Delphi Basics : Case command

DelphiBasics

Case
Keyword

A mechanism for acting upon different values of an Ordinal

1	keyword Case(case Ordinal expression of Ordinal value {,Ordinal value...} : Statement; {...} else Statement; end;
2	type Name = record Declarations ... case {Tag :} Ordinal type of Ordinal value {,Ordinal value...} : (Declarations); {...} end;

Description

The Case keyword provides a structured equivalent to a sequence of if statements on the same variable.

The case statement is more elegant, more efficient, and easier to maintain than multiple if nestings.

Version 2. Is used for Records declarations. It is then called a Variant. It provides a means of mapping two or more differing sets of declarations onto the same section of the record.

It is mostly used when handling different types of dataset for a record, where the datasets have mostly the same content. See the example for clarification.

The Tag provides identification of the case element.

Related commands

Else		Starts false section of if, case and try statements
End		Keyword that terminates statement blocks
If		Starts a conditional expression to determine what to do next
Packed		Compacts complex data types into minimal storage
Record		A structured data type - holding fields of data
Then		Part of an if statement - starts the true clause

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Example code : Standard case statement usage

var
  colour : TPrimary;
  number : Integer;

begin
  // Show the colour before it has an assigned value
  ShowColour(colour);

  // Now set the colour and try again
  colour := Green;
  ShowColour(colour);

  // Calculations can also be used in the case statement
  number := 17;
  Case number mod 2 of
    0 : ShowMessage(IntToStr(Number)+' mod 2 = 0');
    1 : ShowMessage(IntToStr(Number)+' mod 2 = 1');
  else ShowMessage(IntToStr(Number)+' mod 2 is unknown');
  end;
end;

// Procedure to show the colour of a passed
procedure TForm1.ShowColour(colour : TPrimary);
begin
  // Use a case statement to see the colour of the passed var
  // Note how important the else clause is, even though we have
  // apparently covered all TPrimary values!
  Case colour of
       Red : ShowMessage('The colour is Red');
     Green : ShowMessage('The colour is Green');
      Blue : ShowMessage('The colour is Blue');
    Yellow : ShowMessage('The colour is Yellow');
  else ShowMessage('The colour is Unknown!');
  end;
end;

Show full unit code

  The colour is Unknown!
  The colour is Green
  17 mod 2 is 1

Example code : Case within a record

type
  // Declare a fruit record using case to choose the
  // diameter of a round fruit, or length and height ohterwise.
  TFruit = record
    name : string[20];
    Case isRound : Boolean of// Choose how to map the next section
      True  :
        (diameter : Single);  // Maps to same storage as length
      False :
        (length   : Single;  // Maps to same storage as diameter
         width    : Single);
  end;

var
  apple, banana, fruit : TFruit;

begin
  // Set up the apple as round, with appropriate dimensions
  apple.name     := 'Apple';
  apple.isRound  := True;
  apple.diameter := 3.2;

  // Set up the banana as long, with appropriate dimensions
  banana.name    := 'Banana';
  banana.isRound := False;
  banana.length  := 7.65;
  banana.width   := 1.3;

  // Show the attributes of the apple
  fruit := apple;
  if fruit.isRound
  then ShowMessage(fruit.name +' diameter = '+
                   FloatToStrF(fruit.diameter, ffFixed, 2, 1)+'"')
  else ShowMessage(fruit.name +' length = '+
                   FloatToStrF(fruit.length, ffFixed, 2, 1)+'" width = '+
                   FloatToStrF(fruit.width, ffFixed, 2, 1)+'"');

  // Show the attributes of the banana
  fruit := banana;
  if fruit.isRound
  then ShowMessage(fruit.name +' diameter = '+
                   FloatToStrF(fruit.diameter, ffFixed, 2, 1)+'"')
  else ShowMessage(fruit.name +' length = '+
                   FloatToStrF(fruit.length, ffFixed, 2, 1)+'" width = '+
                   FloatToStrF(fruit.width, ffFixed, 2, 1)+'"');
end;

Show full unit code

Apple diameter = 3.2"
Banana length = 7.7" width = 1.3"