An Introduction to Ada

1
An Introduction to Ada

• Programming Languages
• Fall 2002

2
Basic Structure of a Program

• A program is a collection of units
• Packages
• Functions
• Procedures
• Bound together to form a program
• Typically a unit is stored in a file
• Units reference other units

3
Procedure

• procedure H (M Integer) is
  declarationsbegin statements returnend
  H
• Typical use is procedure Main is which defines
  the main program

4
Function

• function Max (A Integer B Integer) return
  Integeris Result Integerbegin if A gt
  B then Result A else Result
  B end if return Resultend Max

5
Packages

• Packages define a related collection of types,
  and subprograms (procedure and functions)
• A package provides a set of services to a client
• A package may be a client of another package

6
Package Spec (Declaration)

• package X is declarations types
  subprogram specs (but not subprogram
  bodies)end X

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Subprogram Specs

• procedure Print_In_Hex (X Integer)
• function Max (A, B Integer) return Integer
• Note the semicolon instead of is
• A subprogram spec has everything you need to know
  to use the subprogram
• A client needs only the spec

8
Package Bodies

• package body X is declarations subprograms
  local to body variables/constants local to
  body subprogram bodies for subprogram
  specs appearing in the package specbegin
  initialization statementsend X

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How to be A Client

• To access a package, use a WITHwith
  Calendarprocedure Main is Today
  Calendar.Time end Main

10
The Use Clause

• Accessing Stuff in Calendar without dots
• with Calendaruse Calendarprocedure Main is
  Today Time end Main

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Package Bodies as Clients

• with Calendarpackage body Julian_Calendar_Stuff
  is end Julian_Calendar_Stuff
• Here we have the implementation of a package done
  using stuff in another package

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Package Specs as Clients

• A package spec can build on another spec
• with Calendaruse Calendarpackage To_Do_List
  is procedure Enter (T Time M
  String) -- Enter new item in todo list. Time
  is -- deadline. M is description.end
  To_Do_List

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The Idea of a Package Spec

• Write the package spec
• It is like a contract between client and body of
  the spec
• Write the body
• Write the client
• Last two activities are completely independent
  (and should not talk to one another except via
  the spec)

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Integer Type Declarations

• Type Integer is built in
• But you dont want to use it
• Because its range is implementation defined
• Because it is defined to match the machine not
  your problem
• Because it does not take advantage of strong
  typing to prevent errors

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Defining Integer Types

• Define type according to use
• type Day_In_Year is range 1 .. 366type Age is
  range 0 .. 130type Temperature is range -20 ..
  180
• Now we can define variables of the type
• Today_Day Day_In_YearEmployee_Age
  AgeMachine_Room_Temp Temperature

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Why Define Integer Types

• No dependence on implementation
• Unlike type int in C
• Range of types matches problem
• Get an error or warning at compile time
• Age 200
• Or an exception at runtime
• Age Age 1000

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Strong Typing

• Cannot mix integer types
• Current_Temp TemperatureCurrent_Pressure
  Pressure
• Current_Temp Current_Pressure 1
• Error, cannot assign pressure to temperature
• Current_Temp Current_Temp
  Current_Pressure
• Error, cannot add temperature to pressure

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Integer Subtypes

• A subtype creates a limited range
• But is still the same type
• subtype OK_Operating_Range is Temperature range
  70 .. 80Room_Temp TemperatureMachine_Room_Te
  mp OK_Operating_RangeMachine_Room_Temp
  Room_Temp
• Raises exception if Room_Temp out of range

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Catching Exceptions

• You can catch an exception at run time
• begin Machine_Room_Temp Room_Temp
  exception when Constraint_Error gt
  recovery stuffend

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Unsigned (Modular) Types

• Modular types have wrap around
• type M is mod 7 -- values are 0,1,2,3,4,5,6q
  m 6 -- initializationq q 2 -- result
  is 1
• Most common use, conventional unsigned
• type Uns_32 is mod 2 32
• Remember that twos complement arithmetic is
  equivalent to arithmetic mod 2wordsize

21
Real Types

• Float types (control relative accuracy)
• type My_Float is digits 7type Xfloat is digits
  7 range 1.0 .. 10.0subtype F1 is My_Float range
  1.0 .. 5.0
• Digits is decimal digits of relative precision
• There is a formal model for fpt in Ada
• Target independent (parametrized)
• Guarantees minimal accuracy
• Operations defined in terms of model numbers
• Results fall in defined model interval

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Fixed-Point Types

• Fixed-point types are real types where you
  control the absolute accuracy.
• Typically implemented as scaled integers
• type Velocity is delta 0.125 range 0.0 .. 10.0
• Decimal fixed-point types
• Decimal small
• Typical use in financial programming
• type Money is digits 10 delta 0.01 range 0.00
  .. 999_999_999.00

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Character Types

• Built in types
• Character (8-bit Latin-1)
• Wide_Character (16-bit Unicode/ISO 10646)
• Good enough for most purposes, but you can define
  your own types
• type My_Character is (A, B, C, .)
• Note on standard types
• Standard types are in package Standard that is
  automatically visible in every unit.

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Enumeration Types

• An enumeration type is a sequence of ordered
  enumeration literals
• type State is (Off, Powering_Up, On)
• No arithmetic defined
• S1, S2 StateS1 S1 S2 -- Illegal
• Can add/subtract one
• StatePred (S1)StateSucc (S2)
• These are examples of attributes

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Boolean Types

• Predefined enumeration type
• type Boolean is (False, True)
• Expressions of type boolean used in
• if statements
• while loops
• exit statements
• Etc.

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Access Types

• Access types function like pointers
• But are not necessarily implemented that way
• type r is access integertype s is access all
  integer
• The all allows the access value to reference any
  item at all. Without all, you can only reference
  objects specifically allocated for the pool in
  question.

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Using Access Types

• Allocate an object using new
• type AI is access all integerPtr AIPtr
  new Integer -- uninitializedPtr new
  Integer(12) -- initialized
• To obtain value dereference
• V IntegerV Ptr.all

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Array Types

• Arrays can have 1 or more subscripts
• type Vector is
• array (Integer range 1 .. 10) of
  Integertype Matrix is array (Integer range 0
  .. 10, Character range A .. Z)
  of Vector
• VV Vector (others gt 10) -- aggregateMM
  MatrixMM (5, C) VVVV (1 .. 5) VV (2
  .. 6) -- slicing (one dim only)

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Array Bounds and Types

• Are the bounds of an array answer part of the
  properties of the array type?
• Things are much cleaner if we answer yes, as in
  Pascal, but this is very limiting
• For example, a sort routine cannot take a vector
  of any size to sort.
• Instead we consider the bounds to be like the
  range of an integer type

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Unconstrained Arrays

• Unconstrained array type has no bounds
• type UA is array (Int range ltgt) of Int--
  cannot use UA to declare a variable-- instead
  must build a subtypesubtype UA5 is UA (1 ..
  5)UAV5 UA5 (6,5,4,3,2)-- can also set
  bounds for a variableUAV2 UA (1 .. 2)

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String Types

• A string type is an array whose elements are a
  character type.
• Two standard built in string types
• type String is array (Natural range ltgt) of
  Charactertype Wide_String is array (Natural
  range ltgt) of Wide_CharacterS String (1 .. 5)
  Hello
• Note Natural is a predefined subtype of Integer
  with bounds 0 .. IntegerLast

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Record Types

• Like struct in C
• type Date is record Year Year_Number
  2002 -- default Month Month_Number Day
  Day_Numberend recordDD DateEE Date
  (2001, 8, Day gt 27)DD.Month EE.Month 1

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Arrays/Records and Access Types

• Access types and records/arrays
• type A is array (Int range 0 .. 10) of Inttype
  AP is access AAV APAV.all (3) AV (4)
  -- can omit .all here
• Similarly do not need .all for fields of records
• DP.Month DP.all.Month 1

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What about Union Types?

• The union type in C is fundamentally unsafe, and
  therefore unacceptable
• union (int, float) puzzle
• Now puzzle has either an int or a float
• But at runtime, cannot tell which
• So we have to trust the programer
• In Ada we are short on trust ?

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Instead, Discriminated Types

• A record can have discriminants
• type IF is (Int, Float)type Int_Float (V IF
  Int) is record case V is when Int gt
  Int_Val Integer when Float gt Float_Val
  Float end caseend record
• Now the value of the discriminant V shows what
  type is currently present

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More on Discriminated Records

• Referencing a discriminanted type
• Puzzle Int_FloatPuzzle (Float, 1.0)F
  Puzzle.Float_Val -- OKI Puzzle.Int_Val
  -- raise exceptionPuzzle.V Int -- not
  allowed!

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More on Discriminated Records

• Can make subtypes
• subtype PuzzleI is puzzle (Int)-- this type
  only holds int values
• Can dimension arrays from discriminant
• Subtype Vlen is Integer range 1 .. 10type Vstr
  (Vlen Integer 0) is record Data String
  (1 .. Vlen)end recordVV Vstr (5,
  hello)

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Other Types

• Derived types (copying a type)
• Extended types (object oriented stuff)
• Task types (active threads)
• Protected types (passive synchronization)
• More on all these later!

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Statement Forms

• Assignment statement (is not an expression)
• Variable expression
• If statements
• if condition then statementselsif condition
  then statementselse statementsend if

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Statement Forms (cont)

• Loops
• for J in Integer range 1 .. 10 loop
  statementsend loop
• while condition loop statementsend loop
• loop statementsend loop

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Statements (cont)

• Exit statement
• exitexit when condition
• Can only be used in a loop
• Can use labels
• Outer loop Inner loop exit Inner when
  Done end loop Innerend loop Outer

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Statements (cont)

• Case statements
• case expression is when 0 gt statements
  when 1 10 gt statements when 2 .. 9 gt
  statementsend case
• All values in when branches must be static
• All possible values of expression must be
  included exactly once
• Can use when others gt to cover rest

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Statements (cont)

• Return statement
• return -- procedurereturn expression --
  function
• Only in procedure/function
• Function must have at least one return
• Raise statement
• raise exception-name

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Declaring and Handling Exceptions

• Declaring an exception
• Error, Disaster exception
• Raising an exception
• raise Disaster-- strips stack frames till a
  handler is found
• Handling an exception
• exception when Disaster gt statements

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More on Exception Handling

• Anywhere we have begin end, we can do
• begin statementsexception when handler gt
  statements when handler gt statementsend

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Block Statement

• Block statement can be used anywhere
• declare -- declare section optional
  declarationsbegin statementsexception
  -- exception section optional handlersend

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Summary

• Thats enough to get you on your way!
• First assignment will be to do one of the ACM
  programming competition problems
• You do the program and make test data
• Your choice of examples
• See website for URL