Revisit Stack ADT with Exceptions

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Revisit Stack ADT with Exceptions

• CSE116

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Exceptional Handling

• Create an exception class or hierarchy of
  exception classes to handle application-dependent
  exceptions. Ex RunTimeException,
  EmptyStackException
• Define the method that has a potential of causing
  the exception with throws primitive at the
  header and inside the method test the condition
  for throwing exception and throw the
  exception (object). throws clause is optional
  (See pop() in Stack)
• throws clause in the method definition forces
  the method call to be enclosed within try block
  followed by catch blocks where the exception can
  be caught and handled.

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java.util.Stack

• It has EmptyStackException which is a subclass of
  RunTimeException
• Lets study the code for the EmptyStackException
  class, Stack class, and an application.

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Applications

• Stack
• Balancing parenthesis
• Postfix expression evaluation
• Language recognizers
• Traversing a maze
• Queue
• Infix expression evaluation
• Queuing analysis

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Arithmetic expressions

• Infix expressionOperator in between operands
• Ex A B, (A B (C D)B)
• Postfix expression Operator after the operand
• Prefix Operator in front of the operands
• Note Operators are binary operators with two
  operands.

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Postfix Evaluation

• 1. Scan elements of the postfix expression left
  to right
• 1.1 If element is operand s.push(element)
• 1.2 else if element is operator
• 1.2.1 op2 s.pop()
• 1.2.2 op1 s.pop()
• 1.2.3 result op1 operator op2
• 1.2.4 s.push(result)
• 2. Value of the postfix expression s.pop()

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Infix to postfix conversion

• Infix expression of format
• ( 5 6 ) ( 8 10 (8 4))
• Use a Queue q and Stack s
• Only binary operations operations with two
  operands

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Algorithm

• Scan elements of expression from left to right.
• Case element of
• 2.1 operand q.add(element)
• 2.2 ( s.push(()
• 2.3 ) 2.3.1 x s.pop()
• 2.3.2 while (x ! ()
• q.add(x)
• x s.pop()

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Algorithm (contd.)

• 2.4 operator
• 2.4.1 x s.top()
• 2.4.2 while !(xs precedence lt operator) x
  ( s.empty() )
• q.enque(s.pop())
• x s.top()
• 2.4.3 s.push(operator)
• 3. while (!s.empty())
• q.enque(s.pop())
• 4. Queue q has the postfix expression.

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To be continued.