CS 170 Java Programming 1. Week 7: More on Logic

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1 CS 170 Java Programming 1 Week 7: More on Logic

2 What s the Plan? Topic 1: A Little Review Use relational operators to compare values Write functions using if and else to make decisions Topic 2: New Logical Concepts How combine decisions with the logical operators How to make multi-way decisions Topic 3: Introducing ACM Graphics Write graphical applications and use graphical objects

3 Your "IC" or "Lab" Document Use Word or OpenOffice to create a new document Save the file as IC07.doc (Office compatible) Place on your network U: drive or on a thumb drive Put your name and today's date at the top of the sheet Title it "CS 170 Lab Exercises Week 7" Start DrJava and we'll review a few basic logic concepts

4 Review: the boolean Type The last of the four primitive types is boolean (Note that there is also a Boolean wrapper class) boolean literals are written as true and false Create boolean variables just like other variables boolean visible = false; Boolean expressions often compare values using the six relational operators: <, <=, >, >=, ==,!= Relationship between two values? true or false

5 Comparing Numbers To review, we'll use the DrJava Interactions Pane 1. Create a pair of double variables, a and b 2. Initialize a by multiplying.1 by Initialize b by adding.1 ten times 4. Write an expression telling you if a is equal to b 5. Tell me if a is equal to b, compared to 10 decimals Exercise 7.1 snap a pic and paste into your IC doc

6 Real Comparisons Floating-point numbers are approximations of real numbers You have limited precision: about 15 digits with double You should not use == or!= with floating-point Mathematically equivalent calculations may have differing results when done in binary Some quick-and-dirty comparisons (to 3 places) Math.round(a * 1000) == Math.round(b * 1000) (long)(a * 1000) == (long)(b * 1000)

7 Comparing Strings Now, let's look at comparing Strings 1. Create a Scanner object (remember the import) 2. Create two String variables, password and user 3. Assign password a password of your choice 4. Use the Scanner to read a value into user 5. Print "Accepted" if the correct password is entered 6. Print "Access Denied" if it doesn't match Exercise 7.2 snap a pic and paste into your IC doc

8 Comparing Strings With Strings, you should never use == or!= Instead, use the Stringequals() method if (str.equals("hello")) NOT if (str == "hello") For "not-equals" use the NOT logical operator if (! str.equals("hello")) Also a version called equalsignorecase()

9 Review: Making Decisions In Java, selection statements allow us to have different code run under different conditions Simplest selection statement is the if statement If the condition is true, then the code in the body is executed If the condition is false, the code in the body is skipped

10 Review: Logic Functions Locate sleepin2 problem in Week06Review.java Notice that this function returns true or false We call these predicate functions Start by creating a result variable and setting to false Then return the variable Run to see what tests fail, then fix Exercise 7.3 snap a pic and paste into your IC doc Exercise 7.4 now do the same thing with seecolor

11 Topic 2 More Complex Logic and Decisions

12 New Decision Concepts this Week Decisions involving multiple branches or tests Logical operators allow us to test several different conditions to produce a single output value Sequentialif statements allow us to produce several different outputs, based on interdependent tests Nestedif statements allow us to produce several different outputs, based on leveled test criteria The switch statement allows us to produce different output based on a simple integer expression evaluation

13 Logical Operators Many times, you need to test several values at once Check if an input falls between 1 and 100, for instance Java's logical operators can to combine several tests Logical operators work on boolean values One unary logical operator: NOT (!) Five binary logical operators Two versions of AND (&&, &) Two versions of OR (, ) One exclusive OR (^)

14 The AND Operators The symbol used by Java for AND is && or & Corresponds to normal use of word "and" Both conditions must be true for a true result If either condition (or both) is false, result is false if (units >= 123 && GPA >= 1.5) yougraduate();

15 The OR Operators The symbol used by Java for inclusive OR is or Corresponds to natural understanding of OR If any condition is true, the result is true Both conditions must be false for a false result if (quiz > 75 homework > 95) finalgrade = 'B'; The logical exclusive OR operator (^) is rarely used If exactly one condition is true, the result is true

Cigar Party Exercise 7.5 Open LogicalOperators.java, solve cigarparty problem At OCC, when squirrels get together for a party, they like to have cigars (just like mice and cats do, of course).

16 Cigar Party Exercise 7.5 Open LogicalOperators.java, solve cigarparty problem At OCC, when squirrels get together for a party, they like to have cigars (just like mice and cats do, of course). A party is successful when the number of cigars is between 40 and 60, inclusive. Unless it is the weekend, in which case there is no upper bound on the number of cigars. Return true if the party with the given values is successful, or false otherwise.

How Much to Evaluate? Sometimes, you "know" the answer to an expression without evaluating the entire thing. Wheel of Fortune when the answer is obvious No?

17 How Much to Evaluate? Sometimes, you "know" the answer to an expression without evaluating the entire thing. Wheel of Fortune when the answer is obvious No? How about this: what is the value of0 * x? You know the value is 0, whatever value x has Expressions using logical operators are similar The value == 5 && a > b is false The values stored in a and b are irrelevant

18 Short-circuit Evaluation When using the and && operators, Java stops evaluating once it can determine the result This is called short-circuit or lazy evaluation The expression is evaluated from left to right Evaluation stops when the result is assured For && stops when a false value is encountered For stops when a true value is encountered and & evaluate both sides regardless

Multi-Way Branching Many problems involve branching statements with

grades based on a numeric score Income taxes based on filing status

different techniques Sequentialif statements: related comparisons

19 Multi-Way Branching Many problems involve branching statements with more than just the two alternatives handled by if-else Calculating grades based on a numeric score Income taxes based on filing status and income Writing menu-type code based on a user selection Three different techniques Sequentialif statements: related comparisons Nestedif statements: levels of decision making The switch statement: numbered branches

20 Sequential If Statements Consider assigning an A-F grade, based on a percentage if (pct >=.8 && pct <.9) grade = 'B'; Inefficient: all six conditions are tested each time Better: Multiple Selection Algorithmic Pattern AKA as ladder-style if-else statements Pitfall: order is significant

21 Try It Out Exercise 7.6 Open MultipleChoice.java, solve datefashion problem You and your date are trying to get a table at a restaurant. The variable you is the stylishness of your clothes, in the range 0..10, and date is the stylishness of your date's clothes. The result getting the table is either 0=no, 1=maybe, or 2=yes. If either of you is very stylish, 8 or more, then the result is yes. With the exception that if either of you has style of 2 or less, then the result is no. Otherwise the result is maybe.

22 Nested if Statements Can also combine Boolean expressions with nesting One if statement embedded inside the body of another if (x > 0) { if (y <= 0) quadrant = SIRIUS; else quadrant = OMEGA; } A nested if statement The nested if can appear in the else part as well

23 Using Nested if Statements Use nested ifs when you have levels of decisions Calculating income tax, for instance First decide on filing status, then decide on income

24 Try It Yourself Exercise 7.7 solveparrottrouble with nestedifs Notice that we have multiple tests Is the parrot talking? Is it before 7AM? Is it after 10PM? Yes Parrot talking? No Yes Before 7 AM OR After 10 PM No No trouble Got trouble No trouble

25 Menu-Style Code With ladder-style if-else-if, you might sometimes find yourself writing menu-style code that looks like this: int choice = getuserchoice(); if ( choice == 1 )... else if (choice == 2)... else if (choice == 3)... else... Notice that each condition is testing the same variable Each test is made against a literal expression Java has a more efficient way to do this: the switch

26 What is the switch? A selection statement based on integer evaluation Named after the switches used by the phone company Dial a number and get connected to your party Think also, of a menu or vending machine Open SimpleMenuSwitch

27 Parts of the switch The parts of a switch statement are The test condition or switch selector A body surrounded by braces (required) A set of labeled code fragments called case blocks A "flow-of-control" statement called break

28 Topic 3 ACM Graphical Applications

29 The ACM Class Libraries ACM Java libraries were designed for students ACM: Association for Computing Machinery Computer Science Professional and Academic Organization Student membership is about $20 per year Java Task Force (JTF) Eric Roberts (Stanford) chair Review the Java language Develop a stable collection of libraries for teaching We ll use several graphics classes from library

30 ACM GraphicsPrograms Open MyFirstAcmGuiProgram.java in the week07 folder Graphical application that uses the ACM libraries I've added the libraries to our copy of DrJava import statements make ACM classes accessible Builds on, or extendsgraphicsprogram class Normally put one-time code into init() method Sequential code goes into run()

31 ACM Graphic Objects Graphics program is just a "blank" canvas Instead of commands, OOP programs create objects, and then ask them to carry out different actions Instead of using methods like Graphics drawoval() we want to start using independent graphical objects ACM acm.graphics package has a family of objects Let's use one class (GLabel) to display a message Similar to "hello world" programs we started with

32 Using Graphical Objects To use an ACM graphical object (GObject) you: 1. Create a variable to refer to the object 2. Construct and customize the object 3. Add the object to the surface of your app 4. Send messages to the object as your program runs Exercise 7.8 Add a label and snap a pic 1. Create a GLabel variable, mylabel 2. Create a GLabel object: "Hello, I love you!" 3. Use add() to place it on surface of canvas at 50, 50.

33 Sending Messages Use a font descriptor: type-weight-size mylabel.setfont("serif-bold-36"); Change the color by using the setcolor() method mylabel.setcolor(color.red); Change the caption with the setlabel() method mylabel.setlabel("hi, my name is ");

34 Other Shapes and Images GLine, GOval and GRect work like Graphics drawing methods, except they are persistent objects Exercise 7.9 add oval and rectangle to the canvas Closed shapes implement GFillable interface setfillcolor(), setfilled(true/false) Fill your rectangle and your oval GImage: load images from gif/jpg/png files Place files in current directory or in images folder Exercise 7.10 create a GImage using "Earth.gif" image

35 Finishing Up To complete your IC "lab" document: Complete Exercise 7.11 in the Logical Operators lesson, Exercise 7.12 in the Multiple-Choice lesson and Exercise 7.13 in the GImage and Friends lesson Submit to Blackboard when done Reading for this week: Gaddis: Finish Chapter 3, Use JTF tutorial for ACM material

36 Homework and Quiz Homework, Quiz and Assignments Deadline is Monday at noon Twelve homework problems 10 small decision function problems 2 ACM graphics problems Upload to Assignment Dropbox. Make sure to press Submit Midterm Exam Next Week Sample exam available on BBV You may make up one Proficiency Quiz after the exam

CS 170 Java Programming 1. Week 10: Loops and Arrays

CS 170 Java Programming 1. Week 10: Loops and Arrays CS 170 Java Programming 1 Week 10: Loops and Arrays What s the Plan? Topic 1: A Little Review Use a counted loop to create graphical objects Write programs that use events and animation Topic 2: Advanced