1. CS 106X – Programming Abstractions in C++ Cynthia Bailey Lee CS2 in C++ Peer Instruction Materials by Cynthia Bailey Lee is licensed under a Creative Commons Attribution-NonCommercial- ShareAlike 4.0 International License. Permissions beyond the scope of this license may be available at http://peerinstruction4cs.org.Cynthia Bailey LeeCreative Commons Attribution-NonCommercial- ShareAlike 4.0 International Licensehttp://peerinstruction4cs.org

2. Today’s Topics 1. ADTs/Containers: what are they? 2. Stack 3. Compare Stack and Vector implementations of the same code 4. [Maybe Monday] Stack example: Reverse Polish Notation evaluator 2

3. ADTs

4.  Programming language independent models of common containers  They encompass not only the nature of the data, but ways of accessing it  They form a rich vocabulary of nouns and verbs, often drawing on analogies to make their use intuitive, and to give code written in them a certain literary quality

5. "Hope" is the thing with feathers BY EMILY DICKENSON “Hope” is the thing with feathers - That perches in the soul - And sings the tune without the words - And never stops - at all - And sweetest - in the Gale - is heard - And sore must be the storm - That could abash the little Bird That kept so many warm - I’ve heard it in the chillest land - And on the strangest Sea - Yet - never - in Extremity, It asked a crumb - of me.

6. Some ADTs implemented in the Stanford Libraries  Vector  Grid  Graph  Map (and HashMap)  Set (and HashSet)  PriorityQueue  Queue  Stack

7. ADT: Grid static void clearBoard(Grid & board); int main(){ Grid board(8,8); clearBoard(board); //not strictly necessary // …more code to come… return 0; } static void clearBoard(Grid & board){ for (int i=0; i<board.numRows(); i++){ for (int j=0; j<board.numCols(); j++){ board[i][j] = false; } queensafety.cpp

8. Stacks

9. New ADT: Stack template class Stack { public: Stack(); virtual ~Stack(); int size() const; bool isEmpty() const; void clear(); void push(ValueType value); ValueType pop(); ValueType peek() const; std::string toString(); private: }; stack.h -Redacted-

10. New ADT: Stack template class Stack { public: Stack(); virtual ~Stack(); int size() const; bool isEmpty() const; void clear(); void push(ValueType value); ValueType pop(); ValueType peek() const; std::string toString(); private: }; stack.h -Redacted- This image was released by the National Cancer Institute, an agency part of the National Institutes of Health, with the ID 8368National Cancer InstituteNational Institutes of Health8368

11. Using a Stack to buffer file input void mystery(ifstream& infile) { Stack lines; string line; while (getline(infile,line)) { lines.push(line); } infile.close(); while (!lines.isEmpty()) { cout << lines.pop() << endl; } What does this code do? A. Prints all lines of a file to cout B. Prints only the first line of a file to cout C. Prints only the last line of a file to cout D. Prints all lines of a file to cout in reverse E. All/ none/ more than one of the above

12. void mystery(ifstream& infile) { Stack lines; string line; while (getline(infile,line)) { lines.push(line); } infile.close(); while (!lines.isEmpty()) { cout << lines.pop() << endl; } Using a Stack to buffer file input What does this code do? A. Prints all lines of a file to cout B. Prints only the first line of a file to cout C. Prints only the last line of a file to cout D. Prints all lines of a file to cout in reverse E. All/ none/ more than one of the above Why do I need Stack? I could have done that with a Vector!

13. Stack or Vector? Vector lines; lines.insert(lines.size(), line); cout << lines[lines.size()-1] << endl; lines.remove(lines.size()-1); void mystery(ifstream& infile) { Stack lines; string line; while (getline(infile,line)) { lines.push(line); } infile.close(); while (!lines.isEmpty()) { cout << lines.pop() << endl; }

14. Vector version void mystery2(ifstream& infile) { Vector lines; string line; while (getline(infile,line)) { lines.insert(lines.size(),line); } infile.close(); while (!lines.isEmpty()) { cout << lines[lines.size()-1] << endl; lines.remove(lines.size()-1); }

15. Applications of Stacks We’ve seen one (buffering input and giving it back in reverse—LIFO—order). What else are Stacks good for?

16. Operator Precedence and Syntax Trees  Ignoring operator precedence rules, how many distinct results are there to the following arithmetic expression?  3 * 3 + 3 * 3 A. 1 B. 2 C. 3 D. 4 E. More than 4

17. Reverse Polish Notation  Ambiguities don’t exist in RPN  Also called “postfix” because the operator goes after the operands  Postfix (RPN):  4 3 * 4 3 * +  Equivalent Infix:  (4*3) + (4*3) http://commons.wikimedia.org/wiki/File:Hewlett-Packard_48GX_Scientific_Graphing_Calculator.jpg

18. Reverse Polish Notation  This postfix expression:  4 3 * 7 2 5 * + +  Is equivalent to this infix expression: A. ((4*3) + (7*2)) + 5 B. (4*3) + ((7+2) + 5) C. (4*3) + (7 + (2*5)) D. Other/none/more than one http://commons.wikimedia.org/wiki/File:Hewlett-Packard_48GX_Scientific_Graphing_Calculator.jpg

19. Stacks and RPN  Evaluate this expression with the help of a stack  Encounter a number : PUSH it  Encounter an operator : POP two numbers and PUSH result  4 3 * 7 2 5 * + + 4 3 4 12 * 7 2 7 * ? ? ? ? ? ? Contents of the stack, reading from top down: A. 7, 12 B. 2, 7, 12 C. 10, 7,12 D. 10, 5, 2, 7, 12 E. Other 5 2 7 12

20. Stacks and RPN: What does that look like in code?  Evaluate this expression with the help of a stack  Encounter a number : PUSH it  Encounter an operator : POP two numbers and PUSH result 43*725*++