Decision Making and Looping

By Amrit Hallan
2005-06-01


Decision Making and Looping
A common feature of any programming script/language is, its ability to perform repeat tasks until a condition is satisfied, and, its ability to take decision. After all, there can be no logic if a program cannot decide what to do and what not to do (sounds like a Shakespearean statement).

Decision Making

VBScript too can take if-then-else decisions. An example:

<%
Dim num1, num2
num1=45
num2=33

if num1<num2 then
%>

  <p><%=num1%> is less then <%=num2%></p>

<%
elseif num1=num2 then
%>

  <p><%=num1%> is equal to <%=num2%></p>

<%
else
%>

  <p><%=num1%> is greater than <%=num2%></p>

<%
end if
%>

Side Note: In the above program, we already know the values of num1 and num2, but if a user enters these values using an HTML form, then only this sort of program will be able to tell which value is greater.

We can also check for multiple conditions, for instance, "if this is Jack's room and Jack is in the room and the room is in his house then Jack is at home." Programmatically:

<%
Dim JacksRoom, RoomOfJacksHouse, JackInRoom, JackAtHome
JacksRoom=true
JackInRoom=true
RoomOfJacksHouse=true

if JacksRoom=true and JackInRoom=true and RoomOfJacksHouse=true then
  JackAtHome=true
Else
  JackAtHome=false
End if
%>

So for JackAtHome to be true, all preceding variables must be true too. So if we put an AND, all the conditions have to satisfy the "if" condition.

If Jack can be at home if even one of the conditions is true, then we can have

<%
if JacksRoom=or JackInRoom=or RoomOfJacksHouse=true then
  JackAtHome=true
End if
%>

In case of an OR, any one of the conditions can satisfy an if structure.

You can see that the if-then-else-end-if statement is almost similar to the real-world decision-making structure, and hence, is quite self-explanatory. Just take care that each if-then-else-end-if is properly indented, so that you can have multiple sub-if-then-else-end-if's without getting confused or bogged down.

These are also called logical structure, for, if a and b are not equal, then some condition being a surely means that the condition is not b.

Similar to if-then-else-end-if is the Select Case structure, which is much better organized then the if structures. Let's see how. (Note that whenever we have to insert remarks in the VBScript code, it is preceded by a single-quote.)

<%
' Assuming a visitor chooses a particular food recipe to read and assume the value is being stored in a variable called recipe. So we use the Select Case in the following manner:

Select Case recipe
  Case "Egg Curry" ' recipe contains "Egg Curry"
    ' take the appropriate action
  Case "Mango Pie" ' recipe contains "Mango Pie"
    ' take the appropriate action
  Case Else ' All other recipes have been checked for but the one left
    ' take the appropriate action
End Select
%>

Looping

Many a time you have to perform a programming task again and again. For example, the program should keep reading the data rows of a database file until it encounters the end of the file. Or, a shopping cart script should keep adding items to the cart until the user clicks on "checkout" button.

Primarily, in VBScript, there are four loop structures, namely, for…next, do…until…loop, while…wend, and do…while…loop. Let us see them one by one…continued on the next page.

Tutorial Pages:
» Decision Making and Looping
» Decision Making and Looping...Continued

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