Control Structures

Control structures are statements that modify the running state of your program - that is, they turn your program into a state machine and allows it to perform logical decisions. Your program can be in a different state depending on various conditions. For instance, if you're hungry then you are empty, but if you eat, you are full. Or if you're not hungry or empty, then you are something else that is not defined. Notice how the state can change depending on the conditions that are set. We call these conditionals and they impart intelligent logic into our programs.

In FreeBASIC, as well as other programming languages, there are three types of control structures.

• Decision Making Structures - makes a series of decisions using if,then,else statements
• Branching Structures - makes a decision based on an array of conditions using case and case select statements
• Looping Structures - Performs looping while a condition is a certain state

In this section we will only focus on Decision Making Structures and Branching Structures. In the next section where we discuss loops, we'll delve more into Looping Structures.

It may be tempting to humanize your program and believe that there is a “maybe” state. There are no maybes. A conditional statement can only be either true or false. We call this Boolean Logic.

First some basic theory…

Computers “think” in binary terms. A bit can only be either a 0 or 1, therefore it's running state is absolute. In fact, at the most fundamental level, a computer processor is a series of transistor gates that can only work in a black or white way.

Conditional structures have two types of operators - relational and bitwise. Together we can create very powerful statements that represent a wide variety of computational logic.

Don't let bitwise operators intimidate you. For the purpose of tutorial we'll be focusing on the AND and OR operators.

A usual conditional structure will look as follows:

dim as string myName
 
input "What is your name? ", myName
 
if myName = "Praetor" then
  print "Ave Praetor!"
else
  print "You are not praetor"
end if

Let's break down this example.

dim as string myName 

create a variable called myName as we learned from the previous section

input "What is your name? ",myName

input is a new keyword that asks the user for input and stores it in a variable, myName in this case.

if myName = "Praetor" then
  print "Ave Praetor!"

if myName is “Praetor” then, perform some function. In this case, print “Ave Praetor” to the screen

else
  print "You are not praetor"
end if

if myName is anything else besides “Praetor”, perform another function. This time print “You are not praetor” to the screen and end the statement.

But notice if you tell your program your name is “praetor” instead of “Praetor”. It will tell you that your name is not Praetor! But why? The “=” operator in FreeBASIC matches strings as string literals. This is different behavior from some other programming languages. So it will only respond when myName is exactly 'Praetor'. While there are many ways to solve this, the easiest for our purposes is to make myName all upper-case, like so

if ucase(myName) = "PRAETOR" then...

But what if you want it to match it exactly? This requires regular expressions which matches a string against a format. Regular expressions are out of the scope of this beginner's tutorial, but there are plenty of resources you can turn to.

What if you have many things you wish to test? Well you can do that with the elseif statement, like so:

if Ucase(myName) = "PRAETOR" then
  print "Ave Praetor!"
elseif Ucase(myName = "SMJ") then
  print "Howdy SMJ!"
else
  print "I don't know you"
end if

You can have as many elseif statements as you wish, and while you don't necessarily need an else statement, it is considered good form to have one to catch any unknowns a user may have. You'll see why when we discuss functions and subroutines.

We can also use bitwise operators like AND, or OR to test multiple things as well. Like such.

if Ucase(myname) = "PRAETOR" or "SMJ" then
   print "I know you!"
...

OR states that if myName is either PRAETOR or SMJ, then the condition is true.