Storing data values

Variables

The intent of a program is usually to model some aspect of the real world

A data object (variable) which the program manipulates thus represents some object in the real world. The data type should best reflect the properties of the object.

In Ada, a variable has a

• name
  • an Identifier
  • what does the variable represent?
• data type
  • what values can the variable have?
  • what operations can be performed on it?

Main predeclared data types in Ada:

        INTEGER
        FLOAT
        CHARACTER
        STRING
        BOOLEAN

(There is also a type called FIXED, for real numbers. We will stick to FLOAT for real numbers.)

Constants

Constants are data values that do not change

• can be strings or numbers

    answer        : constant STRING := "forty two";
    medicare_rate : constant FLOAT := 1.4;
    pi            : constant FLOAT := 3.1415926536;

Form:

• name : constant type := value;

Benefits:

• easier to read

    english_drink := metric_drink * 0.568;

    litres_to_pints : constant float := 0.568;
    english_drink := metric_drink * litres_to_pints;

• easier to modify (only one line to change)

Type Coercion

Ada has strong typing - you cannot mix data types

   3 + 4                ok
   3.0 / 4.0            ok
   1.0 > 0           error
   3 * 4.0              error

mixed arithmetic - must convert one type to another

   1.0 > FLOAT(0)    ok
   FLOAT(3) * 4.0       ok
   3 * INTEGER(4.0)     ok

FLOAT (integer value)

• some accuracy may be lost

INTEGER (float value)

• round-off
• fails if outside INTEGER'FIRST ... INTEGER'LAST

An example program demonstrates type coercion, among other things.