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

Aaron Tan

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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

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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?

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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: 82 + 77 + 36 + 05 + 24 + 13 +

52 = 16+49+18+0+8+3+10 = 104 Step 2: Divide step 1 result by 11 to obtain

the remainder. Eg: 104 % 11 = 5

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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’.

1 2 3 4 5 6 7 8 9 10 11

A B C D E F G H I Z J

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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’

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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");

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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");

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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!

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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.

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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.

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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.

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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.

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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

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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.

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Announcement

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

2359hr.

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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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