CS 1101 Programming Methodology http www comp nus

CS 1101: Programming Methodology http: //www. comp. nus. edu. sg/~cs 1101 x/ 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’ § Testing and Debugging 2

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

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 4

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 1 2 3 4 5 6 7 8 9 10 11 A B C D E F G H I Z J § Eg: The check code corresponding to 6 is ‘F’. § Therefore, the check code for 8730215 is ‘F’. 5

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’, ‘flag’. § Use ‘is. Valid’, ‘is. Odd’, ‘to. Continue’ 6

Using real numbers (1/2) § Arithmetic operations of real numbers may yield inaccurate results. § Download Real. Numbers. java double real. Num = 0. 1; double sum = 0. 0; for (int i=1; i<=10; i++) sum += real. Num; 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"); 7

Using real numbers (2/2) § Accept some inaccurancy with tolerance. final double EPSILON = 0. 0000001; double real. Num = 0. 1; double sum = 0. 0; for (int i=1; i<=10; i++) sum += real. Num; 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"); 8

Exercise #1 (Simple loop) § Write a program Odd. Integers. 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! 9

Exercise #2 (Nested loops) § Download the programs Loops. Ex 1. java, Loops. Ex 2. java and Loops. Ex 3. 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. 10

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 Bad. GCD. java. § Can you improve on this program? § See next slide for the Euclidean algorithm that computes GCD. 11

Euclidean algorithm n 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. 12

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

Coin Change n n 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 14

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

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

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 17

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