Detect if number change from negative to positive or positive to negative

Question

How to detect if positive or nagative changes ?

Example :

1 change to 2 = false

0 change to 1 = false

Because both are positive numbers

1 change to -1 = true

0 change to -1 = true

Because positive change to negative

-1 change to 0 = true

-1 change to 1 = true

Because negative change to positive

I do like..

var a_test,b_test;
if(a<0) {
    a_test = 'negative';
} else {
    a_test = 'positive';
}
if(b<0) {
    b_test = 'negative';
} else {
    b_test = 'positive';
}

if(a_test!==b_test) {
    alert('Yeah!');
}

For test : http://jsfiddle.net/e9QPP/

Any better coding for do something like this ?

---

Wiki : A negative number is a real number that is less than zero

Answer 1

According to the Zen of Python ,

Readability counts.

So, I present more readable and code-review passing version

if (a < 0 && b >= 0 || a >= 0 && b < 0) {
    alert("Yeah");
}

Answer 2

You seem to want

if (a*b<0) alert('Yeah!');

If you want to consider 0 as a positive number, you may use

if (a*b<0 || (!(a*b) && a+b<0)) alert('Yeah!');

Answer 3

Taking a suitable sign function :

function sign(x) {
  return typeof x === 'number' ? x ? x < 0 ? -1 : 1 : x === x ? 0 : NaN : NaN;
}

Then your problem can be expressed in clear and (hopefully) simple terms:

if (sign(a) !== sign(b)) {
  // Your code here
}

Answer 4

Based on your criteria, it seems you simply want to compare the signs of the two numbers. The sign is stored in the highest bit of the number, therefore, to compare the signs, we can simply shift all the other bits off the number, and compare the signs.

Numbers in JavaScript are 64 bit ( double ), so we need to shift off the 63 bits preceding the sign:

if (a >>> 63 !== b >>> 63) alert('Yeah!');

Here is a jsFiddle demo

Here is a jsPerf comparison based on the 4 methods offered here.

Please note that this assumes that the numbers are 64 bit. I don't know if the spec restricts it to 64-bit, but it's plausible that there are browsers out there (or will be one day) where numbers are represented by perhaps a 128-bit number or greater.

Answer 5

Maybe a bit late, but I had the same problem.

Assuming you now that a and b are numbers why not:

if(a < 0 ? b >=0 : b < 0){
   // your code here
}