Delphi Language Guide Delphi for Microsoft Win32 Delphi for the Microsoft. Net framework

If Statements
There are two forms of if statement: if...then and the if...then...else. The syntax of an if...then statement is
ifexpressionthenstatement
where expression returns a Boolean value. If expression is True, then statement is executed; otherwise it is not. For
example,
if J <> 0 then Result := I / J;
The syntax of an if...then...else statement is
ifexpressionthenstatement1elsestatement2
where expression returns a Boolean value. If expression is True, then statement1 is executed; otherwise
statement2 is executed. For example,
if J = 0 then 
   Exit
else
   Result := I / J;
The then and else clauses contain one statement each, but it can be a structured statement. For example,
if J <> o then
  begin
    Result := I / J;
    Count := Count + 1;
  end
else if Count = Last then
        Done := True
else
  Exit;
            
Notice that there is never a semicolon between the then clause and the word else. You can place a semicolon after
an entire if statement to separate it from the next statement in its block, but the then and else clauses require nothing
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more than a space or carriage return between them. Placing a semicolon immediately before else (in an if statement)
is a common programming error.
A special difficulty arises in connection with nested if statements. The problem arises because some if statements
have else clauses while others do not, but the syntax for the two kinds of statement is otherwise the same. In a series
of nested conditionals where there are fewer else clauses than if statements, it may not seem clear which else
clauses are bound to which ifs. Consider a statement of the form
ifexpression1thenifexpression2thenstatement1elsestatement2;
There would appear to be two ways to parse this:
ifexpression1 then [ ifexpression2thenstatement1elsestatement2 ];
ifexpression1then [ ifexpression2thenstatement1 ] elsestatement2;
The compiler always parses in the first way. That is, in real code, the statement
if ... { expression1} then
  if ... {expression2} then
    ... {statement1}
  else
    ... {statement2}
is equivalent to
if ... {expression1} then
  begin
    if ... {expression2} then
      ... {statement1}
    else
      ... {statement2}
end;
The rule is that nested conditionals are parsed starting from the innermost conditional, with each else bound to the
nearest available if on its left. To force the compiler to read our example in the second way, you would have to write
it explicitly as
if ... {expression1} then
  begin
   if ... {expression2} then
     ... {statement1}
   end
end
else
   ... {statement2};