IF

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Revision as of 16:42, 5 January 2010 by Kees (talk | contribs)

3Ca - IF (author: Tao Yue, state: unchanged)

The IF statement allows you to branch based on the result of a Boolean operation. The one-way branch format is: <delphi> if BooleanExpression then

 StatementIfTrue; 

</delphi>

If the Boolean expression evaluates to true, the statement executes. Otherwise, it is skipped.

The IF statement accepts only one statement. If you would like to branch to a compound statement, you must use a begin-end to enclose the statements: <delphi> if BooleanExpression then begin

 Statement1;
 Statement2

end; </delphi>

There is also a two-way selection: <delphi> if BooleanExpression then

 StatementIfTrue

else

 StatementIfFalse; 

</delphi>

If the Boolean expression evaluates to FALSE, the statement following the else will be performed. Note that you may not use a semicolon after the statement preceding the else. That causes the computer to treat it as a one-way selection, leaving it to wonder where the else came from.

If you need multi-way selection, simply nest if statements: <delphi> if Condition1 then

 Statement1

else

 if Condition2 then
   Statement2
 else
   Statement3;

</delphi>

Be careful with nesting. Sometimes the computer won't do what you want it to do: <delphi> if Condition1 then

 if Condition2 then
   Statement2

else

 Statement1;

</delphi>

The else is always matched with the most recent if, so the computer interprets the preceding block of code as: <delphi> if Condition1 then

 if Condition2 then
   Statement2
 else
   Statement1;

</delphi>

You can get by with a null statement: <delphi> if Condition1 then

 if Condition2 then
   Statement2
 else

else

 Statement1;

</delphi>

Or you could use a begin-end block. But the best way to clean up the code would be to rewrite the condition. <delphi> if not Condition1 then

 Statement1

else

 if Condition2 then
   Statement2;

</delphi>

This example illustrates where the not operator comes in very handy. If Condition1 had been a Boolean like: (not(a < b) or (c + 3 > 6)) and g, reversing the expression would be more difficult than NOTting it.

Also notice how important indentation is to convey the logic of program code to a human, but the compiler ignores the indentation.


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