Class Notes 9/21/06 – courtesy of Shawn

Exceptions

l Define exception

l Raise exception

l Handle exception

l Some are pre-defined

l Constraint error

We looked at the code Daniel shared with us for version of stack.adb and discussed it and modified it.

-- Assignment : Homework due Sept. 21st, 2006
-- Programmer : Daniel 
-- Course : CS 240
-- Date : Sept. 20th, 2006
-- Compiler : gcc with gnat
-- Professor : Elizabeth Adams
-- Filename : stack.adb
-- Executable :
-- Version : 0.2
 
-- Purpose: To implement the operations of the stack data type as dicussed in class.
 
-- Input : none
 
-- Output
-- To Screen : Messages in the event of an error pushing or
-- popping from a full or empty stack, respectively.
 
-- Modifications:
--
-- Version 0.2 : A typo fixed in the peek declaration!
 
with ada.text_io;
-- use ada.text_io;
 
 
package body stack is
 
-- pre-Condition : theStack exists and theValue exists
-- post-Condition : theValue has been added to the stack
-- raises error if theStack is full
 
 procedure push ( theStack : in out stackType;
 theValue : in Integer ) is
 begin
 if not (theStack.top = theStack.MAXDATA ) then
   theStack.top := theStack.top + 1;
   theStack.aStack(theStack.top) := theValue;
 else
 ada.text_io.put_line ( "A number could not be added to the stack, because it is full.");
 end if;
 end push;
 
 
 
 procedure pop ( theStack : in out stackType;
 theValue : out Integer ) is
 begin
 if ( theStack.top > 0 ) then
 theValue := theStack.aStack(theStack.top);
 theStack.top := theStack.top - 1;
 else
 ada.text_io.put_line 
 ("A number could not be popped off of the stack, because it is empty.");
 end if;
 end pop;
 
 procedure peek ( theStack : in stackType;
 theValue : in out Integer ) is
 begin
 if ( theStack.top /= 0 ) then
 theValue := theStack.aStack(theStack.top);
 else
 ada.text_io.put_line ("A number could not be peeked at, because the stack is empty.");
 end if;
 end peek;
 
 procedure makeEmpty ( theStack : in out stackType ) is
 begin
 theStack.top := 0;
 end makeEmpty;
 
 function isFull ( theStack : in stackType ) return boolean is
 
 result : boolean;
 begin
 result := false;
 
 if ( theStack.top > theStack.MAXDATA ) then
 result := true;
 end if;
 
 return result;
 end isFull;
 
 function isEmpty ( theStack : in stackType ) return boolean is
 
 result : boolean;
 begin
 result := false;
 
 if ( theStack.top = 0 ) then
 result := true;
 end if;
 
 return result;
 
 end isEmpty;
 
 
end stack;

procedure push

l Preconditions - were added in class

l The stack exists

l The value exists

l The stack is NOT full

l Postconditions – were added in class

l The value has been added to the stack

l There is an error if the stack was full

l Removed the USE clause and added fully qualified references

l Corrected the test for truth of a boolean variable

l Discussed whether to increment the stack top before or after pushing a value on the stack

l Dan had modified the specification by adding a constant MAX value to the stack size instead of using a literal . We added the comments to each procedure and function specification.

-- Assignment : Homework due Sept. 21st, 2006
-- Programmer : Daniel 
-- Course : CS 240
-- Date : Sept. 20th, 2006
-- Compiler : gcc with gnat
-- Professor : Elizabeth Adams
-- Filename : stack.ads
-- Executable :
-- Version : 0.2
 
-- Purpose: To specify the stack data type as discussed in class with its data types and operations
 
-- Input : none
 
-- Output : none
 
-- Modifications: The MAX constant determines the maximum size of a given
-- stack.
--
-- Version 0.2 : An off-by-one error fixed in the type declaration of
-- my array, and a typo fixed in the peek declaration.
package stack is
 
 MAX : constant Integer := 20;
 
 type myArray is array (1..MAX) of Integer;
 
 type stackType is record
 top : Integer; -- alternate choice is top := 0 : Integer; 
 aStack : myArray;
 MAXDATA : Integer := MAX; -- added to original specification 
 end record;
 
 
 procedure push ( theStack : in out stackType; -- procedure to put an item on the stack 
 theValue : in Integer); -- modifies stack
  
 procedure pop ( theStack : in out stackType; -- procedure to pop an item off the stack
 theValue : out Integer);  -- modifies stack by removing the top value
  
 procedure peek ( theStack : in stackType;  -- procedure to look at top of stack
 theValue : in out Integer); -- leaves stack unchanged
  
 procedure makeEmpty ( theStack : in out stackType ); -- creates an empty stack
   -- alters stack by setting top pointer
   
 function isFull ( theStack : in stackType ) return boolean; -- tests the stack for fullness
   -- doesn't alter stack
   
 function isEmpty ( theStack : in stackType ) return boolean; -- tests the stack for emptiness
   -- doesn't alter stack
 
end stack;

Designing the Test Program

First, before writing any code DESIGN THE TEST.

Things to test:

1. isEmpty

2. peek

3. push

4. pop

5. isFull

6. makeEmpty

 
Note: lines in red show the declaration and instantiation of an enumerated type. They are not part of this assignment. They are just an illustration.

-- Assignment : inClass program to test stack package
-- Programmer : entire class
-- Course : CS 240
-- Date : Sept. 21th, 2006
-- Compiler : gcc with gnat
-- Professor : Elizabeth Adams
-- Filename : stackTest.adb
-- Executable :
-- Version : 0.1
 
-- Purpose: test all of the methods in the stack package to see if they
--  work as desired
 
-- Input : integers to be pushed onto the stack
 
-- Output
-- messages telling what procedure/function is being tested (labels)
-- data values being pushed onto and popped from stack
-- data values at top of stack
-- status of stack
 
-- Modifications:
--
-- Version 0.1 : 
 
WITH ada.text_io;
WITH ada.integer_text_io;
WITH stack;
 
PROCEDURE stackTest is
 status: boolean;
 testStack : stack.stackType;
 
-- the following two lines do not belong in THIS program and should be removed
 type fruit is (apple, berry, cherry, banana);
 myfruit : fruit;
 
-- the following line is an instantiation for an enumerated type which allows us to read and
-- write values of that type
package boolean_io is new Ada.Text_io.Enumeration_io (boolean);
 
-- the following line desn't belong in THIS program : it should be removed
package fruit_io is new Ada.Text_io.Enumeration_io (fruit);
 
BEGIN
-- test the status of the stack to see if is empty
ada.text_io.put 
 ( " testing the status of the stack - expect it to be empty output should be TRUE ");
 
-- call function
status := stack.isEmpty (testStack);
 
boolean_io.put (status);
 
ada.text_io.new_line(1);
 
-- the following lines don’t belong in THIS program : they should be removed
myfruit := apple;
fruit_io.put (myfruit);
 
END stackTest;

Other Notes

l In general, you don't need to worry as much about adding headings to .ads files.

l Comments should be added to a package specification file to explain what each procedure does, i.e.

procedure push (theStack : in out stackType; -- procedure to put and item ...

l Use IF NOT when doing a boolean test. Never ever ever in any programming language compare a boolean variable to the actual boolean value.

l DO NOT change specifications when working on a project unless discussing it with someone.

· Reserved words should be in uppercase.

· Ada is not case sensitive.

· A boolean is an enumerated type: {false true} (false is less than true.)

· Instantiation for boolean type: package boolean_io is new Ada.ext_io.Enumeration_io (boolean);

· You can either define in your record that the top of the stackType has a starting value of 0, or you can require users of the stackType to call makeEmpty for each new stack which will do it.

boolean variable done

done : boolean;

if not (done) then -- shows test of boolean variable

-- do what you want to do here

end if;

if (done) then -- use this if you don’t remember how to use NOT

null;

else

-- do what you want to do here

end if;

DeMorgan's Laws

If NOT(A and B) == If (NOT A or NOT B)

If NOT (A or B) == if (NOT A AND NOT B)

Homework: re-write your code so it works for floating point numbers. Bring printouts of integer version and floating point versions to class on Tuesday.