Course overview: Introduction to programming concepts

Transcription

1 Course overview: Introduction to programming concepts What is a program? The Java programming language First steps in programming Program statements and data Designing programs. This part will give an introduction to general programming concepts, and to the Java programming language. 1

2 What is a program? A program is a set of instructions which control a computer (laptop, desktop, tablet, etc) Programs can be written for huge variety of tasks: performing complex computations; financial trading; computer graphics and games; medical diagnosis and data processing; aircraft autopilot, etc Programs can read data from computer keyboard, mouse movements and actions, from data files, databases and any other sensors/data sources on the computer and from internet if connected. Programs can present information graphically on computer screen, can write to text files, or update any other device connected to computer if permitted to do so. 2

3 What is a program? Example of simple program: read a number X; read a number Y; calculate Z = (X + Y) divided by 2; display Z; This computes average of two numbers, eg: for X = 203, Y = 1965, displays

4 What is a program? Another name for programs is software as opposed to hardware, the physical computer and devices. Programs are written in text files in the format of some programming language The most widely-used programming languages are C, C++, C# and Java. We ll use Java, as it s simplest of the popular languages. Java was invented in 1995, originally for web browsers and internet software (eg., to animate web pages). Now in general use. 4

5 The Java programming language We write Java programs in text files (eg., using WordPad or JCreator), with.java file extension, eg.: Program1.java public class Program1 { public static void main(string[] args) { int X = 203; int Y = 1965; int Z = (X + Y)/2; System.out.println("The answer is: " + Z); Here, three integer-valued variables X, Y, Z are declared. X and Y are given values 203 and 1965, then Z is calculated from them, and then displayed. 5

6 Running Program1.java 6

7 First steps in programming Open the Windows console (Start; All Programs; Accessories; Command Prompt) In the Windows console, cd to the directory cllexamples where Program1.java is (on memory stick) Compile the program with javac, and run it with java: javac Program1.java java Program1 Alternatively, open Program1.java with JCreator, or use 7

8 Data in computer 8

9 First steps in programming A Java program in a file Name.java can have the text public class Name { public static void main(string[] args) { // Your program code goes here The text between the inner { brackets can change for different programs. Give programs meaningful names: Average.java would be better name for our first program. Program statements: individual instructions and steps the computer should take. 9

10 Eg.: int X = 203; Introduce an integer variable called X, and assign the value 203 to it. int Z = (X + Y)/2; Introduce an integer variable called Z, and assign (X + Y) divided by 2 to it. Statements end with a ; Try changing the values assigned to X, Y, re-compile and re-run. 10

11 First steps in programming Of course, a more useful program is one that can read inputs from user: import javax.swing.joptionpane; // dialog support public class Average { public static void main(string[] args) { String xvalue = JOptionPane.showInputDialog( "Enter 1st integer value:"); int X = Integer.parseInt(xvalue); String yvalue = JOptionPane.showInputDialog( "Enter 2nd integer value:"); int Y = Integer.parseInt(yvalue); 11

12 int Z = (X + Y)/2; System.out.println("Their average is: " + Z); The dialogs prompt user for the X, Y values. Input from user is stored in String variables xvalue, yvalue these store pieces of text. Then converted to numbers by Integer.parseInt. Eg., string 334 is converted to number

13 Average.java input dialog 13

14 First steps in programming If we enter some wrong data (not integers), our program crashes a professional program should be robust & continue despite errors Dialogs can be used to show results: JOptionPane.showMessageDialog(null, "The result is: " + Z); The User Interface of a program consists of the visual components used to exchange data between program and users in this case the input/output dialogs. Also called Graphical User Interface (GUI). 14

15 First steps in programming Alternatively, read values from console: import java.util.scanner; // input support class AverageConsole { public static void main(string[] args) { Scanner input = new Scanner(System.in); System.out.println("Enter 1st integer value:"); int X = input.nextint(); System.out.println("Enter 2nd integer value:"); int Y = input.nextint(); int Z = (X + Y)/2; System.out.println("Their average is: " + Z); 15

16 First steps in programming Average2.java: import javax.swing.joptionpane; // dialog support public class Average2 { public static void main(string[] args) { String xvalue = JOptionPane.showInputDialog( "Enter 1st integer value:"); int X = Integer.parseInt(xvalue); String yvalue = JOptionPane.showInputDialog( "Enter 2nd integer value:"); int Y = Integer.parseInt(yvalue); int Z = (X + Y)/2; 16

17 JOptionPane.showMessageDialog(null, "Their average is: " + Z); Try changing program to calculate average of three input values. 17

18 Output from Average2.java 18

19 Program Statements Programs consist of program statements individual processing instructions, operating on variables Assignment statements assign a value to a variable: X = 530; Can also declare/introduce the variable: int X = 530; Conditional statements enable decisions to be made: if a condition is true, one behaviour is executed; if condition is false another behaviour is executed. 19

20 Max.java: import javax.swing.joptionpane; // dialog support public class Max { public static void main(string[] args) { String xvalue = JOptionPane.showInputDialog( "Enter 1st integer value:"); int X = Integer.parseInt(xvalue); String yvalue = JOptionPane.showInputDialog( "Enter 2nd integer value:"); int Y = Integer.parseInt(yvalue); int Z; if (X < Y) 20

21 { Z = Y; else { Z = X; JOptionPane.showMessageDialog(null, "The largest value is: " + Z); 21

22 The statement if (X < Y) { Z = Y; else { Z = X; tests the condition X < Y, ( is X less than Y? ), if this is true then the statement Z = Y; is executed, otherwise (if X equals Y or is larger than Y), the statement Z = X is executed. Effect is to always set Z to be the larger of X and Y. 22

23 Console version MaxConsole.java: import java.util.scanner; // input support public class MaxConsole { public static void main(string[] args) { Scanner input = new Scanner(System.in); System.out.println("Enter 1st integer value:"); int X = input.nextint(); System.out.println("Enter 2nd integer value:"); int Y = input.nextint(); int Z; if (X < Y) { Z = Y; else { Z = X; 23

24 System.out.println("The largest value is: " + Z); 24

25 Conditions can be as complex as needed, and multiple conditions can be tested: import javax.swing.joptionpane; // dialog support public class StudentMarks { public static void main(string[] args) { String markvalue = JOptionPane.showInputDialog( "Enter the student s mark:"); int mark = Integer.parseInt(markvalue); String result; if (mark < 40) { result = "Fail"; else if (mark >= 40 && mark < 50) { result = "Pass -- grade D"; 25

26 else if (mark >= 50 && mark < 60) { result = "Pass -- grade C"; else if (mark >= 60 && mark < 70) { result = "Pass -- grade B"; else { result = "Pass -- grade A"; JOptionPane.showMessageDialog(null, "The student s result is: " + result); The symbols && mean and. result is a String variable it stores a piece of text. 26

27 Conditional Statements Examples of different cases: Input Output 10 Fail 42 Pass grade D 55 Pass grade C 67 Pass grade B 70 Pass grade A Try executing StudentMarks with other example input values. 27

28 Loop Statements Loop statements repeat some instructions over & over again until a task is completed for statements loop for a fixed number of times: for (int i = a; i <= b; i++) { statements means execute the statements with i = a, then with i = a+1, i = a+2,..., then with i = b If a > b, does nothing If a equals b, just one iteration, for x = a. 28

29 Loop Statements import javax.swing.joptionpane; // dialog support public class SumNumbers { public static void main(string[] args) { String nvalue = JOptionPane.showInputDialog( "Enter the number to sum:"); int n = Integer.parseInt(nvalue); int sum = 0; for (int i = 1; i <= n; i++) { sum = sum + i; JOptionPane.showMessageDialog(null, "The sum is: " + sum); 29

30 calculates sum of numbers from 1 to n. Results for different input values: 30

31 n sum

32 Graphics/drawing example: import javax.swing.jframe; import java.awt.graphics; import java.awt.color; import javax.swing.jpanel; // drawing support public class Drawings extends JPanel { public void paintcomponent(graphics g) { super.paintcomponent(g); for (int i = 0; i < 10; i++) { if (i % 2 == 0) { g.setcolor(color.pink); g.drawoval(10 + i*10, 10 + i*10, 50 + i*10, 50 + i*10); else 32

33 { g.setcolor(color.green); g.drawrect(10 + i*10, 10 + i*10, 50 + i*10, 50 + i*10); public static void main(string[] args) { JFrame app = new JFrame(); app.add(new Drawings()); app.setsize(300, 300); app.setvisible(true); drawoval(x, y, X, Y ) draws an oval at x, y position, of size X, Y. 33

34 Output from Drawings.java 34

35 Drawing example Notice that y=0 is at top: y increases from top to bottom, x increases from left to right. The condition if (i % 2 == 0) is true if i is an even integer (0, 2, 4, 8) in this case a pink oval is drawn at position (10 + i 10, 10 + i 10). Condition is false if i is odd (1, 3, 5, 7, 9) in this case a green rectangle is drawn at (10 + i 10, 10 + i 10). Try changing the positions & sizes to see the effect. 35

36 Graphics/drawing example: import javax.swing.jframe; import java.awt.graphics; import java.awt.color; import javax.swing.jpanel; // drawing support public class Drawings2 extends JPanel { public void paintcomponent(graphics g) { super.paintcomponent(g); for (int i = 0; i < 10; i++) { if (i % 2 == 0) { g.setcolor(color.pink); g.filloval(10 + i*10, 10 + i*10, 50 + i*10, 50 + i*10); else 36

37 { g.setcolor(color.green); g.fillrect(10 + i*10, 10 + i*10, 50 + i*10, 50 + i*10); public static void main(string[] args) { JFrame app = new JFrame(); app.add(new Drawings2()); app.setsize(300, 300); app.setvisible(true); filloval(x, y, X, Y ) draws a filled oval at x, y position, with size X, Y. 37

38 Output from Drawings2.java 38

39 Drawing spirals: import javax.swing.jframe; import java.awt.graphics; import java.awt.color; import javax.swing.jpanel; // drawing support public class Drawings3 extends JPanel { public void paintcomponent(graphics g) { super.paintcomponent(g); int centerx = getwidth()/2; int centery = getheight()/2; int radius = 20; int w = 2*radius; int h = 2*radius; g.drawarc(centerx, centery, w, h, 0, 180); int radius2 = 30; 39

40 g.drawarc(centerx - 20, centery - 10, 2*radius2, 2*radius2, 0, -180); int radius3 = 40; g.drawarc(centerx - 20, centery - 20, 2*radius3, 2*radius3, 0, 180); int radius4 = 50; g.drawarc(centerx - 40, centery - 30, 2*radius4, 2*radius4, 0, -180); int radius5 = 50; g.drawarc(centerx - 40, centery - 40, 2*radius5, 2*radius5, 0, 180); public static void main(string[] args) 40

41 { JFrame app = new JFrame(); app.add(new Drawings3()); app.setsize(300, 300); app.setvisible(true); drawarc(x, y, w, h, 0, 180) draws an anticlockwise semicircle within the box from x, y position to x+w, y+h. drawarc(x, y, w, h, 0, 180) draws a clockwise semicircle. 41

42 Output from Drawings3.java 42

43 Summary so far Simple program structure: public class Program {... Variables of integer type (int x;) these can store values of integers, eg.: -214, 0, 1, 55, 1000, , etc. Variables of String type (String s;) these can store strings/text, eg.: Result, Enter a value Assignment statements var = value; Conditional statements if (Condition) { statement1 else { statement2 Bounded loops: for (int var = a; condition; var++) { statement Simple graphics code. 43

44 More advanced Java double variables (rational numbers) Unbounded loops (while statements) What is programming? Multiple operations; recursion Multiple classes in a program Array variables. 44

45 Calculations with rational numbers double variables can store fractional values: 0.5, 2.25, , etc. Java provides functions for square root: Math.sqrt(d), x to power y, etc: Math.pow(x,y) These functions return double values in general but integers can be used as doubles (1.0, -3.0, etc). Example: calculate total amount earned if invest deposit (eg, 100) for n years at interest rate rate (eg. 5%, or 0.05). 45

46 Calculating compound interest deposit grows with interest to: total = deposit (1 + rate) n after n years at rate rate. public class Investment { public static void main(string[] args) { double amount = 100.0; // total money double deposit = 100.0; // original investment double rate = 0.05; // interest rate for (int year = 1; year <= 10; year++) { amount = deposit * Math.pow(1.0 + rate, year); System.out.println( "After " + year + " years, total is: " + amount); 46

47 Amounts after n years: n amount

48 Calculating compound interest How many years does it take for investment to double? Need while loop: public class Investment2 { public static void main(string[] args) { double amount = 100.0; // total money double deposit = 100.0; // original investment double rate = 0.05; // interest rate int year = 1; while (amount < 2*deposit) { amount = deposit * Math.pow(1.0 + rate, year); System.out.println( "After " + year + " years, total is: " + amount); 48

49 year = year + 1; A while (E) { C loop repeats C until E is false but may run forever! Here, termination when year = 15. Try changing the rate and see the effect on the result. 49

50 What is Programming? Given: a problem ( draw a spiral, find how many years it takes to double an investment at 5% interest ) First: idea for solution Second: plan an algorithm to carry out the solution Third: code up algorithm in a programming language (Java) Fourth: test/correct your code. 50

51 What is Programming? Problem to draw a spiral Idea for solution: draw semi-circles of increasing radius, joined end-to-end Algorithm: draw semi-circle 1 with radius 20, centered on panel center X, Y; draw semi-circle 2 with radius 30, displaced to X-20, Y-10, etc. Code in Java: use drawarc to draw the semi-circles Test/correct the code. Algorithms can be expressed in words before coding. 51

52 What is Programming? Problem to find number of years until investment doubles Idea for solution: calculate total amount earned for successive years, stopping when this amount is at least twice the original deposit Algorithm: compute total earned to year by amount = deposit (1.0 + rate) year for year = 1, 2, 3,... stopping when amount 2 deposit Code in Java: use while loop to repeat calculation until the stopping condition is true Test with different rates and deposits. 52

53 Reading data from files Programs can read data from files (usually, text files) Eg.: to read student marks and classify these into grades Idea is that program reads lines of text from file (should be numbers), and converts each to a grade, until end of file (eof) is reached. 53

54 import java.io.*; public class StudentMarksFile { public static void main(string[] args) { File file = new File("marks.txt"); /* default input file */ BufferedReader br; String s = ""; boolean eof = false; try { br = new BufferedReader(new FileReader(file)); catch (FileNotFoundException _e) { System.out.println("File not found: " + file); return; while (!eof) 54

55 { try { s = br.readline(); catch (IOException _ex) { System.out.println("Reading failed."); return; if (s == null) // the file has ended { eof = true; break; int mark = Integer.parseInt(s); String result; if (mark < 0 && mark > 100) { result = "Invalid mark " + mark; else if (mark < 40) { result = "Fail"; else if (mark < 50) 55

56 { result = "Pass -- grade D"; else if (mark < 60) { result = "Pass -- grade C"; else if (mark < 70) { result = "Pass -- grade B"; else { result = "Pass -- grade A"; System.out.println( "The student s result is: " + result); // ends the while loop over file 56

57 Programs with multiple operations/classes So far we ve written all code in main method of one class. This is method where execution starts. Also possible to write several operations in one class, called from main method: factorial example. For larger programs, best to create separate classes + call their code from main method. We ll show how to do this with lottery example. 57

58 Calculating factorials For integer n > 0, its factorial is product: n (n 1) public class Factorial { public static int fact(int n) { if (n <= 1) { return 1; else { return n*fact(n-1); public static void main(string[] args) { for (int n = 1; n <= 14; n++) { System.out.println( "Factorial " + n + "! = " + fact(n)); 58

59 Recursion is another way of looping: fact(5) calls fact(4), which calls fact(3), etc. return e; ends the call and returns value of e to the caller. 59

60 Calculating factorials Notice that factorials for n > 12 don t appear correct: the int type is too small to represent them, need to use long integer type instead: public class Factorial { public static long fact(int n) { if (n <= 1) { return 1; else { return n*fact(n-1); public static void main(string[] args) { for (int n = 1; n <= 21; n++) { System.out.println( "Factorial " + n + "! = " + fact(n)); 60

61 All data on a computer is finite, int variables can hold numbers up to 2,147,483,

62 Programs with multiple classes Problem to Carry out lottery with three numbers in range 1 to 30 for user to guess with five guesses Idea for solution: generate random numbers, ask user to guess these, and count correct guesses Algorithm: Generate 3 random integers in 1..30; Ask user for 5 guesses; Count and display the correct guesses Code in Java: use Random class, and nextint(30) + 1 to generate the random numbers Test with different cases of correct/incorrect guesses. 62

63 import java.util.random; // to generate random numbers import javax.swing.joptionpane; // dialog support public class Lottery { public static void main(string[] args) { Random rand = new Random(); int ball1 = 1 + rand.nextint(30); // first lottery number int ball2 = 1 + rand.nextint(30); // second lottery number int ball3 = 1 + rand.nextint(30); // third lottery number int correct = 0; for (int i = 1; i <= 5; i++) { String sguess = JOptionPane.showInputDialog("Enter your guess:"); int guess = Integer.parseInt(sguess); if (guess == ball1) { System.out.println("Correct guess of ball 1"); correct++; 63

64 else if (guess == ball2) { System.out.println("Correct guess of ball 2"); correct++; else if (guess == ball3) { System.out.println("Correct guess of ball 3"); correct++; else { System.out.println("Sorry, wrong guess!"); System.out.println("You guessed " + correct + " correct"); Random is a built-in class, rand is an instance of Random which we use in our program. 64

65 Random rand = new Random(); declares rand as variable storing an instance of Random, and assigns a new instance of Random to rand. rand.nextint(30) generates a random integer in the range 0 to

66 Array variables So far, we ve seen variables that store single items an individual integer, rational, string, or class instance Sometimes useful to have one variable storing group of items such as the 3 balls in lottery game Array variables store group of items of same kind. Eg.: int[] balls = new int[3]; declares balls as an array of 3 integers. Individual items are written as balls[0], balls[1], balls[2]. 66

67 Data in array variables 67

68 Lottery with array import java.util.random; // to generate random numbers import javax.swing.joptionpane; // dialog support public class Lottery2 { public static void main(string[] args) { Random rand = new Random(); int[] balls = new int[3]; // 3 lottery balls for (int i = 0; i < 3; i++) { balls[i] = 1 + rand.nextint(30); int correct = 0; for (int j = 1; j <= 5; j++) { String sguess = JOptionPane.showInputDialog("Enter your guess:"); int guess = Integer.parseInt(sguess); 68

69 for (int k = 0; k < 3; k++) { if (guess == balls[k]) { System.out.println("Correct guess of ball " + (k+1)); correct++; // end of if // end of inner for-loop // end of outer for-loop System.out.println("You guessed " + correct + " correct"); A loop of form for (int i = 0; i < n; i++) {... code for array[i]... is often used to process array of size n. Notice that numbering starts from 0! Array lottery version has fewer lines of code. Also, easy to change number of balls. 69

70 Summary Program structure, class definitions Variables of integer, rational, string, class instance types Array variables Assignment, conditional, loop statements Simple graphics code. 70

71 Further resources We hope you have enjoyed this course. The following are useful for further study: Java JDK can be downloaded from: oracle.com/technetwork/java/javase/downloads/ Choose the JDK option, this will provide the javac compiler and java execution tool. JCreator is a visual tool for creating and running Java programs, download from jcreator.com Java library information is here: 71