CS 101 Computer Programming

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1 CS 101 Computer Programming Statements Review

2 Variables Declare once, use multiple times, support variations Use meaningfull names with appropriate length Case sensitive, e.g., speed, currentcoursecode, carlength Start with a letter, cannot be a keyword (int) Constants, e.g., NUM_RECTANGLES Make use of arithmetic operations (- + / *) int x = 10; int h = 50; GRect r1 = new GRect(x, 10, 50, h); GRect r2 = new GRect(x+10, 20, 50, h); GRect r3 = new GRect(x+20, 30, 50, h); GRect r4 = new GRect(x+30, 40, 50, h); add(r1); add(r2); add(r3); add(r4); CS 101 Özyeğin University 2

3 Order Of Execution public class Squares extends GraphicsProgram { public void run() { int x = 10; GRect r1 = new GRect(x, 10, SIZE, SIZE); GRect r2 = new GRect(x+10, 20, SIZE, SIZE); GRect r3 = new GRect(x+20, 30, SIZE, SIZE); GRect r4 = new GRect(x+30, 40, SIZE, SIZE); add(r1); add(r2); add(r3); add(r4); private static final int SIZE = 50; variable definitions in the class run() CS 101 Özyeğin University

4 Order Of Execution public class Squares extends GraphicsProgram { public void run() { int x = 10; GRect r1 = new GRect(x, 10, SIZE, SIZE); GRect r2 = new GRect(x+10, 20, SIZE, SIZE); GRect r3 = new GRect(x+20, 30, SIZE, SIZE); GRect r4 = new GRect(x+30, 40, SIZE, SIZE); add(r1); add(r2); add(r3); add(r4); private static final int SIZE = 50; variable definitions in the class run() CS 101 Özyeğin University

Order of statements Order of method run and variable declarations does not matter Order of statements within the run method does matter public class Squares extends GraphicsProgram { private static

5 Order of statements Order of method run and variable declarations does not matter Order of statements within the run method does matter public class Squares extends GraphicsProgram { private static final int SIZE = 50; public void run() { int x = 10; GRect r1 = new GRect(x, 10, SIZE, SIZE); GRect r2 = new GRect(x+10, 20, SIZE, SIZE); GRect r3 = new GRect(x+20, 30, SIZE, SIZE); GRect r4 = new GRect(x+30, 40, SIZE, SIZE); add(r1); add(r2); add(r3); add(r4); Özyeğin University CS 101 Computer Programming 5

6 Conditional Selection if(condition) { dosomething; dootherstuff; true condition false dosomething dootherstuff CS 101 Özyeğin University

7 Conditional Selection if(condition) { dosomething; else { dosomethingelse; dootherstuff; true dosomething condition false dosomethingelse dootherstuff CS 101 Özyeğin University

8 Boolean Expressions as Conditions x is 3, y is 5. Expression Value x - y > 0 false (x % 2) == 0 false ((x+1) % 2) == 0 true x + y >= 8 true!(y x > 0) false

9 Example: logical operators if (x == 10 && y == 20) {... if (x == 10 y == 20) {... CS 101 Özyeğin University

10 Common mistakes 0 < x < 5 error You should use 0 < x && x < 5 Use == for comparison, not =. The latter is for assignment. =< and => are syntax errors. Do not use & or instead of && and. & and are bitwise And and Or operators. Do not use == to compare floating point numbers (because float representation is not exact. Use >= and <= instead) CS 101 Özyeğin University

11 Exercise Does the following boolean expression evaluate to true or false? Assume x is 10, y is (x > 0 && x < 100) y > 1000 CS 101 Özyeğin University 11

12 Exercise Does the following boolean expression evaluate to true or false? Assume x is 10, y is (x > 0 && x < 100) y > 1000 true true true false true CS 101 Özyeğin University 12

13 Boolean Variables Datatype for the Boolean values true and false. Common to use with control statements to determine the flow of a program boolean condition = true; boolean isbigger = (x > y);... if(condition && isbigger) {... CS 101 Özyeğin University

14 Exercise What would be the output of the following? boolean draw = false; if (!draw) { GRect rect = new GRect(30, 20, 50, 50); add(rect); draw = true; if (draw) { GLine line1 = new GLine(20, 10, 90, 80); GLine line2 = new GLine(20, 80, 90, 10); add(line1); add(line2); CS 101 Özyeğin University 14

15 Exercise What would be the output of the following? boolean draw = false; if (!draw) { GRect rect = new GRect(30, 20, 50, 50); add(rect); draw = true; if (draw) { GLine line1 = new GLine(20, 10, 90, 80); GLine line2 = new GLine(20, 80, 90, 10); add(line1); add(line2); CS 101 Özyeğin University 15

16 Loops while(condition) { dosomething domorestuff condition true false dosomething domorestuff CS 101 Özyeğin Üniversitesi

17 Example: boolean variables boolean screenborderreached = false; int y = 0; while(!screenborderreached) { GLine line = new GLine(0, y, getwidth(), y); add(line); y = y + 10; if(y > getheight()) { screenborderreached = true; CS 101 Özyeğin University

18 boolean screenborderreached = false; int y = 0; Example while(!screenborderreached) { GLine line = new GLine(0, y, getwidth(), y); if(y > getheight()/4 && y < (getheight()/4)*3) { line.setcolor(color.red); else { line.setcolor(color.green); add(line); y = y + 10; if(y > getheight()) { screenborderreached = true; CS 101 Özyeğin University

19 Nested if s if, if-else statements can be nested within each other if (<condition>) { if(<another condition>) {... else {... if(<yet another condition>) {... CS 101 Özyeğin University

1 Example: equivalent results 2 if(y > getheight()/4 && y < (getheight()/4)*3) { line.setcolor(color.red); else { line.setcolor(color.green); 3 if(y > getheight()/4) { if(y < (getheight()/4)*3) { line.

20 1 Example: equivalent results 2 if(y > getheight()/4 && y < (getheight()/4)*3) { line.setcolor(color.red); else { line.setcolor(color.green); 3 if(y > getheight()/4) { if(y < (getheight()/4)*3) { line.setcolor(color.red); else { line.setcolor(color.green); else { line.setcolor(color.green); 4 line.setcolor(color.green); if(y > getheight()/4) { if(y < (getheight()/4)*3) { line.setcolor(color.red); line.setcolor(color.green); if(y > getheight()/4 && y < (getheight()/4)*3) { line.setcolor(color.red); CS 101 Özyeğin University

21 Nested loops int x = 0; while (x < 100) { int y = 0; while(y < 100) { GRect rect = new GRect(x, y, 10, 10); add(rect); y = y + 10; x = x + 10; CS 101 Özyeğin University 21

22 Exercise Modify the program to come up with the result below int x = 0; while (x < 100) { int y = 0; while(y < 100) { GRect rect = new GRect(x, y, 10, 10); /* TODO:??? */ add(rect); y = y + 10; x = x + 10; CS 101 Özyeğin University 22

23 Exercise int x = 0; while (x < 100) { int y = 0; while(y < 100) { GRect rect = new GRect(x, y, 10, 10); if(x > 30 && x < 60 && y > 30 && y < 60) { rect.setfilled(true); rect.setfillcolor(color.red); add(rect); y = y + 10; x = x + 10; CS 101 Özyeğin University 23

24 for vs. while The for statement for ( init ; test ; step ) { statements to be repeated is functionally equivalent to the following code using while: init; while ( test ) { statements to be repeated step; The advantage of the for statement is that everything you need to know to understand how many times the loop will run is explicitly included in the header line.

25 Nested for Statements The body of a control statement can contain other statements. Such statements are said to be nested. Many applications require nested for statements. The next slide, for example, shows a program to display a standard checkerboard in which the number of rows and number of columns are given by the constants N_ROWS and N_COLUMNS. The for loops in the Checkerboard program look like this: for (int i = 0; i < N_ROWS; i++) { for (int j = 0; j < N_COLUMNS; j++) { Display the square at row i and column j. Because the entire inner loop runs for each cycle of the outer loop, the program displays N_ROWS x N_COLUMNS squares.

26 The Checkerboard Program public void run() { double sqsize = (double) getheight() / N_ROWS; for (int i = 0; i < N_ROWS; i++) { for (int j = 0; j < N_COLUMNS; j++) { double x = j * sqsize; double y = i * sqsize; GRect sq = new GRect(x, y, sqsize, sqsize); sq.setfilled((i + j) % 2!= 0); add(sq); sqsize i j x y Execute these inner statements loop six seven more times to complete the checkerboard. row. sq Checkerboard skip simulation

27 The End

Programming Lecture 4

Programming Lecture 4 Five-Minute Review 1. What are classes and objects? What is a class hierarchy? 2. What is an expression? A term? 3. What is a variable declaration? 4. What is an assignment? What is precedence? 5. What