ADT Stack & Queue - Case Studies TCP1201: 2013/2014

ADT Stack: Checking for Balanced Braces  A stack can be used to verify whether a program contains balanced braces o An example of balanced braces - abc{defg{ijk}{l{mn}}op}qr o An example of unbalanced braces - abc{def}}{ghij{kl}m Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley. Ver 5.0.

ADT Stack: Checking for Balanced Braces  Requirements for balanced braces  Each time you encounter a “}”, it matches an already encountered “{”  When you reach the end of the string, you have matched each “{”  Every time we see a start ‘{', push onto the stack.  Every time we see an end ‘}', pop off the stack.  So the parentheses are not balanced if: 1. We try to pop, but the stack is empty. 2. When we are done, the stack is not empty Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley. Ver 5.0.

ADT Stack: Checking for Balanced Braces Figure 6-3 Traces of the algorithm that checks for balanced braces Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley. Ver 5.0.

ADT Stack: Checking for Balanced Braces bool isBalanced (string expression) { Stack s; char temp; for (int i=0; i<expression.length(); i++) { if ( expression[i] == '{' ) s.push (expression[i]); if ( expression[i] == '}' ) if ( s.isEmpty() ) return false; else s.pop(temp); } if ( !s.isEmpty() ) return false; return true; }

ADT Stack: Checking for Balanced Braces int main() { string expression; cout << "Enter a string: "; cin >> expression; if (isBalanced(expression)) cout << "Expression is BALANCED!" << endl; else cout << "Expression is NOT BALANCED!" << endl; } OUTPUT:

 In Postfix Notation (RPN), mathematical expressions can be written without parentheses.  * = ((1+2)+4)*3  Some programming languages such as Postscript use Postfix Notation.  C++ is built to interpret the standard Infix Notation.  We can get C++ to interpret Postfix Notation using a stack. ADT Stack: Evaluating Postfix Expressions

8  A postfix calculator  When an operand is entered, the calculator  Pushes it onto a stack  When an operator is entered, the calculator  Applies it to the top two operands of the stack  Pops the operands from the stack  Pushes the result of the operation onto the stack Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley. Ver 5.0. ADT Stack: Evaluating Postfix Expressions

9 Figure 6-8 The action of a postfix calculator when evaluating the expression 2 * (3 + 4) Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley. Ver 5.0. ADT Stack: Evaluating Postfix Expressions

10 ADT Stack: Evaluating Postfix Expressions  To evaluate a postfix expression entered as a string of characters  Use the same steps as a postfix calculator  Simplifying assumptions  The string is a syntactically correct postfix expression  No unary operators are present  No exponentiation operators are present  Operands are single lowercase letters that represent integer values Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley. Ver 5.0. Take Home Exercise Implement the postfix calculator using the ADT Stack.

ADT Queue: Character Categorization Write a program that reads a set of characters as input (each character is separated by a space), and categories the characters into 4 different groups, namely number, small letter, capital letter, and others. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley. Ver 5.0. Input characters (Press ENTER to stop): w 1 ^ B A i 2 l _ 3 4 l N i ^ | a | m | Numbers: Small Letters: w i l l i a m Capital Letters: B A N Others: ^ _ ^ | | |

ADT Queue: Character Categorization Steps of the program: 1. Enqueue each character into an input queue. 2.Categorize the input queue into four queues according to the type of a character: number, small letter, capital letter, and others. 3.Display the characters in each queue by performing dequeue on each queue until it is empty Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley. Ver 5.0.

int main() { Queue input; char temp; char character = '\0'; // STEP 1: Enqueue each character into an input queue cout << "Input characters (Press ENTER to stop):" << endl; while (cin.peek() != '\n') { cin >> character; input.enqueue(character); } cout << endl; ADT Queue: Character Categorization

// STEP 2: Categorise the input queue into four queues // according to the type of a character const int NUM_OF_CATEGORY = 4; Queue category[NUM_OF_CATEGORY]; string caption[] = {"Numbers", "Small Letters", "Capital Letters", "Others"}; while (!input.isEmpty()) { input.peek( temp ); if ( temp >= '0' && temp <= '9') { category[0].enqueue( temp ); } else if ( temp >= 'a' && temp <= 'z'){ category[1].enqueue(temp); } else if (temp >= 'A' && temp <= 'Z') { category[2].enqueue(temp); } else { category[3].enqueue(temp); } input.dequeue(temp); } ADT Queue: Character Categorization

// Display the content of the queue for each category for (int i = 0; i < NUM_OF_CATEGORY; ++i) { cout << caption[i] << ": "; while (!category[i].isEmpty()) { category[i].dequeue(temp); cout << temp << " "; } cout << endl; } ADT Queue: Character Categorization