Class Notes 9/19 – CS 240

l Stack data structure is LIFO. (Last In First Out)

l You do not need a 'with' statement in the body of the package implementation to link it to the specification. That happens automatically

l In the stack specification package:

l package stack is

Both the package specification and implementation headers end with 'is'

l Type

l type myArray is array (1..50) of integer;

l type stackType is record;

An array is homogeneous and a record is heterogeneous

l aStack : myArray;

l top : integer;

l Operations

l procedure push (someStack : IN OUT stackType

someValue : IN integer);

l procedure pop (someStack : IN OUT stackType

someValue : OUT ?);

l procedure makeEmpty (someStack : IN OUT stackType);

l <optional> procedure peek (someStack : IN stackType

someValue : OUT integer);

l function isEmpty (someStack : IN stackType) return boolean;

l function isFull (someStack : IN stackType) return boolean;

l In the stack implementation package:

(Stub this out and compile correctly before attempting to write all of the code.)

(Documentation needs to be added to this to outline the preconditions and postconditions of each procedure and function.)

l package body stack is

l Type does not have to be redefined, the line from the spec will be used.

l Operations

The procedure headers are the same as in the specification, I will use ellipses in place of them.

l procedure push (someStack : IN OUT stackType

someValue : IN integer) is

full : boolean;

begin

--if the stack isn't full, we can push something on to it

--ask whether it's full

full := isFull (someStack);

if not (full) then

--put the value on the stack

someStack.top := someStack.top + 1;

someStack.myArray(someStack.top) := someValue;

end if;

end push;

l procedure pop (...) is

begin

null;

end pop;

l procedure peek (...) is

begin

null;

end peek;

l procedure makeEmpty (...) is

begin

someStack.top := 0;

end makeEmpty;

l function isEmpty (...) return boolean is

result : boolean;

begin

result := false;

if someStack.top = 0 then

result := true;

end if;

return result;

end isEmpty;

<only have one return statement per function>

l function isFull (...) return boolean is

result : boolean;

begin

result := false;

if (someStack.top = 20) then

result := true;

end if;

return result;

end isFull;