Stacks.ppt

Stacks
CS 308 – Data Structures

What is a stack?
• It is an ordered group of homogeneous items of elements.
• Elements are added to and removed from the top of the
stack (the most recently added items are at the top of the
stack).
• The last element to be added is the first to be removed
(LIFO: Last In, First Out).

Stack Specification
• Definitions: (provided by the user)
– MAX_ITEMS: Max number of items that might be on
the stack
– ItemType: Data type of the items on the stack
• Operations
– MakeEmpty
– Boolean IsEmpty
– Boolean IsFull
– Push (ItemType newItem)
– Pop (ItemType& item)

Push (ItemType newItem)
• Function: Adds newItem to the top of
the stack.
• Preconditions: Stack has been
initialized and is not full.
• Postconditions: newItem is at the top
of the stack.

Pop (ItemType& item)
• Function: Removes topItem from stack and
returns it in item.
• Preconditions: Stack has been initialized
and is not empty.
• Postconditions: Top element has been
removed from stack and item is a copy of
the removed element.

Stack Implementation
#include "ItemType.h"
// Must be provided by the user of the class
// Contains definitions for MAX_ITEMS and ItemType
class StackType {
public:
StackType();
void MakeEmpty();
bool IsEmpty() const;
bool IsFull() const;
void Push(ItemType);
void Pop(ItemType&);
private:
int top;
ItemType items[MAX_ITEMS];
};

Stack Implementation (cont.)
StackType::StackType()
{
top = -1;
}
void StackType::MakeEmpty()
{
top = -1;
}
bool StackType::IsEmpty() const
{
return (top == -1);
}

Stack Implementation (cont.)
bool StackType::IsFull() const
{
return (top == MAX_ITEMS-1);
}
void StackType::Push(ItemType newItem)
{
top++;
items[top] = newItem;
}
void StackType::Pop(ItemType& item)
{
item = items[top];
top--;
}

Stack overflow
• The condition resulting from trying to push
an element onto a full stack.
if(!stack.IsFull())
stack.Push(item);
Stack underflow
• The condition resulting from trying to pop
an empty stack.
if(!stack.IsEmpty())
stack.Pop(item);

Implementing stacks using
templates
• Templates allow the compiler to generate
multiple versions of a class type or a
function by allowing parameterized types.
• It is similar to passing a parameter to a
function (we pass a data type to a class !!)

Implementing stacks using templates
template<class ItemType>
class StackType {
public:
StackType();
void MakeEmpty();
private:
int top;
ItemType items[MAX_ITEMS];
};
(cont.)

Example using templates
// Client code
StackType<int> myStack;
StackType<float> yourStack;
StackType<StrType> anotherStack;
myStack.Push(35);
yourStack.Push(584.39);
The compiler generates distinct class types
and gives its own internal name to each of
the types.

Function templates
• The definitions of the member functions must be
rewritten as function templates.
StackType<ItemType>::StackType()
{
top = -1;
}
void StackType<ItemType>::MakeEmpty()
{
top = -1;
}

Function templates (cont.)
bool StackType<ItemType>::IsEmpty() const
{
return (top == -1);
}
bool StackType<ItemType>::IsFull() const
{
return (top == MAX_ITEMS-1);
}
void StackType<ItemType>::Push(ItemType newItem)
{
top++;
items[top] = newItem;
}

Function templates (cont.)
void StackType<ItemType>::Pop(ItemType& item)
{
item = items[top];
top--;
}

Comments using templates
• The template<class T> designation must
precede the class method name in the
source code for each template class method.
• The word class is required by the syntax of
the language and does not mean that the
actual parameter must be the name of a
class.
• Passing a parameter to a template has an
effect at compile time.

Implementing stacks using dynamic
array allocation
class StackType {
public:
StackType(int);
~StackType();
void MakeEmpty();
private:
int top;
int maxStack;
ItemType *items;
};

Implementing stacks using
dynamic array allocation (cont.)
StackType<ItemType>::StackType(int max)
{
maxStack = max;
top = -1;
items = new ItemType[max];
}
StackType<ItemType>::~StackType()
{
delete [ ] items;
}

Example: postfix expressions
• Postfix notation is another way of writing arithmetic
expressions.
• In postfix notation, the operator is written after the
two operands.
infix: 2+5 postfix: 2 5 +
• Expressions are evaluated from left to right.
• Precedence rules and parentheses are never needed!!

Example: postfix expressions
(cont.)

Postfix expressions:
Algorithm using stacks (cont.)

Postfix expressions:
Algorithm using stacks
WHILE more input items exist
Get an item
IF item is an operand
stack.Push(item)
ELSE
stack.Pop(operand2)
stack.Pop(operand1)
Compute result
stack.Push(result)
stack.Pop(result)

Write the body for a function that replaces each copy of an
item in a stack with another item. Use the following
specification. (this function is a client program).
ReplaceItem(StackType& stack, ItemType oldItem,
ItemType newItem)
Function: Replaces all occurrences of oldItem with
newItem.
Precondition: stack has been initialized.
Postconditions: Each occurrence of oldItem in stack has
been replaced by newItem.
(You may use any of the member functions of the
StackType, but you may not assume any knowledge of
how the stack is implemented).

{
ItemType item;
StackType tempStack;
while (!Stack.IsEmpty()) {
Stack.Pop(item);
if (item==oldItem)
tempStack.Push(newItem);
else
tempStack.Push(item);
}
while (!tempStack.IsEmpty()) {
tempStack.Pop(item);
Stack.Push(item);
}
}
1
2
3
3
5
1
1
5
3
Stack
Stack
tempStack
oldItem = 2
newItem = 5

Exercises
• 1, 3-7, 14, 12, 15, 18, 19

Stacks.ppt

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