javascript - comparing two floats to see if they're both negative, or both positive - Stack Overflow

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Hay guys, i have 2 floats, which both es from input boxes.

I need to pare these 2 floats, if one is negative and one is positive thrown an error. If they're both positive, or both negative, thats fine.

Any ideas?

Thanks

Hay guys, i have 2 floats, which both es from input boxes.

I need to pare these 2 floats, if one is negative and one is positive thrown an error. If they're both positive, or both negative, thats fine.

Any ideas?

Thanks

• javascript
• floating-point
• pare

Share Improve this question Follow edited Jun 18, 2012 at 16:12 mskfisher 3,41644 gold badges3737 silver badges4949 bronze badges asked Jan 6, 2010 at 14:49 dottydotty 41.5k6666 gold badges153153 silver badges197197 bronze badges 1

• 2 What if one is (or both are) zero? – balpha ♦ Commented Jan 6, 2010 at 14:52

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4 Answers 4

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Multiply them together.

If the answer is positive then they are both the same sign.

If the answer is negative then they are of opposite sign.

If the answer is zero (within some value to take care of rounding error) then one or both are zero and you'll have to check them individually. You'll then have to decide whether 0 to be treated as positive or negative in your scenario.

Although detection of the sign of the product can be done, it's not what you are interested in. Especially if you're going to use it on large volumes of floats (eg to detect a zero crossing in a time stream).

The simplest way is to exactly express what you ask for: is the sign of a equal to the sign of b?

function samesign( a, b ) {
  var aPositive = a >= 0;
  var bPositive = b >= 0;
  return aPositive == bPositive;
}

Or shorter:

function samesign( a, b ) { return (a>=0) == (b>=0); }

Just do something like:

if float1*float2<0
    Error

function isSameSign(a,b){
  var r = a*b; 
  return (r >= 0)
}

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