You ll be reading more about tools for branching in Sections 3.4, and We are skipping Sections 3.5, 3.11 and 3.13-end of chapter.

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1 CS Preview for Session 6 You ll be reading more about tools for branching in Sections 3.4, and We are skipping Sections 3.5, 3.11 and 3.13-end of chapter. Section 3.4 Logical Operators When using relational operators like > or <=, sometimes it is helpful to combine relational expressions in one if or else statement. There are three logical operators: && And True if ALL relational expressions are true Or True of at least ONE relational expression is true! Not True if the relational expression is false Examples of each operator follow. For these cases, assume integers a = 5, b = 6 and c = 7. Examples of the && logical operator: if (a < b && b < c) { // True because both relational expressions are true if (a < b && b > c) { // False because the second relational expression is false if (a < b && b < c && a < c) { // True because all three relational expressions are true. Note that the third // expression is not needed. Why? Examples of the logical operator: if (a < b b < c) { // True because both relational expressions are true if (a < b b > c) { // True because the first relational expression is true even // though the second is false Examples of the! logical operator: if (! (a < b)) {

2 // False because a < b is true, so reversing it makes it false if (! (a < b && b > c)) { // True because the second relational expression is false, rendering the // expression inside the outer parentheses false, rendering the entire // expression true because of the! Section 3.5 Switch Statements Section 3.6 Boolean Data Type The powerful boolean data type enables evaluating and remembering the value of relational expressions with or without logical operators. The boolean data type holds the values true or false and is declared and used as with this example: boolean ishumid; boolean ishot; ishumid = (humidity > 0.50); ishot = (temperature >= 90); This sets ishumid to true if the humidity is > 0.50, false otherwise. Similarly, ishot is set to true if the temperature is 90 or more, false otherwise. Note that the statement can also be written as: if (humidity > 0.50) { ishumid = true; else { ishumid = false; Boolean variables are often combined with logical operators to form a more easily readable program. Example: if (ishumid && IsHot) { // Do something Section 3.7 String Comparisons

3 Because strings are classes, we cannot use the relational operators (like >, >= and so on). Instance variables that refer to classes hold the addresses of the objects and not the value of the objects. So comparing instance variables results in comparing addresses which is useless in Java. Instead, we use methods to compare strings. Assume these declarations: String str1 = Hello ; String str2 = Goodbye ; The first method is.equals that returns true if the two strings are exactly identical. That means the strings are the same lengths and contain exactly the same characters. So, str1.equals(str2) returns false str1.equals( Hello ) returns true str1.equals( HELLO ) returns false The method.equalsignorecase does just that: ignores case in alphabetic character comparisons. So, str1.equalsignorecase(str2) str1.equalsignorecase ( Hello ) str1.equalsignorecase ( HELLO ) returns false returns true returns true The.compareTo method is more powerful and enables checking to see if one string is greater than (sorts after), equal to (sorts the same) or less than (sorts before) another string. Respectively, the.compareto methods returns a number > 0, = 0, or < -0. Note that we don t know the exact number; we know just the sign. So, str1.compareto(str2) returns > 0 str1.compareto( Hello ) returns = 0 str1.compareto( HELLO ) returns > 0 With the third example, note that lower case letters are > upper case letters so the former sort after the latter. For example, the code for A is 65 while the code for a is 97. There is also a version of.compareto that ignores case. So, str1.comparetoignorecase(str2) returns > 0 str1.comparetoignorecase ( Hello ) returns = 0 str1.comparetoignorecase ( HELLO ) returns = 0 Section 3.8 String Access Operations Section 3.9 More String Operations

4 The key methods in these two sections were covered in Lab Please see that lab and the file that went with it, LabCh02StringMethods.java. Read Sections 3.8 and 3.9 for more string methods. Section 3.10 Character Operations The key character-related methods are shown below. Assume char ch; isletter(ch) True if ch is a letter: ch is a-z or A-Z isdigit(ch) True if ch is a digit: ch is 0-9 iswhitespace(ch) touppercase(ch) True if ch is whitespace (blank, tab, enter) Converts an alphabetic letter to upper case tolowerchase(ch) Converts an alphabetic letter to lower case For reasons we will learn later, when using character methods, you must code the Character class name before a character method call. For example: if (Character.isLetter(ch)) { // Do something Section 3.11 Conditional Expressions Section 3.12 Floating Point Comparison We ve seen junk introduced to the lower end digits of double calculations. Therefore, we can t check for exact equality when comparing two numbers. So we need to compare the absolute value of the difference between the two numbers to some arbitrarily small number. For example, consider this code: 1 public class DeleteMe { 2 public static void main(string[ ] args) { 3 double x = 0.1; 4 double y = 0.2; 5 double z; 6 7 z = x + y; // z should be exactly 0.3

5 8 9 System.out.println(z); The program prints for the value of z the number That 4 comes from imprecise storing of double numbers and double calculations. So comparing z to 0.3 would produce a false result even though algebraically z = 0.3. So double comparisons are done as follows: double epsilon = ; // Or any other small number if (Math.abs(z 0.3) < epsilon) { // Then assume z = 0.3 Sections

Getting started with Java

Getting started with Java Magic Lines public class MagicLines { public static void main(string[] args) { } } Comments Comments are lines in your code that get ignored during execution. Good for leaving