This does not work in the case where x = Integer.MIN_VALUE and will silently fail*. To be safe, you should check for this case and throw an exception, or use the library function negateExact, which does exactly that. – rdllopes Mar 14 '16 at 13:06
4

@rdllopes The same applies to practically every answer here. – user207421 Mar 15 '16 at 00:18
Note though that it won't compile if x is short or byte, whereas `x *= -1` will. Was a bit surprised when I discovered that which is how I came across this page :) Of course you can cast, but I was auto-generating code where x could be byte, short, int or long... – Dave Griffiths Feb 14 '17 at 10:34
@DaveGriffiths You were autogenerating code to change the sign of a variable? – user207421 Feb 26 '17 at 23:26
Yeah - parsing something where the sign appears after the number :) – Dave Griffiths Feb 28 '17 at 08:07
@rdllopes In fact it applies to every possible answer. When `x == `Integer.MIN_VALUE` *there is no solution.* – user207421 Aug 25 '21 at 10:20
Well, I provided the safe solution. Now, noticed that I just gave a comment for one corner case. As programmers we should have that in mind if the problem might fall in that case. Otherwise, your solution is really good. – rdllopes Aug 25 '21 at 15:26
And if the value is already negative and he wants to convert it to positive? Then your answer fails. – Black Jun 16 '22 at 14:58

Bohemian · Answer 3 · 2014-07-20T23:44:15.337

56

Just use the unary minus operator:

int x = 5;
...
x = -x; // Here's the mystery library function - the single character "-"

Java has two minus operators:

the familiar arithmetic version (eg 0 - x), and
the unary minus operation (used here), which negates the (single) operand

This compiles and works as expected.

edited Jul 20 '14 at 23:44

answered Oct 23 '11 at 23:08

Bohemian

412,405
93
575
722

score 11 · Answer 4 · edited Jun 24 '14 at 19:26

11

Another method (2's complement):

public int reverse(int x){
    x~=x;
    x++;
    return x;
}

It does a one's complement first (by complementing all the bits) and then adds 1 to x. This method does the job as well.

Note: This method is written in Java, and will be similar to a lot of other languages

edited Jun 24 '14 at 19:26

user207421

305,947
44
307
483

answered Mar 02 '14 at 00:16

Shivanshu Goyal

1,374
2
16
22

7

In reality don't do this, just do `x = -x`. From an assembly perspective, `-x` is a simple `neg` instruction while `~x` and `++x` requires a `not` and `add` instruction. – vallentin Sep 27 '16 at 01:49

score 7 · Answer 5 · answered Aug 08 '14 at 12:51

7

No such function exists or is possible to write.

The problem is the edge case Integer.MIN_VALUE (-2,147,483,648 = 0x80000000) apply each of the three methods above and you get the same value out. This is due to the representation of integers and the maximum possible integer Integer.MAX_VALUE (-2,147,483,647 = 0x7fffffff) which is one less what -Integer.MIN_VALUE should be.

answered Aug 08 '14 at 12:51

Salix alba

7,536
2
32
38

11

If you want your code to be robust against this corner case, Java 8 provides [Math.negateExact](http://docs.oracle.com/javase/8/docs/api/java/lang/Math.html#negateExact-int-) that will throw an `ArithmeticException` when trying to negate MIN_VALUE, rather than silently returning the wrong result. – Jeffrey Bosboom Sep 15 '14 at 01:04

score 6 · Answer 6 · edited May 23 '17 at 12:02

6

Yes, as was already noted by Jeffrey Bosboom (Sorry Jeffrey, I hadn't noticed your comment when I answered), there is such a function: Math.negateExact.

and

No, you probably shouldn't be using it. Not unless you need a method reference.

edited May 23 '17 at 12:02

Community

1
1

answered Jan 08 '15 at 16:25

old grouch

81
1
4

Stefan Steiger · Answer 7 · 2018-07-30T10:38:29.470

Necromancing here.
Obviously, x *= -1; is far too simple.

Instead, we could use a trivial binary complement:

number = ~(number - 1) ;

Like this:

import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
        int iPositive = 15;
        int iNegative = ( ~(iPositive - 1) ) ; // Use extra brackets when using as C preprocessor directive ! ! !...
        System.out.println(iNegative);

        iPositive =  ~(iNegative - 1)  ;
        System.out.println(iPositive);

        iNegative = 0;
        iPositive = ~(iNegative - 1);
        System.out.println(iPositive);


    }
}

That way we can ensure that mediocre programmers don't understand what's going on ;)

score 5 · Answer 8 · answered Dec 25 '15 at 17:17

5

original *= -1;

Simple line of code, original is any int you want it to be.

answered Dec 25 '15 at 17:17

K.D. Code

79
1
5

score 4 · Answer 9 · answered Nov 16 '19 at 19:13

4

The easiest thing to do is 0- the value

for instance if int i = 5;

0-i would give you -5

and if i was -6;

0- i would give you 6

answered Nov 16 '19 at 19:13

Jebjosh

83
8

1

somehow I only can think of `* (-1)`, instead of using ` 0 -`. Thanks – Jacky Jan 19 '21 at 04:15

score 1 · Answer 10 · answered Dec 25 '15 at 11:44

1

You can use the minus operator or Math.abs. These work for all negative integers EXCEPT for Integer.MIN_VALUE! If you do 0 - MIN_VALUE the answer is still MIN_VALUE.

answered Dec 25 '15 at 11:44

lovos

21
3

score 0 · Answer 11 · answered Jul 03 '22 at 23:14

0

For converting a negative number to positive. Simply use Math.abs() inbuilt function.

int n = -10;
n = Math.abs(n);

All the best!

answered Jul 03 '22 at 23:14

Aamer

417
3
9

score -2 · Answer 12 · answered Apr 05 '20 at 22:14

-2

In kotlin you can use unaryPlus and unaryMinus

input = input.unaryPlus()

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-int/unary-plus.html https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-int/unary-minus.html

answered Apr 05 '20 at 22:14

Emmanuel Loisance

706
1
12
22

score -4 · Answer 13 · answered Nov 10 '17 at 18:22

-4

You can use Math:

int x = Math.abs(-5);

answered Nov 10 '17 at 18:22

Thiago Araújo

800
9
8

4

How does this turn `5` into `-5`? – QBrute Nov 10 '17 at 19:44
May be `var = Math.abs(var) * -1;` – C-- Feb 02 '18 at 09:41
3

This is blatantly wrong. `Math.abs()` just removes every `-` sign. It does not invert the sign. Use `-x` or `x * -1` instead. – ssc-hrep3 Aug 31 '18 at 13:41

Java math function to convert positive int to negative and negative to positive?

13 Answers13

Linked

Related