COMP2100/2500
Homework 2

Exercise 1

Continue recording all time you spend on COMP2100/2500. Use a fresh Time Recording Log form each week. Same with the Weekly Time Use Summary. This will be part of the homework every week from now on, even if the specification does not say so explicitly.

Exercise 2

Write the following program, making a note of how long it takes you to complete it, and also of how many non-blank lines of code you end up with.

To remind you of how to follow the Initial PSP, check up the notes Lecture 2 and Lab 1 before you write this program.

Write a Java program (in a class Factors) that repeatedly prompts the user for an integer and then prints out the complete prime factorisation of that integer, with the factors in ascending order.

The program should stop when it reads an integer that is less than or equal to 1. You do not have to do any other data checking. In other words, you may assume that the user will type an integer, and it's OK if your program crashes if they type other stuff.

Here is a sample run of the program. User input is in blue, system output in black. Your final output should be formatted exactly like this:

comp2100@partch java Factors
> 15
15 = 3 * 5
> 180
180 = 2 * 2 * 3 * 3 * 5
> 1999
1999 = 1999
> 2000
2000 = 2 * 2 * 2 * 2 * 5 * 5 * 5
> 2001
2001 = 3 * 23 * 29
> 2002
2002 = 2 * 7 * 11 * 13
> 2003
2003 = 2003
> 0
Goodbye

When you have completed this program, test it on a few numbers, large and small, until you would be prepared to stake your reputation on its correctness. Print out a copy of the source code, and store it securely in your lab notebook (engineering journal).

Suggestion: a better solution will use at least one method other than main().

Hints:

1. It makes things a lot simpler when you realise that if you search for factors in increasing order, you will only ever find prime factors. So you don't have to test if the factors are prime. (Maths students: Prove this!)

2. Reading from the keyboard is much easier in Java 1.5 than it was in earlier versions. Take a look at the documentation for class Scanner, particularly the methods nextInt() and hasNextInt(); or use the BufferedReader, InputStreamReader combination as in Schildt, Java the complete reference 7th edition, to read an input line as a string, with Integer.parseInt. (Which version is more robust to the user providing input in different ways?)

Extension task: Think about the efficiency of your program. Would it be more efficient only to test primes? How could you do that without wasting more running time checking which numbers are primes? (can you precalculate a list of primes? how many should you precalculate? can you extend the list if you need more for a larger input?)
What if you just test odd numbers (since there are no even primes after 2)? Any other ideas?

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Deliverables

To make good progress with learning programming and PSP by practice and you should have in your Engineering Journal:

1. A completed Time Log for the past week.

2. A completed Weekly Time Use Summary for the past week.

3. A printout of your completed program .