CS1101: Programming Methodology

Week 8: Strings and Arrays  Previous lecture:  Chapter 7: Defining Your Own Classes – Part 2  This week:  Chapter 9: Characters and Strings  Chapter 10: Arrays and Collections  Next week:  Chapter 10: Arrays and Collections (cont’d)  Chapter 8: Exceptions © CS1101 (AY Semester 1)Week8 - 2

Chapter 9 Characters and Strings Let’ go over Thomas Wu’s slides now… © CS1101 (AY Semester 1)Week8 - 3

Quizzes Are 'A' and "A" the same thing? Why? Can char be used in a switch statement? How about a String object? Show the results, or if it is illegal, how to correct it. (ASCII value of 'a' is 97.) 1. System.out.println('a' + "xyz"); 2. System.out.println('a' + 123); 3. System.out.println('a' + 'b'); 4. if ("a" < "b") if ("abc" < "xx")... © CS1101 (AY Semester 1)Week

Do not overuse String Some students like to use String to solve numerical problems, because there are so many methods available for String.  For example, given an integer, extract its individual digits.  No good: convert the integer to a string, use substring to extract the individual characters, then convert each character to a digit.  Better: use division (/) and modulo (%), like what we did for Matriculation check code. Manipulating strings can use up a lot of space, due to its immutability. Strings are special. There are two ways to create a String object: with or without using ‘new’ statement. © CS1101 (AY Semester 1)Week8 - 5

Two ways of creating String objects © CS1101 (AY Semester 1)Week8 - 6 We can do this because String objects are immutable.

String: Effect of Immutability Example 1: String s = "abc"; s = s + "def";  A new string "abcdef" is created for s in the second statement. (Hence, it uses more memory. What happens to the memory space that stores "abc"?) Example 2: String s1 = "abc"; String s2 = "abc"; s1 = "xyz";  String s2 still refers to "abc". (Imagine the horror if this is not so!) Hence, String objects behave differently from other objects or arrays. © CS1101 (AY Semester 1)Week8 - 7

Exercise: Counting “the” (1/2) Write a program Ch9CountThe.java to count the number of times user entered the word “the”. The program ends when the user enters an empty string. The word “the” may be entered with leading or/and trailing spaces, and it may contain uppercase or lowercase letter. (This question is the same as question 1 in Week 9’s discussion.) © CS1101 (AY Semester 1)Week8 - 8

Exercise: Counting “the” (2/2) A sample run:  User’s inputs are shown in blue  Space is denoted by the character © CS1101 (AY Semester 1)Week8 - 9 Enter a string: tHe Enter a string: quick Enter a string: Enter a string: THE Enter a string: thE Enter a string: Number of times "the" appears: 3 Look up the String API page for appropriate String methods to use.String API page

Take-home lab assignment #4 It has been released more than a week ago. Deadline is Monday, 12 October 2009, 23:59hr. Any questions? © CS1101 (AY Semester 1)Week8 - 10

Motivation (1/3) Task: Find the minimum value of 3 integers int min; if ( (value1 <= value2) && (value1 <= value3) ) min = value1; else if ( (value2 <= value1) && (value2 <= value3) ) min = value2; else if ( (value3 <= value1) && (value3 <= value2) ) min = value3; What if the list is big?  Code will be longer – the logical expression in each ‘if’ statement will be longer and more complex. © CS1101 (AY Semester 1)Week8 - 11

Motivation (2/3) Consider this algorithm int min = value1; if (value2 < min) min = value2; if (value3 < min) min = value3; What if the list is big?  Code will still be longer, but if we make use of the regular pattern and the array structure for the data, we can ‘roll’ the if statements into a loop. © CS1101 (AY Semester 1)Week8 - 12

Motivation (3/3) Using the array structure int min = value[0]; if (value[1] < min) min = value[1]; if (value[2] < min) min = value[2]; Rolling into a loop using array subscript/index: int min = value[0]; for (int k = 1; k < 3; k++) { if (value[k] < min) min = value[k]; } The above code can be easily modified for a bigger list. © CS1101 (AY Semester 1)Week8 - 13

Chapter 10 Arrays and Collections (1/2) In computer science, we deal with very large amount of data  Eg: 3000 integers, 265 days, 1 million real numbers.  Do you want to create so many variables in your program? If the data are homogeneous (of the same type), we can organise them into a single collection (we normally call it a list). Array is an implementation of a list. It is an indexed collection of homogeneous data. © CS1101 (AY Semester 1)Week8 - 14

Chapter 10 Arrays and Collections (2/2) We will cover up to one-dimensional arrays today, and continue from two-dimensional arrays next week. The syntax for array is very simple, yet learning it allows us to solve many interesting problems. Let’ go over Thomas Wu’s slides now… © CS1101 (AY Semester 1)Week8 - 15

Array Declaration Syntax  Array declaration syntax: [] ; Example: double[] values;  Alternative syntax: []; 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. © CS1101 (AY Semester 1)Week8 - 16

Basic Terminology  An array is a form of ordered list.  A list is composed of elements.  List is homogeneous – elements are of the same base type.  Elements in a list have a common name.  The list as a whole is referenced through the common name.  Elements are referenced by subscripting (indexing) the common name. © CS1101 (AY Semester 1)Week8 - 17

Java Array Features  Subscripts/indices are denoted as expressions within square brackets [ ]  Size of array (number of elements) can be specified at run time  Index type is integer and index range must be 0 … n- 1 where n is the array size  Automatic bounds checking  Array is an object  Has features common to all other objects © CS1101 (AY Semester 1)Week8 - 18

Common mistake: length() vs length Do not mix up the two  The length of a String object str is obtained by calling the String method length()  Example: str.length()  The length (size) of an array arr is stored in its data member length  Example: arr.length © CS1101 (AY Semester 1)Week8 - 19

Classic Array Problems  Compute statistics (sum, mean, standard deviation, etc.) of the values in an numeric array.  Find the maximum (or minimum) value in an array.  Search for a value in an array.  Sort the values in an array.  Others © CS1101 (AY Semester 1)Week8 - 20

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 };  Slide 9 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 16 of Chapter 10  (If the array is large, the amount of data will be huge. We may need a file to store the data. We will learn how to read data from a file some other time.) © CS1101 (AY Semester 1)Week8 - 21

Exercise 1: Summing an Array  Write a program SumArray.java to compute the sum of all the values in an array containing type 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.) © CS1101 (AY Semester 1)Week Size of array: 10 Enter 10 values: The sum is  Download SumArray.java from course website, “Resources – Lectures” page.

Exercise 2: Finding Maximum  Write a program FindMax.java to find the largest value in an integer array. (Assume there is at least one element in the array.) © CS1101 (AY Semester 1)Week Size of array: 5 Enter 5 values: 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: 5 Enter 5 values: The largest value is at index 3

Very Common Mistake #1 Array Index Out of Range  Beware of ArrayIndexOutOfBoundsException. © CS1101 (AY Semester 1)Week public static void main(String[] args) { int numbers = new int[10];... for (int i=1; i<=numbers.length; i++) System.out.println(numbers[i]); }

Modular Programming Revisit “Modularize” SumArray.java and FindMax.java so that the computation of sum or maximum is performed in a method. Call your new programs SumArrayModular.java and FindMaxModular.java. © CS1101 (AY Semester 1)Week8 - 25

Exercise 3: Coin Change Download the file Ch3CoinChange.java from the course website, “Resources – Lectures”. Rewrite it using an array of coin denominations ( int[] coins ). Name your program Ch10CoinChange.java. Modularize your program by writing a method computeCoins().  What is its return type?  Does it have any argument(s)? If so, what is/are the type(s) of its argument(s)? © CS1101 (AY Semester 1)Week8 - 26

Exercise 4: Array of Points (Take-home) (1/2) Write a program Ch10PointArray.java to create an array of Point objects, and find out which point in the array is furthest from the origin. If there are more than one such furthest point, output the smallest index among them. You may assume that the array contains at least one point. © CS1101 (AY Semester 1)Week8 - 27

Exercise 4: Array of Points (Take-home) (2/2) Sample output: © CS1101 (AY Semester 1)Week Enter number of points: 5 Enter coordinates for point #0: Enter coordinates for point #1: 1 4 Enter coordinates for point #2: 4 -3 Enter coordinates for point #3: 0 5 Enter coordinates for point #4: -1 1 The furthest point is at index 2

Very Common Mistake #2 (1/2) When you have an array of objects, it’s very common to forget to instantiate the array’s objects. Programmers often instantiate the array itself and then think they’re done – that leads to java.lang.NullPointerException. © CS1101 (AY Semester 1)Week8 - 29

Very Common Mistake #2 (2/2) © CS1101 (AY Semester 1)Week Point[] array = new Point[3]; for (int i=0; i<array.length; i++) { array[i].setLocation(1,2); } array null There are no objects referred to by array[0], array[1], and array[2]! Point[] array = new Point[3]; for (int i=0; i<array.length; i++) { array[i] = new Point(); array[i].setLocation(1,2); } Corrected code: array 0 x 0 y 0 x 0 y x 0 0 y 2 1

Method main() (1/2) Now that we have learned array, let’s check out the main() method. Usual header for main() method: public static void main(String[] args)  args is an array of String objects. Download Ch10MainDemo.java: © CS1101 (AY Semester 1)Week public class Ch10MainDemo { public static void main(String[] args) { for (int i=0; i<args.length; i++) System.out.println("args[" + i + "]: " + args[i]); }

Method main() (2/2) This allows user to specify command line arguments when executing the program. java Ch10MainDemo 10 ABC-D hello "Ice cream" Output: © CS1101 (AY Semester 1)Week args[0]: 10 args[1]: ABC-D args[2]: hello args[3]: Ice cream

Summary for Today Characters and Strings  More String methods  Patterns (regular expressions) Arrays  Syntax of one-dimensional array  Creating and using an array  Passing array into a method © CS1101 (AY Semester 1)Week8 - 33

Announcements/Things-to-do (1/2) Complete the following  Ch10PointArray.java Take-home lab #4 Deadline: 12 October 2009, Monday, 23:59hr To prepare for next lecture  We will continue with 2-dimensional arrays and ArrayList next week.  Read Chapters 10 and 8 and their PowerPoint files before you come for lecture.  We will skip Map for Chapter 10, and Assertions for Chapter 8. © CS1101 (AY Semester 1)Week8 - 34

Announcements/Things-to-do (2/2) Sit-in Lab #2  To be conducted during this week’s discussion session.  Please be punctual.  Make-up lab will only be allowed if you miss the lab with valid reason and proof.  Topics tested include everything up to Chapter 7.  Sit-in lab #2 constitute 5% of your final grade. © CS1101 (AY Semester 1)Week8 - 35

End of File © CS1101 (AY Semester 1)Week8 - 36