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ZBasic Language Reference
31
ZX Microcontroller Family
2.5.8 For-Next Statement
The For-Next compound statement is another form of looping that provides controlled iteration using a
loop index variable. The syntax for a For-Next loop is:
For <var> = <start-expr> To <end-expr> [ Step <step-expr> ]
[<statements>]
Next [ <var> ]
The <var> element, referred to as the loop index variable, must refer to a previously defined scalar
variable (i.e. not an array element) that is either a numeric type or an enumeration type. The <start-
expr>, <end-expr> and optional <step-expr> elements must all be the same type as the loop index
variable.
Example
Dim i as Integer
For i = 1 To 10
Debug.Print CStr(i)
Next i
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
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