4.3.2 JS Array Sort

Reversing an Array

• The reverse() method reverses the elements in an array.
• You can use it to sort an array in descending order:

Example 2: JS Arrays Sort in Reverse

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.sort();        // First sort the elements of fruits
fruits.reverse();     // Then reverse the order of the elements

Numeric Sort

• By default, the sort() function sorts values as strings.
• This works well for strings ("Apple" comes before "Banana").
• However, if numbers are sorted as strings, "25" is bigger than "100", because "2" is bigger than "1".
• Because of this, the sort() method will produce incorrect result when sorting numbers.
• You can fix this by providing a compare function:

Example 3: JS Arrays The sort() Method part 2

var points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return a - b});

Use the same trick to sort an array descending:

Example 4: JS Arrays The sort() Method part 3

var points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return b - a});

The Compare Function

• The purpose of the compare function is to define an alternative sort order.
• The compare function should return a negative, zero, or positive value, depending on the arguments:

function(a, b){return a - b}

• When the sort() function compares two values, it sends the values to the compare function, and sorts the values according to the returned (negative, zero, positive) value.
• If the result is negative, a is sorted before b.
• If the result is positive, b is sorted before a.
• If the result is 0 no changes are done with the sort order of the two values.

Example 5: JS Arrays The sort() Method part 4

• The compare function compares all the values in the array, two values at a time (a, b).
• When comparing 40 and 100, the sort() method calls the compare function(40, 100).
• The function calculates 40 - 100 (a - b), and since the result is negative (-60), the sort function will sort 40 as a value lower than 100.
• You can use this code snippet to experiment with numerically and alphabetically sorting:

<button onclick="myFunction1()">Sort Alphabetically</button>
<button onclick="myFunction2()">Sort Numerically</button>

<p id="demo"></p>

<script>
var points = [40, 100, 1, 5, 25, 10];
document.getElementById("demo").innerHTML = points;

function myFunction1() {
  points.sort();
  document.getElementById("demo").innerHTML = points;
}

function myFunction2() {
  points.sort(function(a, b){return a - b});
  document.getElementById("demo").innerHTML = points;
}
</script>

Sorting an Array in Random Order

Example 6: JS Arrays The sort() Method part 5

var points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return 0.5 - Math.random()});

The Fisher Yates Method

• The above example, array.sort(), is not accurate, it will favor some numbers over the others.
• The most popular correct method, is called the Fisher Yates shuffle, and was introduced in data science as early as 1938!
• In JavaScript the method can be translated to this:

Example 7: JS Arrays The sort() Method part 6

var points = [40, 100, 1, 5, 25, 10];

for (i = points.length -1; i > 0; i--) {
  j = Math.floor(Math.random() * i)
  k = points[i]
  points[i] = points[j]
  points[j] = k
}

Find the Highest (or Lowest) Array Value

• There are no built-in functions for finding the max or min value in an array.
• However, after you have sorted an array, you can use the index to obtain the highest and lowest values.

Sorting ascending:

Example 8: JS Arrays The sort() Method part 7

var points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return a - b});
// now points[0] contains the lowest value
// and points[points.length-1] contains the highest value

Sorting descending:

Example 9: JS Arrays The sort() Method part 8

var points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return b - a});
// now points[0] contains the highest value
// and points[points.length-1] contains the lowest value

Sorting a whole array is a very inefficient method if you only want to find the highest (or lowest) value.

Using Math.max() on an Array

You can use Math.max.apply to find the highest number in an array:

Example 10: JS Array Sort The Highest Number

function myArrayMax(arr) {
  return Math.max.apply(null, arr);
}

Math.max.apply(null, [1, 2, 3]) is equivalent to Math.max(1, 2, 3).

Using Math.min() on an Array

You can use Math.min.apply to find the lowest number in an array:

Example 11: JS Array Sort The lowest number

function myArrayMin(arr) {
  return Math.min.apply(null, arr);
}

Math.min.apply(null, [1, 2, 3]) is equivalent to Math.min(1, 2, 3).

My Min / Max JavaScript Methods

• The fastest solution is to use a "home made" method.
• This function loops through an array comparing each value with the highest value found:

Example 12: (Find Max) JS Arrays sort the highest number

function myArrayMax(arr) {
  var len = arr.length;
  var max = -Infinity;
  while (len--) {
    if (arr[len] > max) {
      max = arr[len];
    }
  }
  return max;
}

This function loops through an array comparing each value with the lowest value found:

Example 13: (Find Min) JS Arrays sort the lowest number

function myArrayMin(arr) {
  var len = arr.length;
  var min = Infinity;
  while (len--) {
    if (arr[len] < min) {
      min = arr[len];
    }
  }
  return min;
}

Sorting Object Arrays

JavaScript arrays often contain objects:

Example

var cars = [
  {type:"Volvo", year:2016},
  {type:"Saab", year:2001},
  {type:"BMW", year:2010}
];

• Even if objects have properties of different data types, the sort() method can be used to sort the array.
• The solution is to write a compare function to compare the property values:

Example 14: JS Arrays sort car objects by age

cars.sort(function(a, b){return a.year - b.year});

Comparing string properties is a little more complex:

Example 15: JS Arrays sort car objects by type

cars.sort(function(a, b){
  var x = a.type.toLowerCase();
  var y = b.type.toLowerCase();
  if (x ⇆ y) {return -1;}
  if (x > y) {return 1;}
  return 0;
});