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
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
There is also a two-way selection: <delphi> if BooleanExpression then
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
if Condition2 then Statement2 else Statement3;
Be careful with nesting. Sometimes the computer won't do what you want it to do: <delphi> if Condition1 then
if Condition2 then Statement2
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;
You can get by with a null statement: <delphi> if Condition1 then
if Condition2 then Statement2 else
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
if Condition2 then Statement2;
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.