If...Then

Control flow statement for conditional branching

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

      Remark: EndIf (without blank) is also supported like in QB for 
      backward compatibility.

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.

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
      

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.

See also
   * ElseIf
   * Do...Loop
   * #if
   * Select Case

