Enumerated Types

Introduction to Enumerated Types

Concept

• Create a new type whose values are a set of NAMES!


• Example:

          type Color is (Red, Blue, Green);
  
          roomColor: Color := Blue;
       

• This is a type:
1. Set of ...:
2. Set of ...:

Enumerated Types Create New Literals!

• We have created new literals in the language!


• What is a literal in a language?
• Something that can be represented directly


• Examples: 3 vs 2 + 1, "Hi Mom", true vs not false

Enumerated Types - Questions

• Questions:
  • What operations are available
  • Case sensitive?
  • Can two types mix names
  • Can variables and types mix names
  • Rules for keeping sets of names distinct
  • Motivation and approach of other languages

Operations Available with Enumerated Types

Some Basic Operations

    -- Illustrates:
    --    declaring variables, assignment, relational operators, in, ranges
    with Ada.Text_io; use Ada.Text_io;

    procedure enum1 is

        type Day is (Sunday, Monday, Tuesday, Wednesday, 
                        Thursday, Friday, Saturday);

        today: Day;

    begin
        today := Thursday;

        if today = Friday then
            put("We made it!");
        end if;

        for d in Sunday .. Saturday loop

            if d in Monday .. Friday then
                put("It's a work day.");

                if <  Wednesday then 
                    put("Just started the week!.");
                end if;

            else
                put("It's the weekend!");
            end if;

        end loop;

    end enum1;
  

I/O of Enumerated Types

• Imagine this code:

            type Color is (Red, Blue, Green);
            c: Color := red;
        begin
            put(c);
      

• Which version of put would be needed here?
• Since type Color is newly created, no put for it can already exist
• Instead, we must create a new IO library for type Color


• A new i/o packages must be created for each new type!

with Ada.Text_io; use Ada.Text_IO;

procedure enum2 is
   type Color is (Red, Blue, Green);

   type Day is (Sunday, Monday, Tuesday, Wednesday, 
                Thursday, Friday, Saturday);

   -- Declare 2 new I/O packages

   package Color_IO is new Ada.Text_IO.Enumeration_IO(Color);

   package Day_IO is new Enumeration_IO(Day);

   -- Use the new packages
   use Color_IO;
   use Day_IO;

   c: Color;
   d: Day;

begin
   c := Red;
   d := Monday;

   put(c); 
   new_line;
   put(d);

end enum2;
SAMPLE RUN:
RED
MONDAY
  

Advantages of Enumeration_IO

• Operates similar to integer IO:
• Skips white space to find input value (ie enumeration literal)
• Error if can't find valid enumeration literal
• Probably a data error

Package Enumeration_IO

• Enumeration_IO is known as a generic package
• because it has a type as a parameter


• Java 1.5 introduces parameterized types, which are similar to generic packages


• We will study generic packages in detail later


• Color_IO.Get is also provided by the generic package


• A slightly more complex example (enum2a.adb) [and prettified] that inputs enumerated type values and strings from the file this data file


• Important: Don't forget that your input file needs to be created using the same operating system that your program is compiled for! (For example, don't create your file using PFE on a PC and then compile using SSH, which creates an executable for a SPARC.)

More Operations: Attributes of Enumerated Types

• Example 1 and (prettified)
  • The usual: Range, First, Last
  • Are plus, minus, etc available No, but these are:
  • Sequence Functions: Succ, Pred

• Example 2 and (prettified)
  • Functions to convert to/from Naturals: Pos, Val
  • Functions to convert to/from Strings: Image, Value
  • Relops

• Example 3 and (prettified)
  • More functions: Min, Max

Enumerated Types Representation

• Enumeration type literals are associated values based on their sequence within the type definition
• For type Color is (red, blue, green); the valuesred, blue, green are associated with values 0, 1, 2.
• It's possible to also associate each value with a specific integer that is used internally for that enumeration literal.
• Attribute functions Enum_Rep and Enum_Val Convert between the literal and its enumeration representation
• Currently available with GNAT compiler, but not part of Ada 2012 standard
• Will become part of the language standard in Ada 2020.
• GNAT also currently supports variable'enum_rep

procedure try_enum_rep is

   type Color is (red, blue, green);
   for Color use (red => 2, blue => 4, green => 16);

   c: Color := red;

begin
   put(Color'enum_rep(c));   -- 2

   c := blue;
   put(Color'enum_rep(c));   -- 4

   c := green;
   put(c'enum_rep);          -- 16 GNAT shorthand

   c := Color'enum_val(4);   -- Inverse: c=Color'enum_val(c'enum_rep)
   put(c'img);               --  GREEN

end try_enum_rep;
      

• Thus Color'val and Color'pos convert between red, blue, green and 0, 1, 2, respectively
• Thus Color'enum_rep and Color'enum_val convert between red, blue, green and 2, 4, 16, respectively

Type Checking with Enumerated Types

Enumerated Types are Strongly Typed

• Types that are distinct cannot be mixed!

        with Ada.Text_io; use Ada.Text_io;

        procedure enum1a is

        type Color is (Red, Blue, Green);

        type Day is (Sunday, Monday, Tuesday, Wednesday, 
                        Thursday, Friday, Saturday);

        c: Color;
        d: Day;
        n: Natural := 1;

        begin
        c := Red;

        d := Friday;

        c := Friday;          -- Compile error
                                
        c := d;               -- Compile error
                                
        c := 1;               -- Compile error
                                
        d := 1;               -- Compile error
                                
        n := c;               -- Compile error
                                
        c := n;               -- Compile error

        if d = Blue then      -- Compile error
            put("It's Friday!");
        end if;

        end enum1a;
  

Ambiguous Values

• What happens when the sets are not distinct
• Example: with Color Blue and Mood Blue?

• Example and (prettified)

Enumerated Type Values and Variables, etc

• What happens here:

       type Color is (Red, Blue);
  
       Red: Integer;
       Blue: Color;
    

• Answer: Legal, but not a good idea `

Array Indices and Subranges

Enumerated Types as Array Indices

• An enumerated type range can be an array index

    type Day is (Sunday, Monday, Tuesday, 
    Wednesday, Thursday, Friday, Saturday);

    type HoursArray is array(Monday .. Saturday) of Float;

    hoursWorked: HoursArray := (others => 0.0);
    ...
    for d in Monday .. Friday loop
        totalHours := totalHours + hoursWorked(d);
    end loop
    totalHours := totalHours + 1.5 * (hoursWorked(Saturday));
      

Subtypes (ie Subranges) of Enumeration

• subtype Weekday is range Monday .. Friday;

• Example and (prettified)

Predefined Enumeration Types

Types Boolean and Character

• Types Boolean and Character are effectively defined as follows:

     type Boolean is (True, False);

     type Character is (..., 'A', 'B', ..., 'a', 'b', ...);
  

• Yes, Enumeration literals can be characters (but this is not quite as useful as you might think)


• Boolean and Character are said to be defined in package Standard
• Package Standard defines what is automatically available
• but with Standard not allowed

Enumerated Types in Other Languages

Java 1.4 and Earlier (and Sometimes Later)

• Java uses final static int for constant values that act as enumeration values
• This does not provide type checking


• Example: Class Calendar has method void set(int field, int value) for setting a specific field in a calendar
• Example:

        myCalendar.set(DAY_OF_WEEK, SUNDAY);    // set day of week of myCalendar to 1
        myCalendar.set(MONTH, JANUARY );        // set month of myCalendar to 0
     

• For the parameter field, class Calendar has int constants for which field to set and for field values:

     class Calendar{
        public static final int YEAR=1, MONTH=2, ..., DAY_OF_WEEK=7, ...;
        public static final int SUNDAY=1, MONDAY=2, ..., SATURDAY=7;
        public static final int JANUARY=0, FEBRUARY=1, ..., DECEMBER=11;
     

• How good is the compile time error checking:

        myCalendar.set(MONTH, 1);             // January, or February?

        myCalendar.set(SUNDAY, DAY_OF_WEEK);  // Which is correct?   
        myCalendar.set(DAY_OF_WEEK, SUNDAY);  // Can compiler catch error?

        myCalendar.set(MONTH, MONDAY);        // Can error even be caught at runtime?
     

• In Java 1.4 and earlier, this was the only way to get enumeration-like values. Java 1.5 added enums (see below)

C and C++

• C, C++ have same problems described above for Java, plus ...


• C has enums which are really are essentially ints, thus little to no type checking

   typedef 
      enum {SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY} 
   week_days;

   week_days today = FRIDAY, tomorrow, dayAfter;

   tomorrow = today + 1;
   dayAfter = today * 2;

   int someInt = tomorrow;

   printf("%d  %d  %d  %d\n", today, tomorrow, dayAfter, someInt);

   // Output: 5  6  10  6



• C++ has enums which give some type checking
• tomorrow = today + 1; gives an error, but ...
• int someInt = tomorrow ; is allowed

Java 1.5 and Later

• Java 1.5 added enums which are similar to Ada enumerated types
• Example:

        enum WeekDay { Monday, Tuesday, Wednesday, Thursday, Friday }
        WeekDay today = Weekday.Monday;
        

• Java enums are implemented using a parameterized class (ie generic) called Enum


• Java enums are objects, not primitive values.

Motivation for Enumerated Types

Motivation

• Write program at level of problem rather than level of machine
• Example: possible to write if roomcolor == blue rather than if roomcolor == 3
• Result: code is easier to write and more readable


• Better type checking to catch more errors

          roomColor *= 3;  
  
          void setDate(int day, int month, int year){...}
          ...
          // How to call setDate for September 8, 2003?
          setDate(9, 8, 2003);   // Which is correct?
          setDate(8, 9, 2003);
  
          // The compiler would not detect the error
       

• If there were a type for Months, the compiler would flag one of these as invalid:

        setDate(September, 8, 2003);   
        setDate(8, September, 2003);
     

A Final Example using Enumerated Types

An Example Using Enumeration Types

• Table5.adb
• Pretty version of Table5.adb


• Purpose: Read desired table bounds and alignment option and then print a table of powers
• Alignment option can be left, center, or right


• Features:
• Define a new Enumerated Type: Alignment
• Enumeration IO
• Global constant: ColumnWidth
• Visible from point of declaration on - thus inside all procedures
• Global constants are okay