ZBasic Language Reference
27
ZBasic Microcontrollers
Do While (val > 1)
factVal = factVal * val
val = val - 1
Loop
Factorial = factVal
End Function
This function implements a mathematical operation called factorial. The value of N factorial (usually
written N!) is the product of all the integers from 1 up to and including N. The value of 0! is, by definition,
1. The value of 3! is 6 and so on.
This example introduces a couple of statements that we haven’t seen yet but they are fairly
straightforward. Notice that a variable named factVal was defined within the function. This variable is
called a “local variable” and it is not visible to any code outside of the function. The other aspect of this
local variable that is different from variables defined at the module level is that this variable only takes up
space while the function is executing. When the function returns, the space used by the local variable is
reclaimed by the system and can be used for other purposes. See Section 3.1 for more information on
the concepts of scope and lifetime of variables.
The second line of the function shows a value being assigned to the variable factVal. Instead of using
the literal constant value as shown on the right hand side of the equal sign, we could just as well have
written an expression that involved several values (perhaps including function invocations) and operators
like addition, subtraction, multiplication, etc. This statement is known as an assignment statement and is
described in more detail in Section 2.5.1.
The third line of code illustrates a useful variation on the Do loop that we have already seen. In this case,
the Do has a condition associated with it. The condition is tested before every pass through the loop and
as long as the conditional expression evaluates to the Boolean value True, the statements within the
loop are executed again. In this case, the condition tests if the val parameter has a value greater than 1,
the statements in the loop will be executed. Otherwise, control will transfer to the first statement following
the Loop statement.
Within the loop, there are two more assignment statements, each of which has an expression on the right
hand side, the first involving multiplication and the second involving subtraction. The meaning of the
expressions should be self-evident.
The last statement of the function, just before the End Function, illustrates how the value to be
returned by the function is set. It is useful to think of there being a variable automatically defined that has
the same name as the function as well as the same type. We call this a return value variable. Within the
function, you can refer to the return value variable, in most cases, just as you would any other variable or
parameter. In the example above, instead of introducing a new variable factVal, we could have instead
simply used the reference to the return value variable Factorial. The function’s code is re-written
below using this idea.
Function Factorial(ByVal val as Long) As Long
Factorial = 1
Do While (val > 1)
Factorial = Factorial * val
val = val - 1
Loop
End Function
As with subroutines, variables and constants may be defined within a function. When this is done, the
variable or constant is private to the function and cannot be directly accessed from any other routine.
Although it is common to place such definitions near the beginning of the function, the definitions may
occur anywhere in the subroutine as long as they occur before their first use.
Note that the normal execution sequence of a function may be altered by using the Exit Function
statement. When executed, this statement causes control to return to the caller immediately. Also, it is
important to know that the return value variable of a function is not automatically initialized. If your
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