Sorting an array that contains both positive and negative zero

Question

I have an array of objects example

var objects = [ 
    { first_nom: 'dave', value: '5'},
    { first_nom: 'roger', value: '-0'},
    { first_nom: 'pete', value: '+0' },
    { first_nom: 'pete', value: '-5' },
];

Sorting this by value would ideally reverse the two zeros.

I have tried the normal javascript/jquery and underscore sorts but they ignore the leading - or + and probably rightly so.

This is what I currently have (this sorts fine but ignores the + and -):

var chartData = _.sortBy(objects, 'value').reverse();

Is there anything else someone can recommend please?

Many thanks

Nev

You'll have to add it as a special case to the sorting function. Show what you have now and someone'll be able to tell how to change it. — JJJ, Oct 31 '14 at 12:44
Don't use strings as value, but just an int. Int's can be negative — Andreas Furster, Oct 31 '14 at 12:45
@AndreasFurster, that still won't sort them properly as it will convert the signed zeros to 'normal' zeros and nothing will happen. — Andy, Oct 31 '14 at 12:47
Are the signed zeros really necessary for what you want to do? — Andy, Oct 31 '14 at 12:48
I don't have access or rather it's not easy to access the ACTUAL value before this formatting was applied so I'm left with this data — Nevnev, Oct 31 '14 at 12:50
@Andy The double precision floating point format distinguishes between positive and negative zeros which is why `1 / 0` is `Infinity` whereas `1 / -0` is `-Infinity`. See my answer below. — Aadit M Shah, Oct 31 '14 at 13:13

score 2 · Answer 1 · answered Oct 31 '14 at 12:51

2

You can special-case them, sorting as strings when both values are 0:

var objects = [ 
    { first_nom: 'dave', value: '5'},
    { first_nom: 'roger', value: '-0'},
    { first_nom: 'pete', value: '+0' },
    { first_nom: 'pete', value: '-5' },
];

var sorted = objects.sort(
  function(a,b) {
    if ((a.value == 0) && (b.value == 0))
      {
        return b.value.localeCompare(a.value);
      }
    else
      return b.value - a.value;
  }
);

console.log(JSON.stringify(sorted));

Results in:

[
  {"first_nom":"dave","value":"5"},
  {"first_nom":"pete","value":"+0"},
  {"first_nom":"roger","value":"-0"},
  {"first_nom":"pete","value":"-5"}
]

answered Oct 31 '14 at 12:51

Paul Roub

36,322
27
84
93

String comparison is probably not the best way to compare numbers. – Aadit M Shah Oct 31 '14 at 13:16
1

@AaditMShah Of course it's not. But in this one (insane) case, where we need to treat `-0` and `+0` as different entities, it's not really that much weirder than relying on different versions of division-by-zero. – Paul Roub Oct 31 '14 at 13:26

Aadit M Shah · Accepted Answer · 2014-10-31T13:41:27.993

Try the following:

var objects = [ 
    { first_nom: "dave",  value: "5"  },
    { first_nom: "roger", value: "-0" },
    { first_nom: "pete",  value: "+0" },
    { first_nom: "pete",  value: "-5" },
];

objects.sort(function (a, b) {
    var x = parseInt(a.value, 10);
    var y = parseInt(b.value, 10);

    if (x === 0 && y === 0)
        return 1 / x - 1 / y || 0;
    else return x - y;
});

alert(JSON.stringify(objects, null, 4));

The reason this works is because 1 / 0 is Infinity whereas 1 / -0 is -Infinity.

For example:

First case: x = 0, y = 0:
```
1 / x = Infinity
1 / y = Infinity

Infinity - Infinity = NaN

NaN || 0 = 0
```
Hence the two numbers are left unsorted with respect to each other.
Second case: x = -0, y = -0:
```
1 / x = -Infinity
1 / y = -Infinity

(-Infinity) - (-Infinity) = NaN

NaN || 0 = 0
```
Hence the two numbers are left unsorted with respect to each other.

Third case: x = 0, y = -0:

1 / x = Infinity
1 / y = -Infinity

Infinity - (-Infinity) = Infinity

Infinity || 0 = Infinity

Hence x will come after y.

Third case: x = -0, y = 0:

1 / x = -Infinity
1 / y = Infinity

(-Infinity) - Infinity = -Infinity

-Infinity || 0 = -Infinity

Hence x will come before y.

That's all that there is to it really.

Edit: To sort it in reverse order just swap x and y as follows:

var objects = [ 
    { first_nom: "dave",  value: "5"  },
    { first_nom: "roger", value: "-0" },
    { first_nom: "pete",  value: "+0" },
    { first_nom: "pete",  value: "-5" },
];

objects.sort(function (a, b) {
    var x = parseInt(a.value, 10);
    var y = parseInt(b.value, 10);

    if (x === 0 && y === 0)
        return 1 / y - 1 / x || 0;
    else return y - x;
});

alert(JSON.stringify(objects, null, 4));

Hope that helps.

@Nevnev You should accept the best answer to your question. Click on the tick mark next to the best answer. — Aadit M Shah, Oct 31 '14 at 13:10
You might want to edit your sample code to sort in the (reversed) order that the Question asks for. — Paul Roub, Oct 31 '14 at 13:27
If I wanted to add a secondary sort on first_nom how can I work that into your code above? @AaditMShah — Nevnev, Dec 03 '14 at 10:46
@Nevnev Just change `return 1 / y - 1 / x || 0` to `return 1 / y - 1 / x || b.first_nom.localeCompare(a.first_nom)`; and change `return y - x` to `return y - x || b.first_nom.localeCompare(a.first_nom)`. — Aadit M Shah, Dec 03 '14 at 11:07
Brilliant. thank you. Sorry about bothering you with such trivial tasks. @AaditMShah — Nevnev, Dec 03 '14 at 11:20

score 0 · Answer 3 · edited May 23 '17 at 10:33

A simple solution is to convert a signed zero to some signed small value, for example:

var objects = [ 
    { first_nom: 'dave', value: '5'},
    { first_nom: 'pete', value: '+0' },
    { first_nom: 'pete', value: '-5' },
    { first_nom: 'roger', value: '-0'},
];

  
convert = function(n) {
  if(n == '-0') return -Number.EPSILON;
  if(n == '+0') return +Number.EPSILON;  
  return parseInt(n);
}
  
objects.sort(function(a, b) {
  return convert(a.value) - convert(b.value);
});
  

document.write(JSON.stringify(objects));

To do that in a pure numeric way, utilize the fact that

Math.atan2(0, -0) === Math.PI 
Math.atan2(0,  0) === 0

(see Differentiating +0 and -0)

var objects = [ 
    { first_nom: 'dave', value: '5'},
    { first_nom: 'roger', value: '-0'},
    { first_nom: 'pete', value: '+0' },
    { first_nom: 'pete', value: '-5' },
];

objects.sort(function(a, b) {
  a = parseInt(a.value);
  b = parseInt(b.value);
  return (a || b) ? (b - a) : (Math.atan2(0, a) - Math.atan2(0, b));
});
  
document.write(JSON.stringify(objects));

This isn't a safe assumption. As soon as you add `{ first_nom: 'greg', value: '-0.000000009' },` you start to see the problem with going down this path. — DRobinson, Oct 31 '14 at 12:57
@DRobinson: very insightful. I can't imagine how I missed that. — georg, Oct 31 '14 at 13:17

Sorting an array that contains both positive and negative zero

3 Answers3