Part 3

JavaScript Comparison Operators

As well as needing to assign values to variables, we sometime need to compare variables or literals.

We do this using Comparison Operators.

Comparison Operators compare two values and produce an output which is either true or false.

For example, suppose we have two variables which are both numeric (i.e., they both hold numbers):

examPasses

and

totalStudents

 

If we compare them, there are two possible outcomes:

 

Therefore, we can make statements like these:

... and then perform a comparison to determine whether the statement is true or false.

 

The basic comparison operator is:

==

(i.e., two equals signs, one after the other with no space in between).

It means 'is equal to'. Compare this with the assignment operator, =, which means 'becomes equal to'. The assignment operator makes two things equal to one another, the comparison operator tests to see if they are already equal to one another.

Here's an example showing how the comparison operator might be used:

  examPasses == totalStudents
 
  If examPasses and totalStudents have the same value, the comparison would return true as a result.
 
  If examPasses and totalStudents have different values, the comparison would return false as a result.

 

Another comparison operator is:

!=

(i.e., an exclamation mark followed by an equals sign, with no space in between).

It means 'is NOT equal to'.

For example:

  examPasses != totalStudents
 
  If examPasses and totalStudents have the same value, the comparison would return false as a result.
 
  If examPasses and totalStudents have different values, the comparison would return true as a result.

 

Two other commonly-used comparison operators are:

<

and

>

 

  < means 'less than'
 
  > means 'greater than'

For example:

  examPasses < totalStudents
 
  If examPasses is less than totalStudents, the comparison would return true as a result.
 
  If examPasses is more than totalStudents, the comparison would return false as a result.

 

Another example:

  examPasses > totalStudents
 
  If examPasses is more than totalStudents, the comparison would return true as a result.
 
  If examPasses is less than totalStudents, the comparison would return false as a result.

As with some of the other 0perators we have encountered, comparison operators can be combined in various ways.

<=

means

'less than or equal to'.

For example:

  examPasses <= totalStudents
 
  If examPasses is less than or equal to totalStudents, the comparison would return true as a result.
 
  If examPasses is more than totalStudents, the comparison would return false as a result.

Also:

>=

means

'greater than or equal to'.

For example:

  examPasses >= totalStudents
 
  If examPasses is more than or equal to totalStudents>, the comparison would return true as a result.
 
  If examPasses is less than totalStudents, the comparison would return false as a result.

 

To summarise, JavaScript understands the following comparison operators:

  == 'is equal to'
  != 'is NOT equal to'
  < 'is less than'
  > 'is greater than'
  <= 'is less than or equal to'
  >= 'is greater than or equal to'

 

If-Else Statements in JavaScript

Much of the power of programming languages comes from their ability to respond in different ways depending upon the data they are given.

Thus all programming languages include statements which make 'decisions' based upon data.

One form of decision-making statement is the If...Else statement.

It allows us to make decisions such as:

  If I have more than 15 left in my account, I'll go to the cinema.
 
  Otherwise I'll stay at home and watch television.

 

This might be expressed in logical terms as:

  If ( money > 15) go_to_cinema
 
  Else watch_television

 

The If-Else statement in JavaScript has the following syntax:

  if (condition)
  {
    statement;
    statement
  }
  else
  {
    statement;
    statement
  };

The condition is the information on which we are basing the decision. In the example above, the condition would be whether we have more than 15. If the condition is true, the browser will carry out the statements within the if... section; if the condition is false it will carry out the statements within the else... section.

The if... part of the statement can be used on its own if required. For example:

  if (condition)
  {
    statement;
    statement
  };

Note the positioning of the semi-colons.

If you are using both the if... and the else... parts of the statement, it is important NOT to put a semi-colon at the end of the if... part. If you do, the else... part of the statement will never be used.

A semi-colon is normally placed at the very end of the if...else... statement, although this is not needed if it is the last or only statement in a function.

 

A practical If-Else statement in JavaScript might look like this:

  if (score > 5)
  {
    alert("Congratulations!")
  }
  else
  {
    alert("Shame - better luck next time")
  };

 

The for Loop

A for loop allows you to carry out a particular operation a fixed number of times.

The for loop is controlled by setting three values:

  - an initial value
  - a final value
  - an increment

 

The format of a for loop looks like this:

  for (initial_value; final_value; increment)
  {
    statement(s);
  }

 

A practical for loop might look like this:

  for (x = 0; x <= 100; x++)
  {
    statement(s);
  }

 

Note that:

  * The loop condition is tested using a variable, x, which is initially set to the start value (0) and then incremented until it reaches the final value (100).
 
  * The variable may be either incremented or decremented.
 
  * The central part of the condition, the part specifying the final value, must remain true throughout the required range. In other words, you could not use x = 100 in the for loop above because then the condition would only be true when x was either 0 or 100, and not for all the values in between. Instead you should use x <= 100 so that the condition remains true for all the values between 0 and 100.

 

Here are some practical examples of for loops.

The first example is a simple loop in which a value is incremented from 0 to 5, and reported to the screen each time it changes using an alert box. The code for this example is:

  for (x = 0; x <= 5; x++)
  {
    alert('x = ' + x);
  }

Click here to see this example working.

The second example is the same except that the value is decremented from 5 to 0 rather than incremented from 0 to 5. The code for this example is:

  for (x = 5; x >= 0; x--)
  {
    alert('x = ' + x);
  }

Click here to see this example working.

 

The start and finish values for the loop must be known before the loop starts.

However, they need not be written into the program; they can, if necessary, be obtained when the program is run.

For example:

  var initialValue = prompt("Please enter initial value", "");
  var finalValue = prompt("Please enter final value", "");
 
  for (x = initialValue; x <= finalValue; x++)
  {
    statement(s);
  }

In this example, the user is prompted to type-in two numbers which are then assigned to the variables initialValue and finalValue. The loop then increments from initialValue to finalValue in exactly the same way as if these values had been written directly into the program.

 

The While Loop

Like the for loop, the while loop allows you to carry out a particular operation a number of times.

The format of a while loop is as follows:

  while (condition)
  {
    statement(s);
  }

 

A practical while loop might look like this:

  var x = 500000;
 
  alert("Starting countdown...");
 
  while (x > 0)
  {
    x--;
  };
  alert("Finished!");

In this example, x is initially set to a high value (500,000). It is then reduced by one each time through the loop using the decrement operator (x--). So long as x is greater than zero the loop will continue to operate, but as soon as x reaches zero the loop condition (x > 0) will cease to be true and the loop will end.

The effect of this piece of code is to create a delay which will last for as long as it takes the computer to count down from 500,000 to 0. Before the loop begins, an 'alert' dialog-box is displayed with the message "Starting Countdown...". When the user clicks the 'OK' button on the dialog-box the loop will begin, and as soon as it finishes another dialog-box will be displayed saying "Finished!". The period between the first dialog box disappearing and the second one appearing is the time it takes the computer to count down from 500,000 to 0.

To see this example working, click here.    

 

The principal difference between for loops and while loops is:

with a while loop, the number of times the loop is to be executed need not be known in advance.

while loops are normally used where an operation must be carried out repeatedly until a particular situation arises.

For example:

  var passwordNotVerified = true;
 
  while (passwordNotVerified == true)
  {
    var input = prompt("Please enter your password", "");
    if (input == password)
    {
      passwordNotVerified = false;
    }
    else
    {
      alert("Invalid password - try again")
    }
  }

In this example, the variable passwordNotVerified is initially set to the Boolean value true. The user is then prompted to enter a password, and this password is compared with the correct password stored in the variable called password. If the password entered by the user matches the stored password, the variable passwordNotVerified is set to false and the while loop ends. If the password entered by the user does not match the stored password, the variable passwordNotVerified remains set to true and a warning message is displayed, after which the loop repeats.

To try this piece of code, click here.    

PS The password is CS7000 - and don't forget that the 'CS' must be capitalised.

 

Testing Boolean Variables

In the while loop example above we used the line:

     var passwordNotVerified = true;

and then tested this variable in a conditional statement as follows:

     while (passwordNotVerified == true)

We could also have written the conditional statement like this:

     while(passwordNotVerified)

In other words, if we don't specify true or false in a conditional statement, the JavaScript interpreter will assume we mean true and test the variable accordingly.

This allows us to make our code a little shorter and, more importantly, to make it easier for others to understand. The line:

     while(passwordNotVerified)

is much closer to the way in which such a condition might be expressed in English than:

     while(passwordNotVerified == true)

 

Logical Operators

We have met a number of operators that can be used when testing conditions, e.g., == , < , > , <= , >= .

Two more operators that are particularly useful with while loops are:

  && Logical AND
 
  || Logical OR

These operators are used to combine the results of other conditional tests.

For example:

    if (x > 0 && x < 100)
 
  means...
    if x is greater than 0 and less than 100...
 
  Placing the && between the two conditions means that the if statement will only be carried out if BOTH conditions are true. If only one of the conditions is true (e.g., x is greater than 0 but also greater than 100) the condition will return false and the if statement won't be carried out.

Similarly:

    if (x == 0 || x == 1)
 
  means...
    if x is 0 or x is 1...
 
  Placing the || between the two conditions means that the if statement will be executed if EITHER of the conditions are true.

 

The ability to combine conditions in this way can be very useful when setting the conditions for while loops.

For example:

  var amount = prompt ("Please enter a number between 1 and 9", "");
 
  while (amount < 1 || amount > 9)
  {
    alert("Number must be between 1 and 9");
    amount = prompt ("Please enter a number between 1 and 9", "");
  }

In this example, the variable amount is initially set to the value typed-in by the user in response to the 'prompt' dialog-box. If the amount entered by the user is between 1 and 9, the loop condition becomes false and the loop ends. If the amount entered by the user is less than 1 or greater than 9, the loop condition remains true and a warning is displayed, after which the user is prompted to enter another value.

To try this piece of code, click here.    

Note that if you enter a correct value immediately, the while loop never executes. The condition is false the first time it is tested, so the loop never begins.