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CS1101: Programming Methodology comp.nus.sg/~cs1101x/

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

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CS1101: Programming Methodology comp.nus.sg/~cs1101x/

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

  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. 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. 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. 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’.

  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. 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. 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. 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. 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. 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. Euclidean algorithm • First documented algorithm by Greek mathematician Euclid in 300 B.C. • To compute the GCD (greatest common divisor) of two integers. • Let A and B be integers with A > B ≥ 0. • If B = 0, then the GCD is A and algorithm ends. • 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. 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. 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: 375Number of coins: 6

  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: 131131 is a prime. • Enter integer: 713713 is not a prime. • Bring your program to class next week. We will discuss it.

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

  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

  18. End of file

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