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Stack writing in Pascal

How can I write procedure empty and puch under the following situation ?
unit StackADT;
{Provides type Stack, a sequence permitting
 insertions and deletions only a the front.}

interface

  const
         MaxStackSize = 100;

  type
         StackElement = Integer;
         StackSize    = 0..MaxStackSize;
         StackIndex   = 1..MaxStackSize;
         Stack = record
                         size : StackSize;
                         val  : array [StackIndex]
                                of StackElement;
                 end;

  procedure MakeEmpty (var s {input/output} : Stack);
  {Make s = the empty stack.}

  procedure Push (var s {input/output} : Stack;
                  e {input} : StackElement);
  {Inserts e at front of stack s.}

  procedure Pop (var s {input/output} : Stack;
                 var v {output} : StackElement);
  {Requires s not to be empty; removes first element of s and
   stores in v.}

  function Top (s {input} : Stack) : StackElement;
  {Requires s not to be empty; returns first element of s.}

  function Empty (s {input} : Stack) : Boolean;
  {Returns (s is empty).}

implementation

  procedure MakeEmpty (var s {input/output} : Stack);
  begin {MakeEmpty}
         *** Student Input ***
  end; {MakeEmpty}

  procedure Push (var s {input/output} : Stack;
                  e {input} : StackElement);
  begin {Push}
         if s.size = MaxStackSize then
                     WriteLn('Error - Can''t push onto full stack.')
         else
               begin
                      *** Student Input ***
                      *** Student Input ***
               end
  end; {Push}

  procedure Pop (var s {input/output} : Stack;
                 var v {output} : StackElement);
  begin {Pop}
         if Empty(s) then
                     WriteLn('Error - Can''t pop empty stack.')
         else
               begin
                      *** Student Input ***
                      *** Student Input ***
               end
  end; {Pop}

  function Top (s {input} : Stack) : StackElement;
  begin {Top}
         if Empty(s) then
                     WriteLn('Error - No top of empty stack.')
         else
               *** Student Input ***
  end; {Top}

  function Empty (s {input} : Stack) : Boolean;
  begin {Empty}
         *** Student Input ***
  end; {Empty}

end. {StackADT}
0
zoe_chui
Asked:
zoe_chui
  • 2
1 Solution
 
scrapdogCommented:
In empty, you have to return a boolean that depends on the condition of size=0.  If size=0, the stack is empty.  Size is your stack pointer.


function Empty (s {input} : Stack) : Boolean;
  begin {Empty}
    Empty := (s.size = 0);      
  end; {Empty}


When you push, you simply increment the stack pointer, then store the value you want to push in that particular part of the array pointed to by the stack pointer.

procedure Push (var s {input/output} : Stack;
                  e {input} : StackElement);
  begin {Push}
         if s.size = MaxStackSize then
                     WriteLn('Error - Can''t push onto full stack.')
         else
               begin
                 s.size := s.size + 1;
                 s.val[size] := e;
               end
  end; {Push}


If you want to pop a value, you do the opposite:  return the value pointed to be size, then decrement the stack pointer.  You already have sufficient error checking in both functions.
0
 
zoe_chuiAuthor Commented:
Hi,

The function empty is O.K. but when the procedure Push was not work, it got a "unknown indentifier" during compile in the following statement :
s.val[size] := e;

But when I change to s.val[1] := e;
It can complie suessfully !
Does it mean I make it correct ?
Pls advise me ?

Thanks

Zoe
0
 
scrapdogCommented:
Change it to s.val[s.size].

My mistake...
0

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