ZBasic Language Reference
Dim i as Integer
i = 1
Do While (i <= 10)
i = i + 1
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
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-
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
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.
BasicX Compatibility Note
In BasicX mode, the For-Next statement is much more restrictive. The loop index variable must be
a scalar integral type and must also be a local variable. Referring to the loop index variable in a
Next statement is not supported. The <step-expr> is restricted to a constant expression that
evaluates at compile time to either 1 or 1. Lastly, For-Next loops may be nested to a maximum
depth of 10 for compatibility.
When Option Strict is enabled, there are additional restrictions that apply. Firstly, the loop index
variable must be local to the routine; it cannot be defined at the module level. Secondly, the loop index
variable is not allowed to be used or modified outside of the For-Next loop except that it can be used as
the loop index variable in a subsequent For-Next loop. Thirdly, inside the For-Next loop the loop index
variable is read-only. Any attempt to modify the loop index variable, or pass it by reference to another
routine will result in an error message from the compiler.