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

Aaron Tan

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This is Week 6

Last week: Week 4’s Exercise 5 (Prime number) A mini programming test! Chapter 5: Using Pre-Built Methods Other classes: Random, DecimalFormat

This week: Chapter 10: Arrays

Only sections 10.1 to 10.6 We will cover the other sections some other time.

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Arrays

In computer science, we deal with very large amount of data. Eg: 3000 integers, 365 days, 1 million real numbers. Do you want to create so many variables?

If the data are homogeneous (of the same type), we can group them into a single collection.

Array is an indexed collection of homogeneous data.

Let’s get to Chapter 10 now!

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Array Declaration Syntax

Array declaration syntax:<element-type>[] <array-variable>;

Example: double[] values;

Alternative syntax:<element-type> <array-variable>[];

Example: double values[];

I prefer the first one, it’s more readable and meaningful. The second form is more commonly used by C/C++ programmers.

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Classic Array Problems Sum the values in an array. Find the maximum (or minimum) value in an

array. Search for a value in an array. Sort the values in an array.

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Loading an Array Before we solve a problem involving array, we need to

first load values into the array! If you know the values before-hand, use array element

initialization Eg: int[] numbers = { 3, 7, -12, 8, 7 }; Slides 12 and 14 of Chapter 10

If not, you need to read the values from the user Use a loop to read in the values Slides 6-7, and 15-16 of Chapter 10

We will learn how to read data from a file some other time.

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Exercise 1: Summing an Array Write a program SumArray.java to compute the sum of

all the values in an array containing double values. Display the sum in 3 decimal places.

Let’s do it into 2 phases: load the array with values first, then compute the sum. (Instead of accumulating the sum as we load the array.)

Size of array: 10Enter 10 values: 5.1 16 3.2 1.8 -4 12.3 8 3.3 -2 9.1The sum is 52.800

Download SumArray.java from course website, “Resources”, “Lectures” page.

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Exercise 2: Finding maximum value

Write a program FindMax.java to find the largest value in an integer array. (Assume there is at least one element in the array.)Size of array: 5Enter 5 values: 10 -20 43 79 8 The largest value is 79

Take home exercise: What if you want to report the index of the largest value, instead of the value itself? (This problem is not well-defined! Why?)Size of array: 5Enter 5 values: 10 -20 43 79 8 The largest value is at index 3

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Common Mistake: Index Out of Range

Beware of ArrayIndexOutOfBoundsException.

public static void main(String[] args) {

int numbers = new int[10]; . . .

for (int i=1; i<=numbers.length; i++) System.out.println(numbers[i]);

}

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Modular Programming (1/5) As our problems get more complex, the main() method

might get too long. It is advisable to split the problem into smaller sub-

problems, and to write appropriate methods for the sub-problems.

In general a problem is solved in 3 steps: input computation output.

It is customary to write a separate method to perform the computation step. (If the computation is complex, it should be split further into smaller steps and each step performed by a method.)

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Modular Programming (2/5) Download CheckNRIC.java program which we did

before. Here’s the partial code:

public static void main(String[] args) { Scanner stdIn = new Scanner(System.in);

System.out.print("Enter 7-digit NRIC number: "); int number = stdIn.nextInt();

int digit7, digit6, digit5, digit4, digit3, digit2, digit1, step1, step2, step3; char checkCode; // computation of check code - code omitted ...

System.out.println("Check code = " + checkCode); }

These variables are used in the computation of the check code.

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Modular Programming (3/5) ‘Modularizing’ CheckNRIC.java:

public static void main(String[] args) { Scanner stdIn = new Scanner(System.in);

System.out.print("Enter 7-digit NRIC number: "); int number = stdIn.nextInt();

char checkCode; // computation of check code checkCode = generateCheckCode(number);

System.out.println("Check code = " + checkCode); }

Delegate the job to the method generateCheckCode().

Pass number into the method generateCheckCode().

What are you expecting generateCheckCode() to return? A character.

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Modular Programming (4/5) How does generateCheckCode() method look like?

public static char generateCheckCode(int num) { // Extract digits int digit7 = num%10; num /= 10; int digit6 = ... ... char code = ... ...

return code; }

The method returns a character.

The method expects an integer argument.

The return statement passes the character to the caller.

Download NewCheckNRIC.java and compare it with CheckNRIC.java.

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Modular Programming (5/5) Let’s see how we can “modularize” our

programs for the previous two exercises. I will show you NewSumArray.java and

NewFindMax.java.

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Exercise 3: Coin Change Download the file CoinChange.java from the

course website, “Resources”, “Lectures”. Rewrite it using an array of coin denominations

(int[] coins). Name your program NewCoinChange.java.

Modularize your program by writing a method computeCoins(). What is its return type? Does it have any argument? If so, what is the type

of its argument?

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Method main() (1/2) Now that we have learnt array, let’s check out the

main() method. Usual signature for main() method:

public static void main(String[] args) args is an array of String objects

Consider this:

public class Demo { public static void main(String[] args) {

for (int i=0; i<args.length; i++) System.out.println("args[" + i + "]: " + args[i]);

} // end main} end Demo

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Method main() (2/2) This allows user to specify command line arguments

when executing the program.java Demo 10 ABC-D hello "Ice Cream"

Output:

args[0]: 10args[1]: ABC-Dargs[2]: helloargs[3]: Ice Cream

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Sorting and Searching I will be covering more topics in every lecture

from now on to make up for the lost lecture on 27 October (Deepavali). Sorting Searching

The above two topics are not included in the mid-term test.

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Sorting Classic computer science problem

Sort an array Three basic (but slow) sorting algorithms

Selection sort Bubblesort Insertion sort

Other faster sorting algorithms (covered in CS1102 and other advanced modules) Mergesort Quicksort Heapsort, etc.

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Selection Sort (1/2)1. Find the smallest element

in the list.0 1 2 3 4 5 6 7 8

23 17 5 90 12 44 38 84 77

minfirst

exchange

0 1 2 3 4 5 6 7 8

5 17 23 90 12 44 38 84 77

sorted unsorted

This is the result of one pass.

2. Exchange the element in the first position and the smallest element. Now the smallest element is in the first position.

3. Repeat Step 1 and 2 with the list having one less element (i.e., the smallest element is discarded from further processing).

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Selection Sort (2/2)0 1 2 3 4 5 6 7 8

5 17 23 90 12 44 38 84 77

sorted

5 12 23 90 17 44 38 84 772

5 12 17 90 23 44 38 84 773

5 12 17 23 38 44 77 84 907

5 12 17 23 38 44 77 84 908

1Pass #

Result AFTER one pass is completed.

Result AFTER one pass is completed.

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Bubble Sort (1/2) Algorithm

Assume array is arr

for (int i = arr.length – 1; i > 0; i--) { for (int j = 0; j < i; j++) { if (arr[j] > arr[j+1] swap arr[j] with arr[j+1] }}

Can you write the code?

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Bubble Sort (2/2)0 1 2 3 4 5 6 7 8

23 17 5 90 12 44 38 84 77

17 23 5 90 12 44 38 84 77

17 5 23 90 12 44 38 84 77

17 5 23 12 90 44 38 84 77

17 5 23 12 44 90 38 84 77

17 5 23 12 44 38 90 84 77

17 5 23 12 44 38 84 90 77

17 5 23 12 44 38 84 77 90

exchange

ok

exchange

exchange

exchange

exchange

exchange

exchange

The largest value 90 is at the end of the list.

The largest value 90 is at the end of the list.

Showing the first pass…

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Searching Another classic computer science problem

Search for a value in a list of items

Two algorithms Sequential search (also called linear search) Binary search (applicable for sorted array) – much

faster

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Announcement/Reminder (1/2) Lab #2

Release: 16 September (Tuesday), 2359hr. Deadline: 1 October (Wednesday), 2359hr.

Identical codes Please do not share codes for your lab assignments!

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Announcement/Reminder (2/2) Consultation

24 September (Wednesday), 10am – 12nn. I will be in PL3.

Mid-term test 4 October, Saturday, 12noon, LT15 (for CS1101X

students) Refer to course website for more info:

http://www.comp.nus.edu.sg/~cs1101x/3_ca/termtests.html

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This is Week 6

Next week? Recess! (Hooray!)

The week after next? Chapter 6 Object-Oriented Programming

(finally!) Mid-term test (argh!)

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