CS2006 - Data Structures I Chapter 7 Stacks I 2 Tutorial Do not teach new materials Review the...
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Transcript of CS2006 - Data Structures I Chapter 7 Stacks I 2 Tutorial Do not teach new materials Review the...
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Tutorial
Do not teach new materials Review the lectures
Hand out simple programming problems Solve them
Use more examples to explain concepts Answer assignment problems
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Topics
ADT Stack Stack Operations Using ADT Stack
Line editor Bracket checking Special-Palindromes
Implementation of ADT Stack
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Stack Of Cups
Add a cup to the stack.
bottom
top
C
A
B
D
E
F
• Remove a cup from new stack.
bottom
top
C
A
B
D
E
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Introduction
Stack: Ordered group of homogeneous items Only one item can be accessed at a time (top) Items addition / removal take place only at the
top Inserting an item is known as "pushing"
onto the stack. Removing an item is known as "popping"
off the stack. Has Last-In-First-Out (LIFO) behavior
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Introduction
Real-life examples: Books Pennies More???
Very useful in computer science Converting numeric strings into equivalent
values Evaluating algebraic expressions
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LIFO
A stack is a LIFO (Last-in-first-out) list
1234
12345
12345
pushpush
1234
1234 5
poppop
toptop
bottombottomstack depth = 5stack depth = 5
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Introduction
Example: Line-editora b c c d d d e e f f g
means backspace will generate the following output:
a b c d e f g
Observations
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Introduction Example: Line-editor
Pseudocode: // Read the line, correcting mistakes along the waywhile (not end of line){
Read a new character chif ( ch is not a ‘ ‘ )
Add ch to the ADTelse
Remove the most recently added item from the ADT
} // end while
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Introduction
Example: Line-editor The following operations are needed
Determine if the line is empty Add a character Remove the most-recently added character Retrieve the most recently added item
These are exactly the operations needed for a stack The ADT Stack can be used to perform the reading
and correction of the line
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The Stack Interface In Java We can define our own stack interface like this:
public interface StackInterface {
public boolean isFull(); // returns if the stack is full
public boolean isEmpty(); // return if the stack is empty public Object top() // return the top element
throws StackException; // if the stack is empty
public void push(Object obj); // push obj onto the stack
public void popAll ();
public Object pop() // return and remove the top element of the stack throws StackException; // if the stack is empty
}
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Using ADT Stack
Read & Correct a text of line: Using stack operations
readAndCorrect ( out aStack : Stack)// Reads input line and either enters characters onto stack S// or corrects contents if the character is the ‘ ‘ symbolaStack.createStackRead newCharwhile ( newChar != eoln ){ if ( newChar != ‘ ‘ )
aStack.Push( newChar)else if (! aStack.isEmpty())
aStack.Pop( )read newChar
} // end whileHow to write the contents in a stack backwards?
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Stack Application
Example: Balanced Braces Assumption
All braces are of the same type Example:
a b c { d e f g { i j k } { l { m n } } o p } q r Balanced
a b c { d e f } } { g h I j { k l } m Unbalanced
Any idea to evaluate?
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Stack Application
Balanced Braces Idea:
keep track for unmatched ‘{‘ by pushing them onto a stack When ‘}’ is encountered , pop one of the unmatched ‘{‘ from
the stack Stop when
The string and stack are both empty Success
Stack is empty, but string still contains closing braces (‘}‘) Failure: more closing braces
String is empty but stack still contains unmatched opening braces (‘{‘’)
Failure: more opening braces
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Stack Application Balanced Braces: Pseudocode
aStack.createStack ( )balancedSoFar = truei = 0while ( balancedSoFar and i < length of aString){ ch = character at position i
++ iif ( ch = ‘{‘)
aStack.push( ‘{‘ )else if ( ch = ‘}’ )
aStack.pop( ) // Pop a matching open brace
else balancedSoFar = false// ignore all characters other that braces
} // end while
if ( balancedSoFar && aStack.isEmpty( ) )aString has balanced braces
elseaString doesn’t have balanced braces
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Stack Application
Algorithm Scan expression from left to right Each time a left bracket is encountered, push it onto
stack When a right bracket is encountered, compare it with
the top item on the stack If it is a match, pop the stack If not, report illegal bracketing If the stack id prematurely empty, report illegal If there are items left on the stack after the expression has
been scanned, report illegal bracketing
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Stack Application
Special-Palindromes L = { w $ w ' : w is either empty or any string not including $,
and w ' = reverse (w) } Idea:
Traverse the first half and push its elements onto the stack When $ is reached, ignore it and start traversing and matching
each element from the input string with that on the stack Termination:
Success: Both stack and string are empty
Failure: Character on input doesn't match character on stack top String empty, but stack isn't Stack empty, but string isn't
Write the pseudo code by yourself
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ADT Stack Implementation
Array Linked List ADT List
Where is the top?
10
20
30
Top10
20
30
Top10
20
30
Top
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An Array-Based Stack We can use arrays for our stacks by requiring
a maximum size N for our stack (e.g. N = 50) The stack would then consist of an N-element
array, s, and the array index of the top element, t
Array indices start at 0,so we will initialize t to -1
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Stack Interface
public interface StackInterface { public boolean isEmpty(); public boolean isFull(); public void push(Object newItem) throws
StackException; public Object pop() throws StackException; public void popAll(); public Object peek/top () throws StackException;
} // end StackInterface
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Stack Exception
public class StackException extends java.lang.RuntimeException { public StackException(String s) { super(s); } // end constructor} // end StackException
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Array Stack Implementation (1)public class ArrayStackBased implements StackInterface { final int MAX_STACK = 50; // maximum size of stack private Object items[ ]; private int top;
public ArrayStackBased() { items = new Object[MAX_STACK]; top = -1; } // end default constructor
public boolean isEmpty() { return top < 0; } // end isEmpty
public boolean isFull() { return top == MAX_STACK-1; } // end isFull
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Array Stack Implementation (1)public class ArrayStackBased implements StackInterface { final int MAX_STACK = 50; // maximum size of stack private Object items[ ]; private int top;
public ArrayStackBased() { items = new Object[MAX_STACK]; top = -1; } // end default constructor
public boolean isEmpty() { return top < 0; } // end isEmpty
public boolean isFull() { return top == MAX_STACK-1; } // end isFull
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Array Stack Implementation (2)
public void push(Object newItem) throws StackException { if (!isFull()) { items[++top] = newItem; } else { throw new StackException("StackException on " + "push: stack full"); } // end if } // end push
public void popAll() { items = new Object[MAX_STACK]; top = -1; } // end popAll
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Array Stack Implementation (2)
public void push(Object newItem) throws StackException { if (!isFull()) { items[++top] = newItem; } else { throw new StackException("StackException on " + "push: stack full"); } // end if } // end push
public void popAll() { items = new Object[MAX_STACK]; top = -1; } // end popAll
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Array Stack Implementation (3) public Object pop() throws StackException { if (!isEmpty()) { return items[top--]; } else { throw new StackException("StackException on " + "pop: stack empty"); } // end if } // end pop
public Object peek() throws StackException { if (!isEmpty()) { return items[top]; } else { throw new StackException("Stack exception on " + "peek - stack empty"); } // end if } // end peek} // end ArrayStack
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Array Stack Implementation (3) public Object pop() throws StackException { if (!isEmpty()) { return items[top--]; } else { throw new StackException("StackException on " + "pop: stack empty"); } // end if } // end pop
public Object peek() throws StackException { if (!isEmpty()) { return items[top]; } else { throw new StackException("Stack exception on " + "peek - stack empty"); } // end if } // end peek} // end ArrayStack
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Array Stack Test public class ArrayStackTest { public static final int MAX_ITEMS = 15; public static void main(String[ ] args) { ArrayStackBased stack = new ArrayStackBased(); Integer items[ ] = new Integer[MAX_ITEMS]; System.out.println("Pushing:"); for (int i=0; i<MAX_ITEMS; i++) { items[i] = new Integer(i); if (!stack.isFull()) { System.out.print(" "+i); stack.push(items[i]); } // end if } // end for System.out.println("\nPopping:"); while (!stack.isEmpty()) { // cast result of pop to Integer System.out.print(" "+(Integer)(stack.pop())); } // end while System.out.println(); } // end main} // end ArrayStackTest
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Array Stack
Questions: Can we push/pop primitive type value onto the
stack? Can we push/pop different objects onto the stack?
Student
Car
How to access each object?
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Review
If the array: 6, 2, 7, 13, 5, 4
is added to a stack, in the order given, which number will be the first number to be removed from the stack? 6 2 5 4
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Review The item that is removed first from a stack
is called the ______ of the stack. front top Base prime
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Review
If the array: 6, 21, 35, 3, 6, 2, 13
is added to a stack, in the order given, which of the following is the top of the stack? 2 6 3 13 35