lab05, CISC105, Fall 2007

Preparation

This lab will go more smoothly if you have read certain sections of the Chapter called "Logical Functions and Control Structures" in your "MATLAB by Holly Moore" textbook.

This is probably Chapter 8 in most of your books.

Sections of that chapter you should read before doing this lab, if possible:

• Section 4, Selection Structures
• Section 5, Repetition Structures: Loops

Overview

This week, in lab05, we will give you some practice with control structures (chapter 8 in the MHM textbook):

• if/else
• for loops
• while loops

In the process, we'll talk about how to refactor some of our test code to make it easier to work with.

We'll learn about a few MATLAB functions:

• mod(), which calculates the remainder after division
• disp() and fprintf(), which are used for output

We'll learn about concatenating character strings by constructing new matrices with [ ]

Goals

By the time you complete this lab, you should be able to:

1. Automate testing routines by writing new MATLAB function M-files
2. State what the output of the mod(x,m) function will be for simple values of x and m
3. State what the output of disp and fprintf will be for certain simple cases (as illustrated later in this lab.)
4. Given examples of simple uses of control statements, state what the output will be:
   1. Given a simple if/else along with fprintf statements, state what the output will be:
   2. Given a simple for loop containing simple fprintf statements, state what the output will be.
   3. Given a while loop controlled by simple arithmetic, containing fprintf statements, state what the output will be.
5. Given a problem statement, and examples, write a function M-file to produce that output using either if/else, for, or while, or a combination of these.

By now we assume you are entirely familiar with these items, and will no longer describe them in detail:

1. Managing directories in MATLAB with the pwd, mkdir, and cd commands.
2. Converting simple mathematical formulas into MATLAB assignment statements
3. Saving a sequence of MATLAB commands into a script M-file

Step-by-Step Instructions

Step 1: Preliminaries

To prepare for this week's lab, do all the following steps. If you are not sure what to do at any stage, you can consult earlier labs for details:

• Create a new directory called ~/cisc105/lab05
• Copy all the files from /www/htdocs/CIS/105/haggerty/07F/labs/lab05 into your new directory ~/cisc105/lab05
• Also create a new directory called ~/cisc105/lab05a
• Copy all the files from /www/htdocs/CIS/105/haggerty/07F/labs/lab05a into your new directory ~/cisc105/lab05a

Because there are lots of files this week, we are dividing them into two directories:

• The files in lab05 include the ones you will turn in for credit
• The ones in lab05a are ones that are only for practice.

Step 2: disp, fprintf, and working with character strings

In this step, you'll learn a bit about two useful MATLAB functions that print output, namely:

• disp()
• fprintf()

You'll also learn about working with MATLAB variables that store text. In MATLAB these variables are called character arrays. They are also sometimes called strings.

There is nothing to turn in for this step, but if you skip it, you'll pay the price...

You could easily skip this step, and your instructor or TA would never know. Until, that is, you

• run into problem with later steps in the lab, where the learning you get from this step is needed
• are unable to answer questions on the exam that pertain to what you were supposed to learn in this step.

Then, we'll know. So, don't skip this step.

Step 2a: disp vs. fprintf, part 1—the newline thingy

So, both disp and fprintf can be used to print out stuff. Try these commands at the MATLAB prompt:

>> disp('hello');

hello

>> fprintf('hello');

hello>> 

Notice how the MATLAB prompt >> comes immediately after the second hello.

Right away, we see one difference between disp and fprintf—disp prints its output and moves to a new line, but fprintf doesn't.

If we want a new line after hello with fprintf, we need to put \n at the end of our string:

>> fprintf('hello\n');

hello

>> 

If we put a \n at the end of a character string with disp, what will happen? You might expect that we would get an extra new line, but that is not what happens:

>> disp('hello\n');

hello\n

>> 

In MATLAB, \n is only interpreted as a new line character when it is used with fprintf. Indeed, if we include \n in a character array, it is interpreted as two separate characters. Notice the size is 1x2.

>> x='\n' 



x =



\n



>> whos x

  Name      Size            Bytes  Class    Attributes



  x         1x2                 4  char               



>> 

(We'll see later that in C++, this is not the case—In C++, as in Java and many other languages, \n is always interpreted as a single character, namely the newline character.)

Step 2b: disp vs. fprintf, part 2 —printing variables

When using disp, if you put something in quotes, it prints out the value literally. If you don't put that thing in quotes, it interprets it as the name of a variable, and tries to print the value. For example, notice the difference between disp('x'); and disp(x);

>> x = 5;

>> disp('x');

x

>> disp(x);

     5



>>     

If we try this with fprintf, we get different results:

>> x = 7;    

>> fprintf('x');

x>> fprintf(x);  

??? Error using ==> fprintf

No format string.



>> 

In the first case, fprintf('x'); prints x just like we expected—and without going to a new line before the MATLAB prompt >>, just as we expected. But in the second case, fprintf(x), we get an error: No format string. What is the reason?

The reason is this: fprintf must always have a string as its first argument.

• This string is treated as the format string.
• If this string is the only argument, it is just printed
• If there are additional arguments, the format string controls how and whether they are printed.

For example, suppose we write an assignment statement to give a value to x. We can then use fprintf to print the string x= followed by the value of x, and then go to a new line, like this:

>> x = 3 + 4;          
>> fprintf('x=%f\n',x);
x=7.000000
>> 

The %f stands in place of where the variable x will be printed, and the \n stands in place of where the new line character will go.

There are many other options for the format string, and it is also possible to print multiple variables in a single format string. Read more about format strings in your "Matlab, by Holly Moore" textbook: Chapter 7, section 2, "Output Options".

You can also type help fprintf and/or help disp at the MATLAB prompt to learn more, as well as doing web searches on "MATLAB fprintf" and "MATLAB disp".

Step 2c: Combining character strings

Another important concept is the concept of combining character strings.

Suppose we assign values as follows:

>> fname = 'Fred';     

>> lname = 'Flintstone';

Then we can combine the two names into one by combining them inside a set of [ ]. This takes the the character arrays and sticks them together:

>> fullName = [fname lname] 



fullName =



FredFlintstone



>>

We sometimes call this operation string concatenation—this is a fancy computer science way of saying "stick 'em together".

Notice that the full name ended up without a space between Fred and Flintstone. We can fix this by including a space when we do the concatenation operation:

>> fullName = [fname ' ' lname]



fullName =



Fred Flintstone



>> 

We'll use this string concatenation trick inside one of the function M-files later in this lab.

Step 3: The if/else statement

The if/else statement is covered in chapter 8 of your MATLAB, by Holly Moore textbook, in Section 4, "Selection Structure". It would be helpful to review that section prior to doing this step in the lab.

Step 3a: Reviewing where we've already seen the if/else

We've already seen the if/else statement in the test scripts from lab02 and lab03. Take a look at each of these, and remind yourself of the places you've already seen the if/else, and how it works.

• lab02/testAreaOfPizza.m
• lab02/testAreaOfRectPizza.m
• lab03/testDistance.m

Step 3b: A function M-file involving if/else

The if/else is also useful in writing certain function M-files. For example, we can write a function M-file that tests whether a number is odd or even. Look at the function M-file isOdd.m that you copied into your lab05a directory in Step 1.

Move into your ~/cisc105/lab05a directory, and try typing each of the following at the MATLAB prompt. Notice that in each case, the answer 1 indicates true, while the answer 0 indicates false.

• isOdd(3)
• isOdd(4)
• isOdd(5)
• isOdd(-7)
• isOdd(-2)
• isOdd(0)

The isOdd() function uses the built-in MATLAB function mod(), which computes the remainder after integer division, to determine whether a number is odd or even. Before we go further, we need to understand this mod function

Step 3c: Understanding the mod function

• mod(x,2) means "divide the number x by 2. If two is evenly divisible by 2, then return 0, but if x is not evenly divisible by 2, return the remainder."
• Try typing mod(27,3) at the MATLAB prompt. Notice that MATLAB returns 0 because Then try:
  • mod(28,3)
  • mod(29,3)
  • mod(30,3)
  • mod(31,3)
  • mod(32,3)
  • mod(33,3)
• Do you see the pattern? Do you understand the concept of "remainder after division"?
• If so, try predicting the value that would be returned by the expressions below. Do as many as of these as you need to until you are comfortable that you can predict the result. Note: a question involving mod may appear on one of the exams.

Some additional notes about the mod function

There are various approaches to using the mod function with negative numbers, as this web page explains:

• http://mathforum.org/library/drmath/view/52343.html

For purposes of CISC105 quizzes/exams, unless the issue arises in some later programming assignment, I don't plan to ask quiz/exam questions about mod involving negatives—I just want you to be aware of the issue. (Thanks to TA Colin Kern for pointing out the issue, and calling my attention to this web page.)

On a related issue, MATLAB actually has two functions for calculating mod. According to the MATLAB reference for the mod function:

mod(X,Y) and rem(X,Y) are equal if X and Y have the
same sign, but differ by Y if X and Y have different signs.

(Thanks to Prof. Terry Harvey for pointing this out.)

Step 3d: Testing scripts for isOdd()

Among the files you copied for this week's lab into ~/cisc105/lab05a are two testing scripts for the isOdd function:

• testIsOdd.m
• test2IsOdd.m

Look first at the script testIsOdd.m. You'll see that this script looks very much like all the testing scripts we've seen before. There is one exception: since we are dealing only with the values false and true, we can directly test if (actual == expected). There is no need to use a tolerance value and test the difference between the actual and expected values, as we did in earlier scripts.

Next, run the testIsOdd.m script, as shown here:

>> testIsOdd

test 1 passed

test 2 passed

test 3 passed

test 4 passed

>> 

You can see that all four tests pass. Now try running the alternative test script, test2IsOdd.m:

>> test2IsOdd

test 1 passed

test 2 passed

test 3 passed

test 4 passed

test 5 passed

>> 

Since test2IsOdd.m has five tests rather than four, you might expect it to be a longer script. But look inside, and there is a surprise—test2IsOdd is actually a much shorter script.

We can measure how much shorter by using the Unix command wc -l filename, which tells us how many lines are in a file.

• Remember to type these at a Unix prompt, not at a MATLAB prompt—or, you can type them inside MATLAB if you precede them with an exclamation point
• Also remember that the letter after the - in wc -l is a lowercase letter l as in lines, not the number 1 as in the integer between 0 and 2.

> wc -l testIsOdd.m

84 testIsOdd.m

> wc -l test2IsOdd.m

13 test2IsOdd.m

> 

This output tells us that testIsOdd.m contains 84 lines, while test2IsOdd.m contains only 13. How is it that test2IsOdd.m gets more work done with 84.5% less code (of course, I computed this using MATLAB...)

>> (84 - 13)/84

ans =
 0.8452
>> 

Look inside the file test2IsOdd.m and you can find the answer—there is another function M-file involved.

Locate this other helper function, and look inside and try to understand how this works.

Inside this helper file, notice especially the line of code:

disp(['test ' testNum ' passed']);

Do you see how this uses the skills we learned in step 2?

Once you've understood how the test2IsOdd.m file and its companion function M-file, you can now tackle the next step.

Step 3e: A revised test script for the distance.m file from lab03

Now move into your ~/cisc105/lab05 directory. We are going to do some work that is part of what you will turn in for credit.

Back in lab03, you worked produced a function M-file called distance.m to compute the distance function. In lab03, a test file called testDistance.m was supplied for you.

This week's lab also contains a version of testDistance.m, but it is different. Use the links below to compare the two files:

• lab03/testDistance.m
• lab05/testDistance.m

The new version of testDistance.m uses a function M-file to partially automate the repeated testing—similar to the way that test2IsOdd.m did —but in this case I have not supplied you with that function M-file. Your job is to supply the missing file.

First, copy your distance.m function M-file from your ~/cisc105/lab03 directory into your ~/cisc105/lab05 directory. (By now, you should know the Unix command to do that—this represents a potential exam question!)

Then, look over the testDistance.m file from this week's lab. Notice the lines such as these:

performDiffTest( distance(1,5,-2,1), 5, 0.001, '1');

performDiffTest( distance(-2,-3,-4,4), 7.28, 0.001, '2');

function result = performDiffTest(actual,expected,tolerance,testNum)

This first line is sometimes called the contract, and is also sometimes called the function prototype. From this function prototype, you can determine the name of the file you will need.

Write this file, and then test it by running the testDistance script at the MATLAB prompt. You should get the following output:

>> testDistance

test 1 passed

test 2 passed

>>

When you get this, you are finished with step 3. You'll document this step later as part of a diary file for this lab.

Step 4: Practice with if/else

This would be a good time to make sure your understanding of the if/else and if/elseif/else statements is solid.

Among the files you copied into the ~/cisc105/lab05a directory are six simple scripts that use the if/else or if/elseif/else statements, named ifElse1.m, ifElse2.m, etc. through ifElse6.m. You

Fair Warning: An upcoming "pop-quiz" may feature questions similar to those represented by these if/else and if/else/elseif files, so don't skip this step even though there is nothing you have to turn in for grading.

A note about these files: they are just for practice

They are not "practical" examples of how to use if/else to solve real world problems. The only purpose of these scripts is to test your understanding of how if/else and if/elseif/else statements work. These scripts are like "kickboard practice" in swimming, or "playing scales" on a musical instrument.

How to practice with these scripts (ifElse1.m through ifElse6.m)

First, change directory to your ~/cisc105/lab05a directory, and locate these scripts.

For each script:

1. Look at the script. Decide what you think the output will be. Write it down.
   To look at the script, you can:
   • use the Unix command more filename,
   • use the MATLAB command type filename,
   • or you can just click on a web link to the lab05a directory and then click on the filename.
2. Then, run the script at the MATLAB prompt and check your results
   • Recall that to run a script M-file, you just type the name of the file without the .m.
   • Also that this is the last time I'll remind you of that fact—and that this fact may show up on the next exam.

Practical Uses of the if/else and if/elseif/else

If you want some example of "practical" uses of the if/else and if/elseif/else, read you can find some in the textbook MATLAB, by Holly Moore text, in the chapter titled Logical Functions and Control Structures (probably Chapter 8)—see section 4 of that chapter.

Step 5: Practice with the while loop

Just as in Step 5, we practiced with the if/else, we can also practice with the while loop, another control structure discussed in Chapter 8 of MATLAB by Holly Moore.

The instructions for Step 5 are the same as for Step 4, except that this time we are working with the files while1.m, while2.m, etc. through while6.m. These files are also in the ~/cisc105/lab05a directory, and can be found at the web link lab05a

The caution about a pop quiz applies equally here. So don't skip this step even though there is nothing you have to turn in for grading.

Step 6: Practice with the for loop

You guessed it—now we practice with for loops in the same manner, using the files for1.m through for6.m. These files are also in the ~/cisc105/lab05a directory, and can be found at the web link lab05a

Be sure your skills with for loops are solid before proceeding, because—you guessed it—there could be a quiz.

Step 7: Using a for loop inside a function M-file

There's nothing to turn in for this step—but later steps in this lab that do require you to turn in something depend on work you do in this step.

• The files for this step are back in the ~/cisc105/lab05 directory.
• All the remaining work we'll do in this lab is in that directory

Step 7a: A for loop in a function M-file: lineOfx.m

We can use a for loop inside a function M-file to print out a line made up of a certain character. For example, see the function M-file lineOfx.m:

function result = lineOfx(howMany)

%lineOfx return a string made up of the letter x (howMany times)

%

% consumes: howMany, a scalar number indicating how many times

%             the letter x should appear

% produces: a string made up of the letter x that many times

%

% if x <= 0, an empty string is returned

%

% Examples:

%    >> lineOfx(3)

%    ans =

%    xxx

%    >> lineOfx(5)

%    ans =

%    xxxxx

%    >> lineOfx(-10)

%    ans =

%           ''

%    >>

% P. Conrad for CISC105, 10/07/2007



  result = '';

  for i=1:howMany

    result = [ result 'x' ];

  end;



  return;



end % function lineOfx

When we run this function at the MATLAB prompt, we get a line of x, just like in the examples:

>> lineOfx(3)


ans =

xxx


>> lineOfx(4)


ans =

xxxx


>> lineOfx(-2)



ans =



     ''





>> 

Note the last example there: the result is an empty matrix. This will end up being an interesting example, as we'll see later in Step 8.

Step 7b: Displaying the output with disp()

We can get rid of all the excess spaces and the the ans = business by making our function call to boxOfx() be an argument to the disp() function:

>> disp(lineOfx(4))
xxxx
>> disp(lineOfx(2))
xx
>> disp(lineOfx(-3))
>> 

Step 7b: Using lineOfx() inside another function M-file: boxOfx()

We can use the lineOfx() function inside another M-file called boxOfx() to produce a box of x characters:

function result = boxOfx(width)

%boxOfx return a square made up of the letter x

%

% consumes: width, a scalar number indicating how many times

%             the letter x should appear

% produces: a square box made up of the letter x

%

% if x <= 0, an empty string is returned

%

% Examples:

%    >> boxOfx(3)

%    ans =

%

%    xxx

%    xxx

%    xxx

%

%    >> lineOfx(5)

%    ans =

%

%    xxxxx

%    xxxxx

%    xxxxx

%    xxxxx

%    xxxxx

%

%    >> lineOfx(-10)

%    ans =

%           ''

%    >>

% P. Conrad for CISC105, 10/07/2007



  result = []; % set result to the empty matrix

  for i=1:width

    result = [ result ; lineOfx(width) ]; %add a row

  end;



  return;

Here's sample output from that function:

>> disp(boxOfx(3))

xxx

xxx

xxx

>> disp(boxOfx(4))

xxxx

xxxx

xxxx

xxxx

>> disp(boxOfx(2))

xx

xx

>> disp(boxOfx(-1))

>> 

So, we might try making a test script for lineOfx.m and boxOfx.m, but we run into some problems. Read on:

Step 8: A test script for boxOfx.m

In the lab05 directory this week, there is a file called testBoxOfx.m, which is a good start at a test script for boxOfx.m. It works for the first two tests, but the third test fails. Your job is to fix this problem.

Step 8a: Discovering the problem

To find the problem, try running the test script. You should get output like the following:

>> testBoxOfx

test 1 passed

test 2 passed
test 3 failed

expected =

 []


actual =

 []

>> 

 

So, this is a bit confusing—expected and actual appear to be the same. We can type whos and see that indeed both are empty matrices:

>> clear

>> whos 

>> testBoxOfx

test 1 passed

test 2 passed

test 3 failed



expected =



     []





actual =



     []



>>  whos

  Name          Size            Bytes  Class     Attributes



  actual        0x0                 0  double              

  expected      0x0                 0  double              



>>

So, why doesn't the test of if (actual == expected) work? We'll look into this more in Step 8b.

Step 8b: About testing with the Empty Matrix

MATLAB is a commercial software product, produced by a company called "The MathWorks". At the company's website http://www.mathworks.com, you can find lots of information about MATLAB.

In particular, there is a page about operations on the Empty Matrix that will shed light on the situation we are facing here. Here's a link to that page:

http://www.mathworks.com/access/helpdesk/help/techdoc/index.html?/access/helpdesk/help/techdoc/matlab_prog/f1-86359.html

Read about the problems with testing the empty matrix for equality. Find the answer to this question:

• What is returned when you compare one empty matrix with another, in a statement such as the following?

if (actual==expected)

Also read about the functions that operate on the empty matrix—in particular, the function:

isempty(A)

Given that test 3 is checking to see whether the actual value is an empty matrix, we can replace the statement:

if (actual==expected)

with some expression involving this isempty() function.

Your task in the next step: figure out what to replace this line of code with so that the test script works properly

Step 8c: Fix the testBoxOfx.m test script.

Using the information you learned in Step 8b, modify the test script so that it works properly for testing the case of returning an empty matrix.

Step 8d: Write a testLineOfx.m test script (from scratch)

Now, write a test script for the function M-file lineOfx.m.

When I say "from scratch", I only mean that I'm not giving you a file to work from. However, you should feel free to start with the testBoxOfx.m script as your starting point—you can use a Unix or a MATLAB command to copy from testBoxOfx.m to testLineOfx.m, and then edit that file.

Once you are finished with both test scripts, you can proceed to the next step—we'll make a diary to show your work later in step 10 of this lab.

Step 9: Converting to decimal to binary using while and if/else

Why are we doing this?

In this step, we'll put together a way of converting from decimal to binary using MATLAB.

• As it turns out, MATLAB has a built in function called dec2bin() that does this conversion—however, that's not the point.
• In fact, one of the challenges of learning programming with MATLAB is that so many of the usual things that one learns to do in an intro programming course are things that MATLAB built-in functions to do already.
• So, please understand that though it may seem like I'm asking you to "reinvent the wheel", that's not the case at all. What I'm ask you to do is to learn how to invent wheels, ramps, levers, etc.
• Besides, the dec2bin() function in MATLAB didn't come from nowhere, or from omniscient, omnipotent deity. Some poor human had to write the instructions—and someday, that person might be you.

So, in this exercise, you need to write a function M-file that converts decimal to binary, but does NOT use the built in dec2bin function. It will be a great way to practice with while loops, for loops and if/else, all at the same time

Step 9a: Reviewing the process of converting decimal to binary: pseudocode

As you may remember, we can convert from decimal to binary by thinking in terms of "coins" that have denominations in powers of two.

For example, to convert 84 from decimal to binary, we first list the powers of two that we will need, starting with 1, and proceeding until have gone past 84:

1,2,4,8,16,32,64,128

We don't actually write down 128, because we realize that it is bigger than 84. Also, we actually need the build the list so that the powers of two go from largest to smallest:

64,32,16,8,4,2,1

One way to express this is with a while loop:

start with an empty list of powers of two
start with the power 1

while (the power is less than the number)     

    add this power to the list (at the very end)
    multiply that power by two  
end

When we write a mixture of english sentences and control structures (things like if/else, while, and for loops), we call is pseudocode. Pseudocode is a helpful tool in writing software.

Of course, to actually convert we then need to process these powers from largest to smallest, subtracting the ones that are smaller than the number we are processing. Here's a list of the steps, as pseudocode:

howMuchLeft = 84

 Is 64 <= 84? Yes! So subtract: howMuchLeft = howMuchLeft - 64 = 20, and write 1 

 Is 32 <= 20? No! so write 0 

 Is 16 <= 20? Yes! So subtract: howMuchLeft = howMuchLeft - 16 = 4, and write 1 

 Is 8  <= 4 No! so write 0 

 Is 4 <= 4 Yes! So subtract: howMuchLeft = howMuchLeft - 4 = 0,  and write 1

 Is 2 <= 0 No! so write 0

 Is 1 <= 0 No! so write 0

 

 And we have written: 1010100, which is indeed 84 in binary. 

So, We can express this second part with a while loop also:

make a vector called powers with all the powers of two, 
  in descending order, from the one just bigger than the one
  we want to convert, all the way down to 1


make a empty result vector

howMuchLeft = the number we want to convert


for i = 1:length(powers)

   if (the ith element of powers <= howMuchLeft)

      subtract this power from howMuchLeft

      add the character '1' to end of the result vector

   else

      add the character '0' to end of the result vector

   end

end

Given this pseudocode, we can write a MATLAB function M-file to solve the problem.

We'll do it in two parts. The first part, I will walk you through. For the second part, you are on your own to figure it out.

• The first part is writing a function that returns a row vector of all the powers of two that are less than or equal to some number x, namely powersOf2LeX(x)
  • In Step 9b, we'll write a test script for this function
  • In Step 9c, we'll write a stub for powersOf2Le(x) to test the test script
  • In Step 9d, we'll finish the function, using the pseudocode above as a guide.
  • In Step 9e, we'll add a step to make sure the function works properly for input where x is less than or equal to 0.
• The second part is writing the function M-file for the function myDec2Bin(dec). For this step, you have the pseudocode above, plus the example of how to turn pseudocode into real code.
  • Of course, you also need to write a test script, and you are strongly encouraged to write the test script first in step 9f, then write a stub in step 9g, and finally write the function M-file in step 9h.
  • I can't make you do the test script first. I can tell you that nearly every student I've asked to do things this way was skeptical at first—so was I when I was first introduced to this approach—but nearly every student that earnestly tried this approach eventually saw the benefits.

So, let's get on with it.

Step 9b: A test script for powersOf2LeX(x)

We'll start by writing the test script—writing the test first is always a good way to be sure that you know exactly what you want the script to do. This test script is in the files you copied in to your lab directory this week:

% testPowersOf2LeX.m   test function powersOf2LeX(x)

% P. Conrad for CISC105, sect 99, 10/07/2007



% Test function that produces a row vector of all powers of 2 <= x



%%%%%%%%%%%%%%%%%

% Run the tests %

%%%%%%%%%%%%%%%%%



% Test 1



actual = powersOf2LeX(80);

expected = [64 32 16 8 4 2 1];



if (actual == expected)

  disp('test 1 passed');

else

  disp('test 1 failed');

  expected

  actual

end



% Test 2



actual = powersOf2LeX(8);

expected = [8 4 2 1];



if (actual == expected)

  disp('test 2 passed');

else

  % signal that test failed, and print both expected and actual

  disp('test 2 failed');

  expected

  actual

end



% end testPowersOf2LeX.m

 

Step 9c: A stub for powersOf2LeX(x)

With this in place, we can proceed to writing the function M-file—first in stub form. The function M-file is not provided for you—you need to start it from scratch. From the test cases above, you should be able to deduce:

• the name that the function M-file must have (the file you need to edit with emacs)
• the function prototype (i.e. the first line of the file, starting with the word function)
• what the function consumes and produces
• the examples you can put in the opening comment.

With that in place, add a stub for the body, like this one, so that we can "test the test script":

...



   result = [42];

   return;

end % end of function M-file 

With the stub in place, we should get this result when running the test script:

>> testPowersOf2LeX

test 1 failed



expected =



    64    32    16     8     4     2     1





actual =



    42



test 2 failed



expected =



     8     4     2     1





actual =



    42



>> 

Step 9d: Finishing the powersOf2LeX(x)

Now, we can replace the stub code (result = [42];) with some real code to calculate our result vector.

You need to work from the pseudocode to figure out the code yourself, but here are a few hints. These hints are all mixed up in terms of the order they come in in the code, though, to force you to think through the sequence of how things need to happen. You can use them to help you figure out how to convert the pseudocode into MATLAB code to calculate the result.

• You'll use a while loop
• You can initialize an empty vector by setting result = [];
• You can add a scalar x to the beginning of the result vector by setting result = [ x result ];
• You can initialize a variable called power to 1, and multiply it by two each time through the while loop (power = power * 2;)
• The loop should continue as long as the condition power <= x is true.

When the test script passes, with a result like this you can move on to Step 9e.

>> testPowersOf2LeX

test 1 passed

test 2 passed

>> 

Step 9e: One more test case in testPowersOf2LeX.m

Now, add a third test case to testPowersOf2LeX.m, one that tests for the case where you pass in a number less than or equal to zero. This should return the empty matrix, so you need to use the special way of testing for the empty matrix that we learned back in step 8c.

Run your test script again and you should get a result like this one:

>> testPowersOf2LeX

test 1 passed

test 2 passed

test 3 passed

>> 

Step 9f: A test script for myDec2Bin(x)

Now, with the powersOf2LeX(x) function working properly, you are ready to tackle the myDec2Bin(x) function. Start by writing a test script, testMyDec2Bin.m

Step 9g: A stub for myDec2Bin(x)

Next, write a stub for myDec2Bin.m. Your stub should contain everything the goes in a function M-file—the first line, the comments with produces/consumes, examples, etc.—everything except the actual code to compute the result.

We will return the binary equivalent of the number x as a string, not as a number (since numbers in MATLAB are expressed in decimal). So, when you write your stub, use a string value as the "evil" value, e.g. result = '42'; (42 obviously cannot be a legal binary value, since a legal binary value contains only zeros and ones.)

Try to get a result where the test fails, but where the test script runs with no MATLAB errors reported.

Step 9h: Finishing myDec2Bin(x)

Then, write the body of myDec2Bin(x), following the pseudocode from step 9a. Some hints:

• You can use the powersOf2LeX(x) function you wrote in step 9d to generate the vector of powers
• You can use a for loop to step through the vector. The one in the pseudocode is pretty much what you need: for i = 1:length(powers)—this is essentially already in MATLAB syntax.
• The ith element of a vector named v is expressed in MATLAB as v(i)
• To add a character 'x' to the end of a string variable s, you can use: s = [s 'x']; You'll use characters '0' and '1' to build up your binary string.
• To initialize an empty string, you can use s='';

The rest is up to you. Good luck, and have fun with it!

Step 10: Make a diary file lab05.txt.

Now, we want to make a diary file called lab05.txt documenting the work from steps 3, 8 and 9 of this week's lab.

Put yourself inside MATLAB, inside the directory ~/cisc105/lab05, and start a diary file called lab05.txt.

Then, do each of the following steps:

To document your work from step 3

• list out the contents of performDiffTest.m and testDistance.m
• run the test script M-file testDistance.m to show that the performDiffTest.m function is working properly.

To document your work from step 8

• list out the contents of testBoxOfX.m
• run the test script testBoxOfX.m to show that it works properly (including for the case of the empty matrix)
• list out the contents of testLineOfX.m
• run the test script testLineOfX.m to show that it works properly (including for the case of the empty matrix

To document your work from step 9

• list out the contents of testPowersOf2leX.m and powersOf2LeX.m
• run the test script testPowersOf2leX.m
• run at least two tests of powersOf2LeX.m by typing in interactive function calls at the MATLAB prompt
• list out the contents of testMyDec2Bin.m and myDec2Bin.m
• run the test script testMyDec2Bin.m
• run at least two more tests of myDec2Bin.m by typing in interactive function calls at the MATLAB prompt

A note about documenting your work

Because for most of you, this is your first programming course, I have been spelling out in detail what steps you need to take to document your work. However, in subsequent weeks, there will be gradually fewer details in the instructions. You'll be expected to know that, in general, you need to:

• list out the files in which you made changes, and perhaps some other key files
• run the script M-files or function M-files needed to show that your work was successful.

What I hope you will begin to do is to

• pay attention to what I ask you for, and
• begin to understand the principles behind what it means to document that the software you developed works properly.

In this way, eventually you won't need me to tell you in such detail what you need to turn in. In fact, in second, third and fourth year programming courses, it is traditional that this is not spelled out in detail—students are, by that time, expected to understand what is needed. So, I want you to begin to acquire that skill, and as such, I'll be weaning you from the level of detail you may have come to expect.

Step 11: Make a zip file lab05.zip of your .m files

Because we have several files this week to submit, we'll make things a bit easier for you and for the TA. Before uploading, we'll create a zip file that contains all of your files for this week.

Here's a list of all the files we want to put in the zip file for this week—all of these files (and only these files) should be in your lab05 directory.

Here's how to do it:

1. Get to a Unix prompt in the directory above ~/cisc105/lab05, i.e. ~/cisc105/.
• Before you go to the next step, type pwd to make sure that you are in ~/cisc105, not ~/cisc105/lab05
2. Type the following Unix command, which will make a zip file consisting of only the .m files in your lab05 directory:

   zip -r lab05 lab05 -i \*.m

   You should get output like this:

   > zip -r lab05 lab05 -i \*.m
   
     adding: lab05/testDistance.m (deflated 41%)
   
     adding: lab05/testBoxOfx.m (deflated 63%)
   
     adding: lab05/testPowersOf2LeX.m (deflated 55%)
   
     adding: lab05/lineOfx.m (deflated 50%)
   
     adding: lab05/boxOfx.m (deflated 52%)
   
     etc...
   
   >

   Afterwards you'll have a file called lab05.zip in your ~/cisc105 directory that you can submit on WebCT.

   You can read about the various options of the zip command by typing man zip at the Unix prompt.

3. To test whether creating the zip file worked or not, you can make a temporary directory, copy the zip file into it, and try unzipping the file, and seeing if it creates a lab05 directory containing the appropriate information. This is optional, but highly recommended.
   • mkdir ~/temp
   • cp lab05.zip ~/temp
   • cd ~/temp
   • unzip lab05.zip
   • ls -l
   • cd lab05
   • ls -l

Step 12: Submit your lab05.txt diary file, and your lab05.zip file

Now you can submit your work on WebCT, and you are done!

Grading

End of lab05 for CISC105, Fall 2007 (150 pts)
Due Date: 10/19/2007