math - Positive Number to Negative Number in JavaScript? - Stack Overflow

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Basically, the reverse of abs. If I have:

if ($this.find('.pdxslide-activeSlide').index() < slideNum - 1) {
  slideNum = -slideNum
}
console.log(slideNum)

No matter what console always returns a positive number. How do I fix this?

If I do:

if ($this.find('.pdxslide-activeSlide').index() < slideNum - 1) {
  _selector.animate({
    left: (-slideNum * sizes.images.width) + 'px'
  }, 750, 'InOutPDX')
} else {
  _selector.animate({
    left: (slideNum * sizes.images.width) + 'px'
  }, 750, 'InOutPDX')
}

it works tho, but it's not "DRY" and just stupid to have an entire block of code JUST for a -.

Basically, the reverse of abs. If I have:

if ($this.find('.pdxslide-activeSlide').index() < slideNum - 1) {
  slideNum = -slideNum
}
console.log(slideNum)

No matter what console always returns a positive number. How do I fix this?

If I do:

if ($this.find('.pdxslide-activeSlide').index() < slideNum - 1) {
  _selector.animate({
    left: (-slideNum * sizes.images.width) + 'px'
  }, 750, 'InOutPDX')
} else {
  _selector.animate({
    left: (slideNum * sizes.images.width) + 'px'
  }, 750, 'InOutPDX')
}

it works tho, but it's not "DRY" and just stupid to have an entire block of code JUST for a -.

• javascript
• math

Share Improve this question Follow edited Apr 25, 2022 at 11:17 Liam 29.5k2828 gold badges137137 silver badges200200 bronze badges asked Apr 6, 2011 at 23:24 Oscar GodsonOscar Godson 32.7k4242 gold badges125125 silver badges206206 bronze badges 0

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13 Answers 13

Sorted by: Reset to default 277

Math.abs(num) => Always positive
-Math.abs(num) => Always negative

You do realize, however, that for your code:

if($this.find('.pdxslide-activeSlide').index() < slideNum-1) {
   slideNum = -slideNum;
}

console.log(slideNum)

If the index found is 3 and slideNum is 3,
then 3 < 3-1 => false
so slideNum remains positive??

It looks more like a logic error to me.

The reverse of abs is Math.abs(num) * -1.

The basic formula to reverse positive to negative or negative to positive:

i - (i * 2)

Javascript has a dedicated operator for this: unary negation.

TL;DR: It's the minus sign!

To negate a number, simply prefix it with - in the most intuitive possible way. No need to write a function, use Math.abs() multiply by -1 or use the bitwise operator.

Unary negation works on number literals:

let a = 10;  // a is `10`
let b = -10; // b is `-10`

It works with variables too:

let x = 50;
x = -x;      // x is now `-50`

let y = -6;
y = -y;      // y is now `6`

You can even use it multiple times if you use the grouping operator (a.k.a. parentheses:

l = 10;       // l is `10`
m = -10;      // m is `-10`
n = -(10);    // n is `-10`
o = -(-(10)); // o is `10`
p = -(-10);   // p is `10` (double negative makes a positive)

All of the above works with a variable as well.

To get a negative version of a number in JavaScript you can always use the ~ bitwise operator.

For example, if you have a = 1000 and you need to convert it to a negative, you could do the following:

a = ~a + 1;

Which would result in a being -1000.

num * -1

This would do it for you.

var x = 100;
var negX = ( -x ); // => -100

Are you sure that control is going into the body of the if? As in does the condition in the if ever hold true? Because if it doesn't, the body of the if will never get executed and slideNum will remain positive. I'm going to hazard a guess that this is probably what you're seeing.

If I try the following in Firebug, it seems to work:

>>> i = 5; console.log(i); i = -i; console.log(i);
5
-5

slideNum *= -1 should also work. As should Math.abs(slideNum) * -1.

If you don't feel like using Math.Abs * -1 you can you this simple if statement :P

if (x > 0) {
    x = -x;
}

Of course you could make this a function like this

function makeNegative(number) {
    if (number > 0) {
        number = -number;
    }
}

makeNegative(-3) => -3 makeNegative(5) => -5

Hope this helps! Math.abs will likely work for you but if it doesn't this little

var i = 10;
i = i / -1;

Result: -10

var i = -10;
i = i / -1;

Result: 10

If you divide by negative 1, it will always flip your number either way.

Use 0 - x

x being the number you want to invert

It will convert negative array to positive or vice versa

function negateOrPositive(arr) {
    arr.map(res => -res)
};

In vanilla javascript

if(number > 0)
  return -1*number;

Where number above is the positive number you intend to convert

This code will convert just positive numbers to negative numbers simple by multiplying by -1

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