Vectors and Grids Eric Roberts CS 106B April 8, 2009

A Heart-to-Heart Talk about Exams I’ve read enough of the warm-up messages coming in for Assignment #1 to know that there is a lot of fear and loathing about exams, and a wish that things could be different. One comment that seems fairly representative is > I loved the class but really disliked the way we were > graded... having to write code for the midterm and > final did not successfully test my knowledge of the > material at all. My immediate response is: “Be careful what you wish for.” We experimented with laboratory exams for a while in the mid-1990s. We abandoned the experiment mostly because the failure rate was way too high. But why not have a take-home exam or just grade CS106 on the homework assignments? Well, there’s a problem... But why not have a take-home exam or just grade CS106 on the homework assignments?

A Modest Proposal For many years, I’ve been thinking about the possibility of reducing the weight of the final if we can do something collectively about the Honor Code problem. Here is my proposal: The weight of the final will be 15% + 5% for each Honor Code case filed this quarter The weight assigned to the homework will be whatever is left after the announced weights are assigned to the various other components, subject to a minimum of 15%. Thus, if no Honor Code cases come up this quarter, the final will count for 15%, which is exactly the same as the midterm. The homework would count for 60%. If we file three Honor Code cases (as we did last quarter), the final will count for 30% and the homework for 45%. And so on...

The Collection Classes For the next three days, we will be learning about the classes in Chapter 4. These classes, for the most part, contain other objects and are called container or collection classes. Here are some general guidelines for using these classes: –These classes represent abstract data types whose details are hidden. –Each class requires a type specification as described on the next slide. –Declaring variables of these types always invokes a constructor. –Any memory for these objects is freed when its declaration scope ends. –Assigning one value to another copies the entire structure. –To avoid copying, these structures are usually passed by reference.

Template Classes The collection classes are implemented using the template facility in C++, which makes it possible for an entire family of classes to share the same code. Instead of using the class name alone, the Vector, Grid, Stack, Queue, and Map classes require a type parameter that specifies the element type. Thus, Vector represents a vector of integers and Grid represents a two- dimensional array of characters. It is possible to nest classes, so that, for example, you could use the following definition to represent a list of chess positions: Vector > chessPositions;

Simple Call-by-Reference Example /* * Function: SolveQuadratic * Usage: SolveQuadratic(a, b, c, x1, x2); * * This function solves a quadratic equation. The coefficients * are supplied as the arguments a, b, and c, and the roots are * returned in x1 and x2, which are reference parameters. */ void SolveQuadratic(double a, double b, double c, double & x1, double & x2) { if (a == 0) Error("The coefficient a must be nonzero"); double disc = b * b - 4 * a * c; if (disc < 0) Error("The solutions are complex numbers"); double sqrtDisc = sqrt(disc); x1 = (-b + sqrtDisc) / (2 * a); x2 = (-b - sqrtDisc) / (2 * a); }

Methods in the Vector Classes vec.size() Returns the number of elements in the vector. vec.isEmpty() Returns true if the vector is empty. vec[i] Selects the i th element of the vector. vec.add(value) Adds a new element to the end of the vector. vec.insertAt(index, value) Inserts the value before the specified index position. vec.removeAt(index) Removes the element at the specified index. vec.clear() Removes all elements from the vector.

The ReadTextFile Function /* * Reads an entire file into the Vector supplied * by the user. */ void ReadTextFile(ifstream & infile, Vector & lines) { while (true) { string line; getline(infile, line); if (infile.fail()) break; lines.add(line); }

AskUserForInputFile /* * Opens a text file whose name is entered by the user. If * the file does not exist, the user is given additional * chances to enter a valid file. The prompt string is used * to tell the user what kind of file is required. */ void AskUserForInputFile(string prompt, ifstream & infile) { while (true) { cout << prompt; string filename = GetLine(); infile.open(filename.c_str()); if (!infile.fail()) break; cout << "Unable to open " << filename << endl; infile.clear(); }

Methods in the Grid Classes grid.numRows() Returns the number of rows in the grid. grid.numCols() Returns the number of columns in the grid. grid[i][j] Selects the element in the i th row and j th column. resize(rows, cols) Changes the dimensions of the grid and clears any previous contents.

Exercise: Crossword Numbering The main problem for today is to write the code to number a crossword grid. The grid itself comes from a text file that has spaces for blank squares and # signs for dark squares: 15x15 # # # # # # ## # ######### # ## # # # # # #

The End