How to sort an array with null values

Question

There are quite a few question related to the topic, but I couldn't find the right solution for my case.

var arr = [a, b, null, d, null]

and am sorting this Array with below logic

return function(a,b){ 

    if(a === null){
      return 1;
    }
    else if(b === null){
      return -1;
    }
    else if(a === b){
      return 0;
    }
    else if(ascending) {
      return a < b ? -1 : 1;
    }
    else if(!ascending) {
      return a < b ? 1 : -1;
    }
  };

I get the following outputs for

Ascending : [a, b,  d, null,null]

Descending : [d, b,  a, null,null]

Expected : [null, null,d, b,  a]

What am I doing wrong?

Your comparison to null does not rely on ascending variable value. PS: it's ugly and confusing when a comparison function is not pure. — zerkms
– zerkms, Commented Nov 13, 2015 at 2:27

royhowie · Accepted Answer · 2015-11-13 02:43:16Z

8

function getSort (ascending) {
  // if ascending, `null` will be pushed towards the end of the array by returning 1
  var nullPosition = ascending ? 1 : -1
  return function (a, b) {
    // if a is null, push it towards whichever end null elements should end up
    if (a == null) return nullPosition
    
    // Note: at this point, a is non-null (previous if statement handled that case).
    //
    // If b is null, it must therefore be placed closer to whichever end the null
    // elements should end up on. If ascending, null elements are pulled towards
    // the right end of the array. If descending, null elements are pulled towards
    // the left.
    //
    // Therefore, we return -nullPosition. If ascending, this is -1, meaning a comes
    // before b; if descending, this is 1, meaning a comes after b. This is
    // clearly the correct behavior, since ascending will push b, which is null,
    // towards the end of the array (with -1) and descending will push b towards
    // the beginning of the array.
    if (b == null) return -nullPosition

    // OTHERWISE, both elements are non-null, so sort normally.
    // if a < b AND
    //     if ascending, a comes first, so return -1 == -nullPosition
    //     if descending, a comes after, so return -nullPosition == -(-1) == 1
    if (a < b) return -nullPosition

    // return the opposite of the previous condition
    if (a > b) return nullPosition
    
    // return 0 if both elements are equal
    return 0
  }
}

function write (arr) { arr.forEach(function (d) { document.write(d + "<br>")})}

var toSort = ['a', 'b', null, 'd', null]

var sortA = getSort(true)
var sortD = getSort(false)

document.write("<br>ASCENDING<br>")
write(toSort.sort(sortA))

document.write("<br>DESCENDING<br>")
write(toSort.sort(sortD))

edited Nov 13, 2015 at 2:43

answered Nov 13, 2015 at 2:30

royhowie

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2 Comments

Peru Over a year ago

what if both rows are null ?

royhowie Over a year ago

@Tharun then either can come first, so if (a == null) return nullPosition will handle that case.

Nina Scholz · Accepted Answer · 2017-08-21 08:18:53Z

0

You could use a two pass approach by checking null values first and then order by string.

To change the sort order, you could swap the parameters or use a negated result of the one of the function.

var data = ['a', 'b', null, 'd', null];

// ascending
data.sort(function (a, b) {
    return (a === null) - (b === null) || ('' + a).localeCompare(b);
});
console.log(data);

// descending
data.sort(function (a, b) {
    return (b === null) - (a === null) || ('' + b).localeCompare(a);
});
console.log(data);

.as-console-wrapper { max-height: 100% !important; top: 0; }

answered Aug 21, 2017 at 8:18

Nina Scholz

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Comments

Noby Fujioka · Accepted Answer · 2019-04-21 20:29:13Z

0

What about the below solution?

var arr = [null, 'e', 'a', 'b', null, 'd', null];

function sortBy(arr, ascending) {
    return arr.sort((a, b) => {
        if(!a) return ascending ? 1 : -1;
        if(!b) return ascending ? -1 : 1;
        if (ascending) return a > b ? 1 : -1;
        return a > b ? -1 : 1;
    })
}

const ascendingArr = sortBy(arr, true);
console.log(ascendingArr);
const decendingArr = sortBy(arr, false);
console.log(decendingArr);

edited Apr 21, 2019 at 20:29

answered Apr 21, 2019 at 20:23

Noby Fujioka

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