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ZBasic Language Reference 32 ZX Microcontroller Family Do Until <boolean-expression> [<statements>] Loop Do [<statements>] Loop While <boolean-expression> Do [<statements>] Loop Until <boolean-expression> As you can see, the difference between the four variations is whether the test is at the top of the loop or at the bottom of the loop and, secondly, the logic sense of the condition. Testing the condition at the top of the loop means that the statements within the loop may be executed zero or more times. Testing the condition at the bottom of the loop means that the statements will always be executed at least once. The difference between using While and Until is nothing more than a logic inversion. The construct Do While Not <boolean-expression> is logically equivalent to Do Until <boolean-expression>. There is no fixed limit on how deeply Do 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 nesting of Do loops is limited to 10 for compatibility. 2.5.7 Exit Statement The Exit statement allows you to terminate the execution of a loop, a function or a subroutine earlier than it otherwise would. This is most commonly used when a condition is detected by the code that prevents further normal processing. The syntax for the Exit statement is: Exit <exit-type> The <exit-type> element may be Do, For, Sub or Function but the use of each is restricted to use within a Do-Loop, For-Next, subroutine and function, respectively. Any other use will result in a compiler error. Example Do <other-statements> If (i > 5) Then Exit Do End If <other-statements> Loop When an Exit Do is executed within nested Do-Loop statements only the innermost Do-Loop that contains the exit statement will be terminated. Control will be transferred to the first statement following the terminated Do-Loop. The same idea applies to an Exit For within nested For-Next statements. 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: