1. CS1101: Programming Methodology http://www.comp.nus.edu.sg/~cs1101x/ http://www.comp.nus.edu.sg/~cs1101x/ Aaron Tan

2. 2 This is Week 4  Last week:  Selection constructs  First part of Chapter 4 Control Statements  ‘if’, ‘switch’, logical operators  This week:  Repetition constructs  ‘while’, ‘do … while’, ‘for’  Testing and Debugging

3. 3 Testing and Debugging  Show me your CheckNRIC program!  Next two slides show the question.  How did you test your program?  How did you debug your program?

4. 4 Last week’s Exercise #4  Algorithm for NRIC check code  NRIC consists of 7 digits.  Eg: 8730215  Step 1: Multiply the digits with corresponding weights 2,7,6,5,4,3,2 and add them up.  Eg: 8  2 + 7  7 + 3  6 + 0  5 + 2  4 + 1  3 + 5  2 = 16+49+18+0+8+3+10 = 104  Step 2: Divide step 1 result by 11 to obtain the remainder.  Eg: 104 % 11 = 5

5. 5 Last week’s Exercise #4  Algorithm for NRIC check code (cont…)  Step 3: Subtract step 2 result from 11  Eg: 11 – 5 = 6  Step 4: Match step 3 result in this table for the check code  Eg: The check code corresponding to 6 is ‘F’.  Therefore, the check code for 8730215 is ‘F’. 1234567891011 ABCDEFGHIZJ

6. 6 Writing good programs  Selection and repetition statements are easy to learn.  But it may not be easy to write good programs with them.  Logic should be clear  Boolean variables should be descriptive  Don’t use ‘b’, ‘f’, ‘flag’.  Use ‘isValid’, ‘isOdd’, ‘toContinue’

7. 7 Using real numbers (1/2)  Arithmetic operations of real numbers may yield inaccurate results.  Download RealNumbers.java double realNum = 0.1; double sum = 0.0; for (int i=1; i<=10; i++) sum += realNum; System.out.println("sum = " + sum); if (sum == 1.0) System.out.println("sum is 1.0"); else System.out.println("sum is not 1.0");

8. 8 Using real numbers (2/2)  Accept some inaccurancy with tolerance. final double EPSILON = 0.0000001; double realNum = 0.1; double sum = 0.0; for (int i=1; i<=10; i++) sum += realNum; System.out.println("sum = " + sum); if (Math.abs(sum - 1.0) < EPSILON) System.out.println("sum is 1.0"); else System.out.println("sum is not 1.0");

9. 9 Exercise #1 (Simple loop)  Write a program OddIntegers.java to print odd integers between 1 and 39 inclusive.  Output: 1 3 5 7 9 11 13 15 17 19 … 37 39  Include 3 versions in your program: using ‘while’ loop, ‘do … while’ loop, and ‘for’ loop.  The ‘for’ loop version is already given in the book!

10. 10 Exercise #2 (Nested loops)  Download the programs LoopsEx1.java, LoopsEx2.java and LoopsEx3.java from the Lectures page in the course website.  Hand trace the programs and write out the outputs without running them.  Verify your answers by running the programs.

11. 11 Exercise #3 (GCD)  The GCD (greatest common divisor) of two non-negative integers a and b, not both zero, is the largest integer that divides both numbers a and b.  Examples: GCD(0,12) = 12; GCD(3,8) = 1; GCD(18,12) = 6; GCD(100,300) = 100.  Download BadGCD.java.  Can you improve on this program?  See next slide for the Euclidean algorithm that computes GCD.

12. 12 Euclidean algorithm First documented algorithm by Greek mathematician Euclid in 300 B.C.  To compute the GCD (greatest common divisor) of two integers. 1.Let A and B be integers with A > B ≥ 0. 2.If B = 0, then the GCD is A and algorithm ends. 3.Otherwise, find q and r such that A = q.B + r where 0 ≤ r < B Note that we have 0 ≤ r < B < A and GCD(A,B) = GCD(B,r). Replace A by B, and B by r. Go to step 2.

13. 13 Exercise #4 (Coin Change)  Write a program CoinChange.java to implement the Coin Change problem we discussed before.  See next slide for problem statement.  Do not use array yet.

14. 14 Coin Change Given this list of coin denominations: $1, 50 cents, 20 cents, 10 cents, 5 cents, 1 cent, find the smallest number of coins needed for a given amount. You do not need to list out what coins are used.  Example 1: For $3.75, 6 coins are needed.  Example 2: For $5.43, 10 coins are needed. You may assume that the input value is in cents.  Enter amount in cents: 375 Number of coins: 6

15. 15 Exercise #5 (Prime Number; Take-home) Primality test is a classic programming problem. Given a positive integer, determine whether it is a prime number or not. A prime number has two distinct factors (divisors): 1 and itself. Examples: 2, 3, 5, 7, 11, … Write a program PrimeTest.java. Some sample runs shown below:  Enter integer: 131 131 is a prime.  Enter integer: 713 713 is not a prime. Bring your program to class next week. We will discuss it.

16. 16 Announcement  Lab #1  Release: 2 September (Tuesday), 2359hr.  Deadline: 10 September (Wednesday), 2359hr.

17. 17 This is Week 4  Next week?  A mini programming test! (argh!)  Chapter 5: Using Pre-Built Methods  The API library  Math class  Wrapper classes  Character class  String methods  Random numbers

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