If statement |
  
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If statement |
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There are two forms of if statement: if...then and the if...then...else. The syntax of an if...then statement is
if expression then statement
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
if expression then statement1 else statement2
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 <> 0 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 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
if expression1 then if expression2 then statement1 else statement2;
There would appear to be two ways to parse this:
if expression1 then [ if expression2 then statement1 else statement2 ];
if expression1 then [ if expression2 then statement1 ] else statement2;
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
else
... { statement2 } ;