Program control statements are the heart of computer operations. Up to now, we have discussed how to store data, add and subtract data, and organize it in procedures. Program control statements such as if-then and for-next allow us to manipulate data.
The following sections define some of the more common program control statements.
If-then-else
This is the most commonly used statement in programming to control program execution.
Syntax:
If condition then action
---a single action
---following
is for multiple actions and else conditions
If condition then
action(s)
Elseif condition then
action(s)
Else
action(s)
End if
The conditional clause must evaluate to True or False.
The conditional clause is based upon boolean logic...
The AND truth table...
| inputs | T | F |
| T | T | F |
| F | F | F |
The OR truth table...
| inputs | T | F |
| T | T | T |
| F | T | F |
The conditions can be quite complex...
Consider variables a, b, c, and d with values of...
 |
a is true |
|
b is -2 |
|
c = "dog" |
|
d = false |
if ((a = false) or (b > 3)) and ((c = "cat") or (d = false))
interprets to...
if (F or F) and (F or T)
if F and T
-->will evaluate to False
Note: groupings of clauseson the other hand...
if ((a = false) or (b > 3) and (c = "cat")) or (d = false))
interprets to...
if (F or F and F) or T
if F or T
--> will evaluate to True
Note: evaluation is in sequence
Remember, at a minimum, you need at least one "If condition then action" for a valid if-then statement.
Example:
If A > B then
Do_this
Else
Do_this_instead
End if
Select-Case
Similar to an if-then, allows you to evaluate a series of conditions (cases) and take action.
Syntax:
Select variable
Case variable-value
action(s)
Case variable-value
action(s)
Case variable-value
action(s)
...
End Select
For-next
Cycles through statements until a value is reached. Starting and end point values must known. Also an increment amount may be set (e.g., increment by one, by two, etc.).
Syntax:
For counter = startval to endval <step stepvalue>
<actions>
next
Counter, startval, endval, stepvalue may be variables or explicit values.
Step is assumed as 1 if not given.
Example:
For I = 0 to 100 'repeat 101 times
<actions>
Next 'increment
Do-loop (While)
Cycles through a loop until the necessary condition is met.
Syntax:
Do <while, until> <condition>
<actions>
Loop <while, until> <condition>
In JavaScript, the syntax is a little different:
Syntax:
Do
<actions>
While <condition>
Condition can be any logical expression.
Example:
I = 0
Do Until I = 1000
Action
Action
I = I + 1
Loop
Do-loops are often used to monitor an on-going process such as reading from data from a file when the file size is unknown. The do-loop continues until the end of the file is reached.
Go-to
Often used in error checking to process unexpected events. Go-to usually jumps to another section of the function or procedure.
Go-to expressions require a label in the code for a destination.
Syntax:
GoTo <label>
…code
<label:>
…code following the label
Example:
Sub subname ()
On error GoTo errfound 'if error found, go to this label
<action>
<action>
Exit sub 'leave if no error detected
errfound: 'label for error processing
<action>
End sub