Enumeration types

Many things are best described using words, not numbers.

Ada lets you define an enumeration type: a data type whose values are a collection of allowed words.

        type days is (Mon, Tue, Wed, Thu, Fri, Sat, Sun);

        type colours is (white, red, yellow, green, brown, blue, pink, black);

        type traffic_colours is (green, yellow, red);

        type suits is (clubs, diamonds, hearts, spades);

The values in an enumeration type are ordered:

        clubs < diamonds

        white < pink

        colours(green) > colours(red)

        traffic_colours(green) < traffic_colours(red)

Note that the same words can be elements in different enumeration types, even in the same program. They are distinguished by reference to the type as well as the element. The last examples show how this is done.

Enumeration types have the following benefits:

• readable programs
• avoid arbitrary mapping to numbers (eg better to use "Wed" than 3 for a day of the week)
• they work well as selectors in case statements

An example program shows the use of enumeration types in a program that determines the effect of mixing primary colours.

Attributes of Enumerated Types

        etype'FIRST     etype'LAST
        etype'PRED(c)   etype'SUCC(c)
        etype'POS(c)    etype'VAL(i)

For example:

        type days is (Mon, Tue, Wed, Thu, Fri, Sat, Sun);

        days'FIRST      is Mon
        days'LAST       is Sun
        days'PRED(Wed)  is Tue
        days'SUCC(Wed)  is Thu
        days'POS(Wed)   is 2
        days'VAL(3)     is Thu

You must ensure the result is legal. A CONSTRAINT_ERROR will occur at run-time otherwise. For example, days'SUCC(Sun) is illegal.

Using Enumerated Types

Can use enumerated types as selectors in case statements.

Can use enumerated types for parameters and result types in functions and procedures:

        function next_day(this_day : week_days) return week_days is

        begin -- next day
            if this_day = sun then
            -- relational comparison of an enumeration type
                return Mon;
            else
                return week_days'SUCC( this_day );
                -- attribute of an enumeration type
            end if;
        end next_day;

Comparisons:

• comparisons between enumerated type values are based on their ordinal value
• comparisons within ranges of values may be simply expressed with the in operator:

     if this_day in weekend_days then
        PUT("Holiday");
     end if;
  
  

Input/output:

• have PUT and GET with enumerated I/O libraries
• GET expects a legal value to be typed by the user.
  • A DATA_ERROR will occur if not.
  • Ada's exception handling mechanism can to ensure safe input of enumeration type values.
• PUT displays an enumerated type value, with the following options

          WIDTH=>n             width of output value
          SET=>UPPER_CASE      use uppercase output 
          SET=>LOWER_CASE      use lowercase output