Using Logic¶

Programming logic is what controls the flow of your code. It allows you to create programs that have different results depending on what inputs the program is given. This makes programming sort of like a “Choose Your Own Adventure” book that has different endings depending on what actions you choose for the character throughout the book.

Basic If/Else Statement¶

Using our previous example, let’s say we wanted to change the text to display “Good Morning” if a browser retrieves the document in the morning and “Good Afternoon” when a browser retrieves the documents in the afternoon. The following code uses an if statement and an else statement to accomplish this:

<html>
   <body>
   [if(date->hour < 12)]
      Good Morning! I am an HTML document.
   [else]
      Good Afternoon! I am an HTML document.
   [/if]
   </body>
</html>

The Lasso code checks to see if the current hour is before noon (date uses 24-hour time) and displays “Good Morning!” if it is, otherwise (else), it displays “Good Afternoon!”.

Using If/Else Statements¶

Let’s say we want to expand our example to say “Good Evening!” as well. The example below defines morning as the hours between 5:00 AM and noon, afternoon as the hours between noon and 5:00 PM, and the evening as everything else:

<html>
   <body>
   [if(date->hour >= 5 and date->hour < 12)]
      Good Morning! I am an HTML document.
   [else(date->hour >= 12 && date->hour < 17)]
      Good Afternoon! I am an HTML document.
   [else]
      Good Evening! I am an HTML document.
   [/if]
   </body>
</html>

A couple of things to notice about this code. First, the use of and in the opening if statement. This is called a “boolean” operator or “logic” operator: it requires that both the expression to its left and to its right evaluate as “true”. In our case it means that the current hour has to be greater than or equal to 5 as well as less than 12. You will sometimes see it as && instead of and as in the second else statement. (See the section Boolean Operations for more information on all the available logical operators.)

The other thing to note is the first else statement takes an expression in parentheses. This is known as an “else if” statement, and it allows for specifying an additional condition that needs to be met for the code inside it to be executed. The first conditional is that the if statement was not met, and then it will evaluate the conditional statement following the else to see if it is met. If not, the code will continue to the else statement (if any) and execute the code inside it. You can have multiple “else if” statements before an optional final else.

So the example page above will say “Good Evening!” from 5 PM–4:59 AM, “Good Morning!” from 5 AM–11:59 AM, and “Good Afternoon!” from 12 PM–4:59 PM.