Conditional Statements--tutorial.jcwcn.com

This chapter describes:

"If" Statements
"If" Statement Example
"Select Case" Statements
"Select Case" Statement Example

"If" Statements

There 4 flavors of "If" statements in VB:

1. Single-statement "If":

   If condition Then statement

where "condition" is Boolean value, and "statement" is any VB statement. The specified statement will be executed, if and only if the specified condition is "True".

2. Multi-statement "If":

   If condition Then 
      statement_block (multiple statements)
   End If

The specified multi-statement block will be executed, if and only if the specified condition is "True".

3. "If ... Else" Statement:

   If condition Then
      statement_block_a
   Else
      statement_block_b
   End If

Two statement blocks are specified. But only one statement block will be executed based on the value of the specified condition. If the specified condition is "True", the first block will be executed. If the specified condition is "False", the second block will be executed.

4. "If ... ElseIf" Statement:

   If condition_1 Then
      statement_block_1
   ElseIf condition_2 Then 
      statement_block_2
   ...
      ...
   Else
      statement_block_n
   End If

Many statement blocks are specified. But only one statement block will be executed based on the value of the condition specified for that block.

"If" Statement Example

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

<html>
<body>
<!-- condition_if.html
   Copyright (c) 2006 by Dr. Herong Yang. http://www.herongyang.com/
-->
<pre>
<script language="vbscript">
   ' Single-statement "If"
   bIsNewYear = True
   document.writeln("")
   If bIsNewYear Then document.writeln("Happy New Year!")

   ' Multi-statement "If"
   bIsLogOn = True
   document.writeln("")
   If bIsLogOn Then
      document.writeln("Open the log file.")
      document.writeln("Write the log message.")
      document.writeln("Close the log file.")
   End If
   
   ' "If ... Else" statement
   bIsAdmin = False
   document.writeln("")
   If bIsAdmin Then
      document.writeln("Display the delete button.")
      document.writeln("Display the edit button.")
   Else
      document.writeln("Display the view button.")
   End If
   
   ' "If ... ElseIf" statement
   iDay = 4
   If iDay = 0 Then
      sOpenHours = "closed"
   ElseIf iDay >= 1 And iDay <=5 Then
      sOpenHours = "open from 9:00am to 9:00pm"
   ElseIf iDay = 6 Then 
      sOpenHours = "open from 9:00am to 5:00pm"
   Else
      sOpenHours = "undefined"
   End If 
   document.writeln("")
   document.writeln("The library is " & sOpenHours)
</script>
</pre>
</body>
</html>

Here is the output:

Happy New Year!

Open the log file.
Write the log message.
Close the log file.

Display the view button.

The library is open from 9:00am to 9:00pm

The output matches my expectation perfectly!

"Select Case" Statements

"Select Case" statement is an alternative way of executing statements selectively based on certain conditions. Here is the syntax of "Select Case" statements:

   Select Case test_value
      Case expected_value_list_1
         statement_block_1
      Case expected_value_list_2
         statement_block_2
      ...
         ...
      Case Else
         statement_block_n
   End Select

where "test_value" is a value used to test against with expected values specified in the "Case" clauses, and "expected_value_list_*" are lists of expected values. The execution of "Select Case" statement goes like this:

Each "Case" clause will be visited sequentially from top to bottom.
If one of the values in the expected value list equals to the test value in a "Case" clause, the statement block in that clause will be executed.
If the statement block of any "Case" clause gets executed, the entire statement ends.
The "Case Else" clause is a special clause. Its statement block will be executed if and only if no statement block of any other "Case" clauses gets executed.

"Select Case" Statement Example

To help you understand how "Select Case" statements work, I wrote the following the example, condition_case.html:

<html>
<body>
<!-- condition_case.html
   Copyright (c) 2006 by Dr. Herong Yang. http://www.herongyang.com/
-->
<pre>
<script language="vbscript">
   iDay = 4
   Select Case iDay
      Case 0
         sDay = "Sunday"
         sOpenHours = "closed"
      Case 1, 2, 3, 4, 5
         sDay = "a weekday"
         sOpenHours = "open from 9:00am to 9:00pm"
      Case 6
         sDay = "Saturday"
         sOpenHours = "open from 9:00am to 5:00pm"
      Case Else
         sDay = "unkown"
         sOpenHours = "undefined"
   End Select
   document.writeln("")
   document.writeln("Today is " & sDay)
   document.writeln("The library is " & sOpenHours)
</script>
</pre>
</body>
</html>

Here is the output:

Today is a weekday
The library is open from 9:00am to 9:00pm

Hope you know how to use "Select Case" statements now.

Conclusions

Remember that:

"If" must be followed by a condition and "Then".
"ElseIf" is one word, no space.
"Case" is followed by a list of expected values.
Unlike C language, no need to break statements in "Case" clauses.

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