If...Thenkeyword

Control flow statement for conditional branching

Control Flow Statementskeyworddocumented

Syntax

If expression Then [statement(s)] [Else [statement(s)]] [End If]

If expression Then : [statement(s)] [Else [statement(s)]] : End If

If expression Then

[statement(s)]

[ ElseIf expression Then ]

[statement(s)]

[ Else ]

[statement(s)]

Description

If...Then is a way to make decisions.
It is a mechanism to execute code only if a condition is true, and can provide alternative code to execute based on more conditions:

Execute code (just behind Then) if a condition is true.
Execute certain code (just behind Then) if a condition is true and execute other (just behind Else) if it's false.
Test other conditions (with ElseIf) if the first condition is false.

expression can be one of several forms:

a conditional expression, for example:

x = 5

multiple conditions separated by logical bit-wise operators, for example:

x >= 5 And x <= 10

multiple conditions separated by logical short-circuit operators, for example:

y <> 0 AndAlso x \ y = 1
(in this case, "x \ y = 1" will only be evaluated if "y <> 0" is True)

any numerical expression, in which case a value of zero (0) represents False, and a non-zero value represents True

Both multi-line and single-line Ifs can be nested. In the latter case, the optional End Ifs can be useful to control where nested Ifs begin and end.

The multi-line syntax allows several Elseifs (but none after a Else) and tests can be nested (there must be in this case as many End Ifs as Ifs).
If the condition of the If is not true, those of Elseifs blocks are tested in succession:

If either of these is true, the corresponding code is executed, then the program skips the following blocks to continue after the End If.
If none are true, the code following the Else (if exists) is executed alone.

In the -lang fb and -lang fblite dialects, colons (:) can be used instead of newlines to construct multi-line If blocks on a single line.

Note: The single-line syntax If...Goto, as shortcut for If...Then Goto, is deprecated and it only exists for compatibility with QB.

Remarks

Dialect Differences

In the -lang qb and -lang fblite dialects, variables declared inside an If..Then block have a procedure-wide scope as in QB
In the -lang fb and -lang deprecated dialects, variables declared inside an If..Then block are visible only inside the block, and cannot be accessed outside it. To access duplicated symbols defined as global outside this block, add one or preferably two dot(s) as prefix: .SomeSymbol or preferably ..SomeSymbol (or only ..SomeSymbol if inside a With..End With block).
In the -lang qb dialect, if there is a new line or a single-line comment (') directly after THEN, then the IF will be multi-line. A colon, a Rem or any other statement will result in a single-line IF.
In the -lang fb and -lang fblite dialects, if there is a new line, a single-line comment ('), a colon (:), or a Rem statement directly after THEN, then the IF will be multi-line. Any other statement will result in a single-line IF.

Differences from QB

END IF was not supported in single-line IFs in QBASIC.

Example

'' Here is a simple "compute the square root" code using a single-line if...then for the decision,

'' but with multiple statements extended with colons (:)



Dim As Double d , r

r = -1

d = 2

'd = -3



If d > 0 Then r = Sqr(d) : Print "square root computed:" Else r = 0 : Print "square root not computed:"

Print r



Sleep

'' Here is a simple "guess the number" game using a multi-line if...then for a decision.



Dim As Integer num, guess



Randomize

num = Int(Rnd * 10) + 1 'Create a random number between 1 and 10...

                

Print "guess the number between 1 and 10 (or CTRL-C to abort)"



Do 'Start a loop



    Input "Guess"; guess 'Input a number from the user



    If guess > 10 OrElse guess < 1 Then  'The user's guess is out of range

        Print "The number can't be greater then 10 or less than 1!"

    ElseIf guess > num Then  'The user's guess is too high

        Print "Too high"

    ElseIf guess < num Then  'The user's guess is too low

        Print "Too low"

    Else                     'The user guessed the right number!

        Print "Correct!"

        Exit Do   'Exit the loop

    End If



Loop 'Go back to the start of the loop



Sleep

Reference

Documented in KeyPgIfthen.html