Loop Statements--tutorial.jcwcn.com

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>  <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>

Here is the output:

Found a prime number: 3
Found a prime number: 5
Found a prime number: 7
Found a prime number: 11
Found a prime number: 13
Found a prime number: 17
Found a prime number: 19
Found a prime number: 23
Found a prime number: 29

The output looks good. Some notes about the sample script:

Nested loops, one loop inside another loop, are used.
The outer loop goes through integers between 3 and 20, and skips even numbers with "Step 2"
The inner loop goes determines if the current integer "i" is a prime or not
The inner loop has a conditional "Exit For" statement to terminate the loop as soon as "i" has been determined as a non-prime number.

"While" Statements

Another type of loop statements is called "While" statement, which has the following syntax:

   While condition
      statement_block (multiple statements)
   Wend

where "condition" is a Boolean value.

The way a "While" statement is executed looks like:

Step 1: Check Boolean value of "condition".

Step 2: If the current value of "condition" is "True", continue with Step 4.

Step 3: If the current value of "condition" is "False", terminate the loop.

Step 4: Execute "statement_block" enclosed in the "While ... Wend" loop.

Step 5: Continue with Step 1.

The logic of a "While" statement is simpler than a "For ... Next" statement. But you have to manage the condition carefully, so that its value will become "False" at some point to terminate the loop.

Notice that there seem to be no "Exit" statement to break "While" loop early.

"While" Statement Example

To help you understand how "While" statements work, I wrote the following the example, loop_while.html:

<html>
<body>
<!-- loop_while.html
   Copyright (c) 2006 by Dr. Herong Yang. http://www.herongyang.com/
-->
<pre>
<script language="vbscript">
   document.writeln("")
   i = 3
   While i <= 30
      bIsPrime = True
      j = 2
      While j <= i\2 And bIsPrime
         bIsPrime = i Mod j > 0
         j = j + 1
      Wend  
      If bIsPrime Then 
         document.writeln("Found a prime number: " & i)  
      End If
      i = i + 2
   Wend
</script>
</pre>
</body>
</html>

Here is the output:

Found a prime number: 3
Found a prime number: 5
Found a prime number: 7
Found a prime number: 11
Found a prime number: 13
Found a prime number: 17
Found a prime number: 19
Found a prime number: 23
Found a prime number: 29

The example worked perfectly. What do you think?

"Do ... Loop" Statements

Personally, I think that two types of loops, "For ... Next" and "While", are enough for any loop programming situations. But the designers of VB decided to give us an extra type of loops, "Do ... Loop" statements, with the following different flavors:

1. "Do While ... Loop" Statements

   Do While condition
      statement_block
   Loop

They are identical logically to:

   While condition
      statement_block
   Wend

2. "Do Until ... Loop" Statements

   Do Until condition
      statement_block
   Loop

They are identical logically to:

   While Not condition
      statement_block
   Wend

3. "Do ... Loop While" Statements

   Do
      statement_block
   Loop While condition

They are identical logically to:

   statement_block
   While condition
      same_statement_block
   Wend

4. "Do ... Loop Until" Statements

   Do
      statement_block
   Loop Until condition

They are identical logically to:

   statement_block
   While Not condition
      same_statement_block
   Wend

I will leave it to you to write some sample scripts for "Do ... Loop" statements as exercises.

Note that VB offers "Exit Do" statements to allow you to terminate "Do ... Loop" statements early.

Conclusions

Question: How many ways of writing a loop in VB? My answer is 7:

"For ... Next".
"While ... Wend".
"Do While ... Loop".
"Do Until ... Loop".
"Do ... Loop While".
"Do ... Loop Until".
"For Each ... Next" - Used with arrays and collections. See the array chapter for details.

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