CS100M Spring 2006
Project 3
Due Thursday 3/9 at 6pm

Submit your files on-line in CMS before the project deadline. See the CMS link on the course webpage for instructions on using CMS. Both correctness and good programming style contribute to your project score.

You must work either on your own or with one partner. You may discuss background issues and general solution strategies with others, but the project you submit must be the work of just you (and your partner). If you work with a partner, you and your partner must register as a group in CMS and submit your work as a group.

Objectives

Completing this project will help you learn about vectors (1D arrays), matrices (2D arrays), strings, and vectorized code. Your best strategy for this project is to decompose each problem into simpler pieces. These simpler pieces correspond to subfunctions (i.e., "helper" functions that reside in the same file as the primary function). 

Comparing Strings & Comparing Vectors

Both of the tasks below require you to compare vectors (recall that a string is a vector of characters). When you compare two vectors using, for instance, "==" the result is a vector of logical values: 0 for false and 1 for true. So what does Matlab do if we try to use a vector comparison in an if-statement or a while-loop?  There isn't a single boolean value; there is a whole vector of boolean values. 

To make sense for an if-statement or a while-loop, we must condense this vector of boolean values into a single value, either true or false. Matlab provides two functions for doing this: any and all. Each of these function takes a single vector (or matrix) as its argument.  Function any returns true if and only if there is some value in the vector that is true (nonzero).  Function all returns true if and only if all values in the vector are true (nonzero).

For example, to check if two strings are equal, we can use the following code.

    if length(strA) == length(strB) && all(strA == strB)
        % Code doing something with the strings
    end

1. Censorship

Write a censoring function: censor(inString, censorWord). The function should return a modified version of inString in which every occurrence of censorWord in inString is replaced with x's. For example, if inString is 'Matt used Matlab to format mathematics' and the censorWord is 'mat' then the resulting string should be 'Xxxt used Xxxlab to forxxx xxxthexxxics'. Note that the result string uses the same capitalization pattern as inString.

Details:

• Argument censorWord should consist entirely of lower case letters. Check this and report an error if necessary.
   
• Argument inString can have both upper- and lower-case letters as well as other kinds of characters.
   
• The returned string must use the same capitalization pattern as inString so that a censored lowercase letter is replaced with x whereas a censored uppercase letter is replaced with X.
   
• Your function should not print anything. The censored string should be returned to the calling program.
   
• Be sure to censor ALL the occurrences of the censored word in inString.
   
• You can use the Matlab functions upper, lower, isletter, and isspace, but not any of other string functions. In particular, do not use findstr or strrep or any of the functions related to regular expressions. The goal here is to practice working with vectors rather than to look up string functions in the Matlab documentation.

Hint: Recall the functions in Section and in Lab that involved DNA pattern matching. This kind of function can be used to accomplish a useful subtask for the censorship problem.

2. Interactive Graphing

Create a steerable worm. The idea is to create a Matlab figure containing a moving "worm"; the worm can be steered using mouse clicks.  We provide the code to create a figure that responds to mouse clicks. Your job is to provide the code that draws the worm and allows it to move.

The worm's current position is represented by a 21-by-2 matrix (called worm) indicating the positions of all 20 segments of the worm; worm(1,:) represents the position of the worm's "head". Each segment of the worm is one unit long and it is oriented either vertically or horizontally. For example, the pairs (1,1), (1,2), (1,3), (1,4), (1,5) correspond to a worm with 4 segments in a vertical position. Note that the worm need not be a straight line! However, in every iteration, the worm must have precisely 20 one-unit-long segments. 

The user steers the worm by clicking somewhere within the plot. A function that detects mouse clicks and the coordinates of the position of the mouse is provided to you (makeClickableFigure.m). Your job is to draw the worm repeatedly to make it move in the selected direction. 

• Your primary function should be goWorm(t) where t is a timing parameter, in seconds, that controls how fast the worm moves. Parameter t should be used in the statement "pause t" within your main loop. Without the pause statement, the worm moves too fast to see. Using t = 0.1 makes for a reasonable worm speed.
   
• Read the comments in makeClickableFigure. It creates a global variable (click) that you will need to use. Be sure to include the statement "global click" in your code.
   
• Your program should have a loop which checks whether a new position was selected. Use the global variable "click" to read the coordinates of the most recent mouse click. Once you've obtained the new value, reset the global variable to [] (the empty vector). This will help you detect a new click. In other words, if your program reads the global variable and finds that it's length is 0, the worm should continue to move in the same direction as in the previous iteration. Otherwise, it changes direction according to the rule below.
   
• Your worm should move either horizontally or vertically. If the vertical distance from the worm's head to the new point is greater than the horizontal distance the worm should move vertically (up or down, depending where the new point is, relative to the worm's head). Otherwise, it should move horizontally (left or right, in the direction of the new point).
   
• Use the "pause" command in the while loop to make the worm move at a reasonable pace. You can control how fast the worm moves (thus making worm-steering more or less difficult).
   
• Redraw the worm in every iteration of the loop. Note that the worm must retain its original size, so when the head moves forward the body follows, and the tail should not get longer.
   
• Draw the worm using the Matlab plot function. Draw each segment of the worm as a line. Draw something to distinguish the head of the worm. Note that each time plot is called, it erases any previous plot before it draws the new one; this is exactly what you want for the worm.
   
• If the worm's head reaches one of the drawing's boundaries, the game ends (exit the loop).
   
• Use a fixed-size window. For example, since each of the 20 worm segments is one unit long, a 40-by-40 area is appropriate. Matlab insists on changing the figure's axis every time something is plotted, so if you want to keep the same axis, you must use the axis function after every plot: axis([xmin xmax ymin ymax]). If you use axis([xmin xmax ymin ymax], 'square') then the axis region will be displayed as a square.

Hint: Break the problem into easier pieces. For instance, start with a "helper function" that draws the worm once using plot, taking care that the figure's axes do not change (i.e., use the axis function as described above).