ZBasic Language Reference
37
ZBasic Microcontrollers
When the For statement begins execution, the <start-expr> is evaluated and the resulting value is
assigned to the loop index variable. Then, before executing any statements contained within the body of
the For-Next statement, the <end-expr> is evaluated and the value of loop index variable is compared
to that value. If the value of the loop index variable is less than or equal to the value of the <end-expr>
the statements within the body of the For-Next are executed. This is called the “loop entry test” because
it controls whether or not the loop statements are executed.
At the bottom of the loop, marked by the Next statement, the loop index variable is modified in
preparation for the next iteration of the For-Next loop. If the optional Step <step-expr> is present its
value is added to the loop index variable, otherwise the loop index variable is simply augmented by 1.
Then control is transferred back to the top of the loop where the loop entry test is performed again.
The logic of the For loop in the example above may be exactly duplicated using other statements as
illustrated below. You can see that the For loop allows you to express the same logic more concisely.
Dim i as Integer
i = 1
Do While (i <= 10)
Debug.Print CStr(i)
i = i + 1
Loop
There are several things to note about the For-Next loop. Firstly, it is important to be aware that although
the values of the <end-expr> and <step-expr> elements are used multiple times, the expressions are
only evaluated once, when the execution of the For-Next begins. This distinction is only important, of
course, if these expressions contain references to other variables that might be modified during loop
execution or if they involve function calls. Secondly, because the loop entry test checks to see if the loop
index variable is less than or equal to the <end-expr> you must choose a data type for the loop index
variable that is capable of representing a value that is greater than the value of the <end-expr> .
Otherwise, the loop entry test will always be true. Consider this errant example:
Dim i as Byte
For i = 0 to 255
<statements>
Next i
This loop will never terminate because the value of i is always less than or equal to 255 since it is a
Byte type. Where possible, the compiler will issue a warning if it detects these problematic conditions.
Even so, you should develop the habit of considering this potential problem every time you code a For-
Next loop.
The third important aspect of the For-Next statement is that if the <step-expr> evaluates to a negative
value, the sense of the loop entry test changes. In this case, the loop index variable is tested to see if it is
greater than or equal to the <end-expr> and, if so, the statements of the loop are executed. Also, in this
case the caveat noted above about the range of the loop index variable changes. The loop index variable
must be capable of representing a value that is less than the value of the <end-expr>. When a For loop
is used with an unsigned data type, the step value is considered to be negative if the most significant bit
of the value is a 1.
One other note: it is permissible for the <step-expr> to evaluate to zero. This will cause the For loop to
execute indefinitely. The For-Next loop may be terminated at any time by using the Exit For
statement.
The presence of the <var> on the Next statement is optional. However, if it is present, it must match the
name of the loop index variable of the For loop with which it is associated. There is no fixed limit on how
deeply For-Next loops may be nested. The actual limit is governed by how much memory is available to
the compiler. For all practical purposes, there is no limit.
 |
 |
 |