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COMP 14 Introduction to Programming Mr. Joshua Stough February 7, 2005 Monday/Wednesday 11:00-12:15 Peabody Hall 218

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Announcements Homework 2 and Program 2 are due Today

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Review GUI I/O –JOptionPane.showInputDialog –JOptionPane.showMessageDialog File I/O –FileReader –FileWriter, PrintWriter String class methods –toUpperCase –toLowerCase

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Review JOptionPane Methods showInputDialog str = JOptionPane.showInputDialog(strExpression); showMessageDialog JOptionPane.showMessageDialog(parentComponent, strExpression, boxTitleString, messageType);

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Review The StringTokenizer Class Default delimiters: –space, tab, carriage return, new line Methods –StringTokenizer (String str) –StringTokenizer (String str, String delimits) –String nextToken() –boolean hasMoreTokens() –int countTokens()

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Review Reading From Text Files String file = "data.dat"; BufferedReader inFile = new BufferedReader (new FileReader (file)); String line = inFile.readLine(); inFile.close();

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Review Writing To Text Files String file = "outfile.dat"; PrintWriter outFile = new PrintWriter (new FileWriter (file)); outFile.print ("Hi"); outFile.println(" There!"); outFile.close();

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Today in COMP 14 Control Structures Relational Operators Comparing Strings Boolean Expressions Textbook Ref: Ch 4 (pgs. 148-164)

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Control Structures Three methods of processing a program: 1. In sequence –statements are executed one after another in order 2. Branching – altering the flow of program execution by making a selection or choice 3. Looping – altering the flow of program execution by repetition of statement(s)

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Flow of Execution

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Examples of Conditionals Note: the following are not actual Java statements 1) if (score is greater than or equal to 90) grade is A 2) if (hours worked are less than or equal to 40) wages = rate * hours otherwise wages = (rate * 40) + 1.5 * (rate * (hours - 40)) 3) if (temperature is greater than 70 degrees and it is not raining) recommended activity is golfing

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Relational Operators Relational Operator –allows you to make comparisons in a program –binary operator needs two operands Condition is represented by a logical (Boolean) expression –expression that has a value of either true or false

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Relational Operators Less than < Greater than > Equal to == –not assignment ‘ = ‘ Not equal to != Less than or equal to <= Greater than or equal to >=

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Comparing Characters In Java, characters are ordered according to the Unicode / ASCII character set (pg. 855) ‘a’ comes before ‘b’ in the character set, so we can say ‘a’ < ‘b’ Order –space character (' ') –digits (‘0’, ‘1’, …) –uppercase –lowercase

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Comparing Characters 'a' > 'A' '6' < 7 ' ' <= 's' 97 > 65 true 54 < 7 false 32 <= 115 true

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Comparing Floating-Point Be careful when using the equality (==) to compare floating point numbers Every bit in the representation must be equal –computer can only store a certain number of digits after the decimal If the numbers are results of computation, it’s unlikely that two floating point numbers will be exactly equal

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Comparing Floating-Point 3.0 / 7.0 2.0 / 7.0 (3.0 / 7.0) + (2.0 / 7.0) + (2.0 / 7.0) == 1 0.4285714285714285 false 0.2857142857142857 If we did this math with exact arithmetic, this expression would be true 0.9999999999999999

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Comparing Strings Strings are compared on a character-by- character basis –first character not in common determines how the strings compare –ex: "Airplane" is less than "Airport" If two strings have different lengths, and one is a substring of the other, the shorter one is evaluated as less –ex: "Air" is less than "Airplane"

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Comparing Strings Don't use relational operators (==, ) for Strings str1.compareTo (str2) Returns an integer value: –< 0 if str1 is less than str2 –0 if str1 is equal to str2 –>0 if str1 is greater than str2 str1.equals (str2) Returns a boolean value of true or false method parameter

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Questions Given String str = "Homer"; str.compareTo("Marge") str.equals("homer") str.compareTo("Bart") negative value false positive value

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Boolean Expressions Remember that booleans in Java are –true –false A boolean expression is an expression that can be evaluated to either true or false Examples of boolean expressions: –2+2 is not equal to 5 –a is equal to 0 –10 is less than 5 Named for George Boole (1815-1864), the founder of symbolic logic

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Boolean Operators NOT ! (unary) !(2+2==5) AND && (binary) (2+2==5) && (1+1==2) OR || (binary) (2+2==5) || (1+1==2) true false true

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NOT Operator

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AND and OR Operators

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Boolean Expressions Examples Simple: –2+2 is not equal to 5 2+2 != 5 –10 is less than 5 10 < 5 –'A' is equal to 7' A' == 7 Compound: –a is equal to 3 and b is equal to 5 (a==3) && (b==5) –x is not equal to 2 or y is less than 4 (x!=2) || (y<4) –s is not equal to b and t is greater than or equal to 3 or t is less than 0 (s!=b) && (t>=3) || (t<0)

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Short Circuited Operators The processing of logical AND ( && ) and logical OR ( || ) is “short-circuited” If the left operand is sufficient to determine the result, the right operand is not evaluated count != 0 && total/count > MAX

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Precedence of Operators

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Boolean Expressions Logical expression that evaluates to true if the value of num is between 0 and 10 0 <= num <= 10 0 <= num && num <= 10 incorrect syntax correct syntax

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Questions 1. What type of primitive variable can the result of a logical expression be stored in? 2. Under what conditions would the expression (ch >= 'A' && ch <= 'Z') evaluate to false? 3. What method do you use to compare two Strings? 4. Why is 'a' greater than 'A'? boolean ch 'Z' compareTo Because 'a' comes after 'A' in the ASCII (or Unicode) character table.

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Questions Given the following declarations, evaluate each Boolean expression: int count = 0, sum = 53; double x = 4.3, y = 1.2; boolean wrong = true; 1. (wrong && sum > 60) 2. ((x > 5) || !(sum == 55)) 3. !((y > 1.0) && (x < 4)) 4. ((count != 4) || (sum > 100) && wrong) false true (false || !(false)) !(true && false) (true || false && true) (true && false) (true || false)

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Next Time in COMP 14 Conditional Statements –if statements –if-else statements –switch statements

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