Transcript Week6 - NUS School of Computing

CS1101: Programming Methodology
http://www.comp.nus.edu.sg/~cs1101/
Week 6: Writing Modular Programs &
Testing and Debugging
 Last week:
 Chapter 5: Selection Statements (cont’d)
 Chapter 6: Repetition Statements
 This week:
 Chapter 6: Repetition Statements (cont’d)
 Writing Modular Programs
 Testing and Debugging
 Next week:
 Recess! (yeah!)
 The week after next:
 Chapter 7: Defining Your Own Class – Part 2
© CS1101 (AY2009-2010 Semester 1)
Week6 - 2
Last Week’s Exercise #4: Prime-Number
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Your DL should have discussed this in last week’s
discussion session.
Write a program PrimeTest.java to check if a positive
integer is a prime.
Sample runs:
Enter a positive integer: 131
131 is a prime.
Enter a positive integer: 713
713 is not a prime.
© CS1101 (AY2009-2010 Semester 1)
Week6 - 3
Modular Programming (1/6)
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Download PrimeTestNonModular.java and
PrimeTest.java. Study and discuss them.
Modular programming
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Goes hand-in-hand with stepwise refinement and
incremental development
Makes the code easier to develop, test and debug
Promotes reusability of codes
In general a problem is solved in 3 steps:
input  computation  output.
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Modular Programming (2/6)
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It is customary to write a separate module to
perform the computation step.
If the computation is complex, it should be
further split into smaller steps and each step
performed by a module.
A ‘module’ (in Java, it is a method)
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Should be well-defined
Should do one task
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Modular Programming (3/6)
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A well-defined module
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Has a good name (for readability and documentation)
Has a clear interface (what parameters does it take?)
May pass back to its caller no result or a single result (what
value does it return?)
It’s nothing new! You have seen them before.
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Example: setColour(String c) in the Ball class
public void setColour(String c) {
colour = c;
}
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Takes in String c.
Does not return
any value (void).
Example: getColour() in the Ball class
public String getColour() {
return colour;
}
© CS1101 (AY2009-2010 Semester 1)
Takes in no
parameter.
Returns a String.
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Modular Programming (4/6)
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Advantages of modular programming:
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Easy to replace
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E.g.: When you discover a better algorithm for
isPrime(int), you just replace that method without
affecting any other parts of the program or other
programs.
Easy to reuse
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E.g.: Suppose you want to write a program to count the
number of prime numbers between two integers a and
b.
Compare how you would need to modify
PrimeTestNonModular.java and PrimeTest.java to do the
above.
© CS1101 (AY2009-2010 Semester 1)
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Modular Programming (5/6)
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Reusability of code
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If isPrime(int) is a very commonly used method,
we could even go a step further…
See Prime.java where we define the Prime class
in which it contains a class method isPrime(int).
See CountPrimes.java which is an application
program that makes use of the Prime class.
 It is so short and sweet!
Any other application that requires the isPrime(int)
method can use the method in a similar fashion.
As the creator of Prime.java, you can hide the
source code and provide only Prime.class.
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Modular Programming (6/6)
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A method should not mix computation with
input/output. (Except for very special situation.)
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Example: The following is not desirable.
public static boolean isPrime(int num) {
boolean isPrime = false;
if (num > 1) {
isPrime = true;
for (int i = (int) Math.sqrt(num); i>1 && isPrime; i--)
if (num % i == 0)
isPrime = false;
}
if (isPrime) System.out.println(num + " is prime.");
else
System.out.println(num + " is not prime.");
return isPrime;

}
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Last Week’s Exercise #2: AsterisksV1.java
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Complete AsterisksV1.java to read in an integer n and
print n asterisks on a single line.
(If n is non-positive, then no asterisk will appear.)
Sample runs:
Enter n: 7
*******
Done!
Enter n: -2
Done!
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Download AsterisksV1Completed.java and study it.
© CS1101 (AY2009-2010 Semester 1)
Week6 - 10
AsterisksV2.java
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Now, write AsterisksV2.java to read in a positive integer
n and display n rows of asterisks in the following
fashion.
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First row has 1 asterisk, second row 3 asterisks, third row 5
asterisks, etc.
Your program should be modular. There should be a
method printStars(int k) to print a row of k asterisks.
Sample runs:
Enter n: 4
*
***
*****
*******
© CS1101 (AY2009-2010 Semester 1)
Enter n: 7
*
***
*****
*******
*********
***********
*************
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AsterisksV3.java
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Now, write AsterisksV3.java to read in a positive integer
n and display n rows of asterisks in a “Christmas Tree”
pattern.
How do you adapt AsterisksV2.java for this?
Sample runs:
Enter n: 4
*
***
*****
*******
© CS1101 (AY2009-2010 Semester 1)
Enter n: 7
*
***
*****
*******
*********
***********
*************
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Take-home lab assignment #3
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It has been released. Deadline is the Monday
when school reopens after the recess, 28
September 2009, 23:59hr.
Any questions?
© CS1101 (AY2009-2010 Semester 1)
Week6 - 13
Programming Errors
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Compilation errors
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Runtime errors
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Occur at runtime.
Java’s exception mechanism can catch such errors.
Logic errors
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Syntax error (example: missing a semi-colon).
Semantic error. (For example, applying modulus % on
floating-point value for certain programming languages. In
Java ,is it fine? Yes!)
Easiest type of errors to fix.
Program runs but produces incorrect result.
Hard to characterize, hence hardest to fix.
Programming errors are also known as bugs
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Origin: a moth in the Mark I computer.
© CS1101 (AY2009-2010 Semester 1)
Week6 - 14
Testing and Debugging (1/5)
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Testing
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Debugging
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To determine if a code contains errors.
To locate the error and fix it.
Documentation
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To improve maintainability of the code.
Include sensible comments, good coding style and clear logic.
Testing
Error?
Yes
Debug
© CS1101 (AY2009-2010 Semester 1)
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Testing and Debugging (2/5)
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Modularization and interfaces
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Problem is broken into sub-problems and each sub-problem
is tackled separately – divide-and-conquer.
Such a process is called modularization.
The modules are possibly implemented by different
programmers, hence the need for well-defined interfaces.
The signature of a method (its return type, name and
parameter list) constitutes the interface. The body of the
method (implementation) is hidden – abstraction.
Good documentation (example: comment to describe what
the method does) aids in understanding.
static double max(double a, double b)
Returns the greater of two double values.
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Testing and Debugging (3/5)
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Manual walkthroughs
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Tracing with pencil-and-paper.
Verbal walkthroughs
Print statements
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Easy to add
Provide information:
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Which methods have been called
The value of parameters
The order in which methods have been called
The values of local variables and fields at strategic points
Disadvantages
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Not practical to add print statements in every method
Too many print statements lead to information overload
Removal of print statements tedious
© CS1101 (AY2009-2010 Semester 1)
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Testing and Debugging (4/5)
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Example on writing print statements
System.out.print("Enter 6-digit Matriculation number: ");
int number = scanner.nextInt();
// Extract
int digit6
int digit5
int digit4
int digit3
int digit2
the digits
= number%10;
= number%10;
= number%10;
= number%10;
= number%10;
number
number
number
number
/=
/=
/=
/=
10;
10;
10;
10;
int step1 = digit2*2 + digit3*6 + digit4*2 + digit5*4 + digit6;
System.out.println("Step 1 = " + step1); // for checking
int step2 = step1 % 13;
System.out.println("Step 2 = " + step2); // for checking
int step3 = 13 - step2;
System.out.println("Step 3 = " + step3); // for checking
:
:
© CS1101 (AY2009-2010 Semester 1)
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Testing and Debugging (5/5)
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Tips and techniques
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Start off with a working algorithm
Incremental coding/test early/fix bugs as you find them
Simplify the problem
Explain the bug to someone else
Recognize common errors (such as using ‘=’ instead of ‘==’,
using ‘==’ instead of equals( ), infinite loop, etc.)
Recompile everything
Test boundaries
Test exceptional conditions
Take a break
© CS1101 (AY2009-2010 Semester 1)
Week6 - 19
Testing Thoroughly (1/3)
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Test your programs with your own data
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Do not rely on CourseMarker to test your programs!
We discussed this in week 4. Richard couldn’t spot the
error in this code (FindMaxV2.java) of his:
// To find the maximum among 3 integer
// values in variables num1, num2, num3.
int max = 0;
if (num1 > num2 && num1 > num3)
max = num1;
if (num2 > num1 && num2 > num3)
max = num2;
if (num3 > num1 && num3 > num2)
max = num3;
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He claimed that he tested it on many sets of data: <3,5,9>,
<12,1,6>, <2,7,4>, etc. and the program works for all these.
What did he miss out in his testing?
© CS1101 (AY2009-2010 Semester 1)
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Testing Thoroughly (2/3)
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In testing your programs thoroughly, do not forget
about boundary or special cases!
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These are the cases where the program may give the wrong
answer – a common error
In the Primality Test problem (checking if an integer is
a prime), what are the boundary cases?
© CS1101 (AY2009-2010 Semester 1)
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Testing Thoroughly (3/3)
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It is also important to test all the paths that your
program control flow can take
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Design test data to check all paths
Example
Test data:
<x=0, z=1> to test path
A, B, G, H;
<x=3, z=3> to test path
E, F, C, D;
etc.
© CS1101 (AY2009-2010 Semester 1)
if (x != 3) {
y = 5;
}
else {
z = z - x;
}
if (z > 1) {
z = z / x;
}
else {
z = 0;
}
if (x != 3)
A
E
y=5
z=z-x
B
F
if (z > 1)
C
z=z/x
D
G
z=0
H
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Debugger
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Debugger usually provides the following
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Stepping
Breakpoint
Watches (inspecting variables)
We will illustrate these on DrJava
© CS1101 (AY2009-2010 Semester 1)
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Other readings
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You will learn other advance testing techniques when
you do Software Engineering.
Some websites
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http://www.cs.wustl.edu/~kjg/CS101_SP97/Notes/Debugging/debug
ging.html
 http://www.csl.mtu.edu/cs2321/www/SELectures/SELeccture3_Deb
ugingAndTesting.htm
Program testing can be used to
show the presence of bugs, but
never to show their absence.
~ Edgar Dijkstra
© CS1101 (AY2009-2010 Semester 1)
Week6 - 24
MasterMind (1/3)
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Let’s play the game!
© CS1101 (AY2009-2010 Semester 1)
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MasterMind (2/3)
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You will be given the following on the module website
(“Resources” – “Lectures”). Look for “Week 6”, “MasterMind”.
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Peg.java
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MMCode.java
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This is the GUI interface. You do not need the source code, but if you
have problem with this class file, you may request for the source code
so that you can compile it.
.gif files
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You are to complete the countSinks(MMCode) and
countHits(MMCode) methods.
You should not change the other methods in the program.
MasterMindGUI.class
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You are not to modify this file.
These are the gif files you need for the GUI interface.
MasterMind.java
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This is the application program (with the main method). You are to
complete this program.
© CS1101 (AY2009-2010 Semester 1)
Week6 - 26
MasterMind (3/3)
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Treat this as your holiday exercise!
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This is actually a past year’s lab exercise. You may see the
write-up on the module website, “CA – Labs”, under “Old
Labs (AY2007/8 Semester 1)”, “Lab #4: Selection and
Repetition II”.
If you do not want the GUI features, just remove
those statements in MasterMind.java which are
suffixed with the comment “– GUI enhancement”.
Note that the programs may contain some Java
syntax/features not yet covered:
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‘this’, overloaded constructors, etc. These will be explained
in the next lecture when we cover Chapter 7.
‘char’ data type which is covered in Chapter 9. You may
read up on your own.
© CS1101 (AY2009-2010 Semester 1)
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Summary for Today
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Writing modular programs
How to test your programs
Using the debugger
© CS1101 (AY2009-2010 Semester 1)
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Announcements/Things-to-do (1/2)
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Complete the following
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MasterMind
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Take-home lab #3 deadline: 28 September
2009, Monday, 23:59hr.
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Next week is recess
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Revise and prepare for mid-term test
Mid-term test on 3 October 2009, Saturday
To prepare for next lecture
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Read Chapter 7 and its PowerPoint file before
you come for lecture.
© CS1101 (AY2009-2010 Semester 1)
Week6 - 29
Announcements/Things-to-do (2/2)
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Sit-in Lab #1
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To be conducted during this week’s discussion session.
Please be punctual.
Make-up lab will only be allowed if you miss the lab with valid
reason and proof.
Topics tested include everything you have learned.
Sit-in lab #1 constitute 5% of your final grade.
Happy
Recess!
© CS1101 (AY2009-2010 Semester 1)
Week6 - 30
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© CS1101 (AY2009-2010 Semester 1)
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