VBScript Tutorials - Herong's Tutorial Notes
Dr. Herong Yang, Version 4.01

Loop Statements

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VB Script Tutorials - Herong's Tutorial Notes © Dr. Herong Yang

Data Types and Literals

Variables

Logic Operations

String Operations

Conditional Statements

Arrays

Loop Statements

Functions and Subroutines

Built-in Functions

Variable Inspection

... Table of Contents

This chapter describes:

  • "For ... Next" Statements
  • "For ... Next" Statement Example
  • "While" Statements
  • "While" Statement Example
  • "Do ... Loop" Statements

"For ... Next" Statements

"For ... Next" is probably the most commonly used loop statement in VB. Here is its syntax: 

   For loop_variable = initial_value To final_value [Step interval]
      statement_block (multiple statements)
   Next

where "loop_variable" is a temporary variable used to control the loop, "initial_value" is the initial value to be assigned to the loop variable, "final_value" is the upper bound to stop the loop, and "interval" is the interval that controls how the loop variable to be updated at each iteration. Note that if "interval" is not specified, "1" will be used as the interval.

If you want to know exactly how the "For ... Next" statement will be executed, here is my understanding:

Step 1: "intial_value" is assigned to "loop_variable".

Step 2: Comparing the current value of "loop_variable" against the "final_value".

Step 3: If the current value of "loop_variable" is less than or equal to the "final_value", continue with Step 5.

Step 4: If the current value of "loop_variable" is greater than the "final_value", terminate the loop.

Step 5: Execute "statement_block" enclosed in the "For ... Next" loop.

Step 6: Update the "loop_variable" with the current value plus the "interval"

Step 7: Continue with Step 2.

Note that the above description is based positive "interval". If "interval" is a negative value, the termination condition will be "the loop variable value is less than the final value.

If you want to terminate the loop early, you can use the "Exit For" statement, which will terminate the loop immediately.

"For ... Next" Statement Example

To help you understand how "For ... Next" statements work, I wrote the following the example, loop_for.html: 

<html>
<body>
<!-- loop_for.html
   Copyright (c) 2006 by Dr. Herong Yang. http://www.herongyang.com/
-->
<pre>
<script language="vbscript">
   document.writeln("")
   For i = 3 To 30 Step 2
      bIsPrime = True
      For j = 2 To i\2
         bIsPrime = i Mod j > 0
         If Not bIsPrime Then Exit For
      Next
      If bIsPrime Then 
         document.writeln("Found a prime number: " & i)  
      End If
   Next
</script>
</pre>
</body>
</html>

(Continued on next part...)

Part:   1   2  3 

Dr. Herong Yang, updated in 2006
VBScript Tutorials - Herong's Tutorial Notes - Loop Statements