13.2.3 The If..then..else statement

The If .. then .. else.. prototype syntax is

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If then statements


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The expression between the if and then keywords must have a Boolean result type. If the expression evaluates to True then the statement following the then keyword is executed.

If the expression evaluates to False, then the statement following the else keyword is executed, if it is present.

Some points to note:

• Be aware of the fact that the boolean expression by default will be short-cut evaluated, meaning that the evaluation will be stopped at the point where the outcome is known with certainty.
• Also, before the else keyword, no semicolon (;) is allowed, but all statements can be compound statements.
• In nested If.. then .. else constructs, some ambiguity may arise as to which else statement pairs with which if statement. The rule is that the else keyword matches the first if keyword (searching backwards) not already matched by an else keyword.

For example:

If exp1 Then
  If exp2 then
    Stat1
else
  stat2;

Despite its appearance, the statement is syntactically equivalent to

If exp1 Then
   begin
   If exp2 then
      Stat1
   else
      stat2
   end;

and not to

{ NOT EQUIVALENT }
If exp1 Then
   begin
   If exp2 then
      Stat1
   end
else
   stat2;

If it is this latter construct which is needed, the begin and end keywords must be present. When in doubt, it is better to add them.

The following is a valid statement:

If Today in [Monday..Friday] then
  WriteLn ('Must work harder')
else
  WriteLn ('Take a day off.');