If...Then...Else
The If, Then, Else statements are used to process one or more instructions conditionally, based on the evaluation of an expression.
Syntax 1 (single line form)
If condition Then thenstatements [Else elsestatements]
Syntax 2 (block form)
If condition1 Then
[statementblock-1]
[ElseIf condition2 Then
[statementblock-2] ]
[Else
[statementblock-n] ]
EndIf
(or End If)
Example
The example illustrates the various forms of the If...Then...Else syntax.
Dim X, Y, Temp( 5 ) 'Declare variables.
X = Temp( 1 )
If X < 10 Then
Y = 1 '1 digit.
ElseIf X < 100 Then
Y = 2 '2 digits.
Else
Y = 3 '3 digits.
EndIf
. . . . 'Run some code.
. . . . 'Run some code.
The Second Example is a full program that shows the basic functionality of the If..Then..Else syntax.
'This program measures the room temperature using a type T thermocouple.
'A string variable called Temp_Status changes between Cold, Normal, and Hot
'depending on the room temperature.
'Declare Public Variables
Public Temp'Variable for temperature
Public Temp_Status As String *50'variable for temperature status
Dim Tref 'variable for reference temperature
'Declare Temperature_Data table
DataTable (Temperature_Data,True,-1)
Sample(1,Temp,FP2)
Sample(1,Temp_Status,String)
EndTable
'Begin main program
BeginProg
Scan (1,sec,2,0)
TCDiff (Temp,1,mv200C,U1,TypeT,TRef,True,0,15000,1.0,0)
'Measure temp of Diff Chan 1
'a temperature between 20 and 25 degrees C is designated normal
If Temp >=20 AND Temp <= 25 Then
Temp_Status = "Normal" 'update Temperature status
'a temperature above 25 degrees C is designated Hot
ElseIf Temp > 25 Then
Temp_Status = "Hot"
'A temperature below 20 degrees C is designated Cold
ElseIf Temp < 20 Then
Temp_Status = "Cold"
EndIf
CallTable Temperature_Data
NextScan
EndProg
Remarks
The single line form shown in Syntax 1 is often useful for short, simple conditional tests. The EndIf statement is not used.
The block form of an If...Then control structure, as shown in Syntax 2, provides more structure and flexibility than the single line form and is usually easier to read, maintain, and debug. This structure allows you to specify an alternate Condition to evaluate if the first Condition is not True by using the ElseIf statement. The statements in a block form control structure cannot have any characters, other than spaces or tabs, before them on a program line. An EndIf statement must close the block form of an If…Then control structure.
The AND and OR instructions can be used along with the If…Then…Else instructions for more flexible (and powerful) programming logic.
Parameters
If
The If statement begins the If...Then control structure.
Condition
The Condition is an expression that evaluates True (nonzero) or False (0 and Null). With the single-line form, you can have multiple statements for a Condition, but they must be on the same line and separated by colons, as in the following statement:
If A > 10 Then A = A + 1 : B = B + A : C = C + B
The implied EndIf occurs at the very end of the line.
Then
The Then statement defines actions to be taken if the Condition is True.
ThenStatements
The ThenStatements are instructions or program branches to be performed when condition is true.
ElseIf
The ElseIf statement is used to specify another Condition to be evaluated if the first Condition is False. If the first Condition is True, the ElseIf statement is not evaluated and control is passed to the portion of code following the Then statement. Multiple ElseIf statements can be used in one If…Then control structure. ElseIf statements must appear before the Else statement.
Else
The Else statement defines actions to be taken if the Condition is False. If the Else statement is not present, control passes to the next statement in the program.
ElseStatements
The ElseStatements are instructions or program branches to be performed when the Condition is False.
EndIf
The EndIf instruction is used to indicate the end of the If/Then/Else construct. It can also be written as End If.
Also see Conditional Compiling.
NOTE: The Select Case instruction may be more useful when evaluating a single expression that has several possible actions.