Website Development I

Class No. 18:

Structuring and Sorting Data Stored in Arrays

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Today’s Goal

To see how to sort an array and use a customized function.

Class Resources

Web Programming book reading for THIS class

• pages 538-542
1. Arrays

JavaScript Array Example 5:  Sorting a Simple Array of Random Numbers (Heines)

• Handout with output and code listing

Web Programming book reading for NEXT class

• pages 335-343
1. Comments and Coding Conventions
2. Event-Handler Attributes
3. onchange, onmouseover, onmouseout
4. Using noscript to Accommodate Disabled JavaScript

Preambles

Questions from our last class or the readings or work between classes?

Creating a Simple Array of Random Numbers

The Math.random() Function

This function returns a pseudo-random decimal number between 0 and 1.

• It is called “pseudo-random” because it is not really completely random.
• Its result is generated by a mathematical algorithm that emulates randomness.

document.writeln( "<ol>" ) ;
for ( var k = 0 ; k < 10 ; k++ ) {
  document.writeln( "<li>" + Math.random() + "</li>" ) ;
}
document.writeln( "</ol>" ) ;

1: Generate Random Numbers

To generate random numbers greater than 1, we multiply the number generated by the Math.random() function by a number greater than 1.  This gives us a range of numbers from 0 to the larger number, such as 100.

document.writeln( "<ol>" ) ;
for ( var k = 0 ; k < 10 ; k++ ) {
  document.writeln( "<li>" + ( Math.random() * 100 ) + "</li>" ) ;
}
document.writeln( "</ol>" ) ;

2: Generate Random Numbers from 0 to 100

To generate random integers, we again multiply the number generated by the Math.random() function by a number greater than 1, but we then pass that result to the Math.round() function.

document.writeln( "<ol>" ) ;
for ( var k = 0 ; k < 10 ; k++ ) {
  document.writeln( "<li>" + Math.round( Math.random() * 100 )  + "</li>" ) ;
}
document.writeln( "</ol>" ) ;

3: Generate Random Integers from 0 to 100

Populating the Array

To populate the array, we use the .push() function for each random integer generated by the above function.

var arr = new Array() ;  // or var arr = [] ;
for ( var k = 0 ; k < 10 ; k++ ) {
  arr.push( Math.round( Math.random() * 100 ) ) ;
}

4: Push Random Integers Into an Array

Sorting the Array

Using the .sort() function

arr.sort() ;

5: Sort the Array

• what is going on here?!?

Writing a Comparison Function

A comparison function is a function that takes two parameters

var comp = function( a, b ) { ... }

The function returns a numeric value based on a decision made on those two parameters

var comp = function( a, b ) { 
    ... // code goes here that decided what number to return
  return number ;
}

• if a should come before b, the function should return a negative number
• if a should come after b, the function should return a positive number
• if no change should be made in the sort order, the function should return 0

// function to sort numbers in **ascending** order
var comp = function( a, b ) { 
  if ( a < b ) {
    return -1 ;
  } else if ( a > b ) {
    return +1 ;
  } else {
    return 0 ;
  }
}

6: Sort the Array Using a Comparison Function

Interestingly, you don’t have to return just +1 or -1.  Since any positive and negative number will have the same effect, we can simplify the comparison function to:

// function to sort numbers in **ascending** order
var comp = function( a, b ) { 
  return a - b ;  // note that b is subracted from a in this formula
}

• for example, if a is 13 and b is 22, the function will return -9, which is a negative number, which means that a should come before b
• if a is 22 and b is 13, the function will return 9, which is a positive number, which means that a should come after b
• if a and b are both 18, the function will return 0
• thus, returning a - b indeed sorts in ascending order
• to sort in descending order, you just reverse the formula: return b - a ;

// function to sort numbers in **descending** order
var comp = function( a, b ) { 
  return b - a ;  // note that a is subracted from b in this formula
}

7: Sort in Descending Order

Using a Comparison Function

To use a comparison function, you pass it to the sort function

arr.sort( comp ) ;

• note that only the name of the function is passed
• you do not pass a and b !!!

Today’s Exercise

Goal:  To create and sort a JavaScript array.

Procedure:

1. Start by working with one of the arrays that you created for our last class.  I recommend that you copy the file you were working on to a new file so that you don’t lose your previous work.
2. Make sure that you can still display your original array using a for loop.
3. Use the sort function to sort your array in ascending order.  Display your array again to make sure that the sorting works.
• Note that your program should display your array twice: once before it is sorted and then again after it is sorted.
• The for loops that you use to display your array should be identical.
• If your array contains string (alphanumeric) data, the sort function should sort your array in alphabetical order.
• If your array contains numeric data, you may have to use a comparison function to get the sort function to sort it in ascending order.
4. Write a comparison function to sort your array in descending order.
• Important Note:  Your comparison function must appear in your code before you use it as an argument to the sort function.
• To ensure that it is defined before you use it, put your comparison function in the <head> section of your HTML file.
5. Use the sort function again, but this time pass it your comparison function to sort your array in descending order.  Display your array a third time to make sure that the sorting works as expected.

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This is Class No. 18.  It was last modified on Saturday, October 21, 2023 at 4:38 PM.  Copyright © 2010-2025 by Jesse M. Heines.  All rights reserved, but may be freely copied or excerpted for educational purposes with credit to the author.