CSCI 51 Introduction to Programming Dr. Joshua Stough February 10, 2009

Basic Review Terms you need to know –variable, type, value, expression, statement, declaration, assignment, initialization, package, class, method, parameter, return type What is the return type of Integer.parseInt(String) double x = *Math.pow(rad,2); You need to learn what every line of code you write does.

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

Review 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

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"

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

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

Today Flow of Execution

Today: Selection if statements if-else statements Nested if statements switch statements Textbook Reference: Chapter 4 up to 4.5

Conditional Statements Let us choose which statement will be executed next –also called selection statements Java's conditional statements: –the if statement –the if-else statement –the switch statement

One-Way Selection Syntax:if (expression) statement

The if Statement if ( condition ) statement; if is a Java reserved word The condition must be a boolean expression. It must evaluate to either true or false. If the condition is true, the statement is executed. If it is false, the statement is skipped.

The if Statement if (sum > MAX) delta = sum - MAX; System.out.println ("The sum is " + sum); First, the condition is evaluated. The value of sum is either greater than the value of MAX, or it is not. If the condition is true, the assignment statement is executed. If it is not, the assignment statement is skipped. Either way, the call to println is executed next.

Block Statements Syntax: { statement1 statement2. statementn } We use curly braces to group a set of individual statements. This way we can have multiple statements execute based on a decision. If we don't use curly braces, only a single statement will be executed based on the result of a decision.

The if Statement if (sum > MAX) delta = sum - MAX; System.out.println ("The sum is " + sum); A if (sum > MAX) { delta = sum - MAX; } System.out.println ("The sum is " + sum); B if (sum > MAX) { delta = sum - MAX; System.out.println ("The sum is " + sum); } C

Curly Braces int num = 87, max = 25; if (num >= max*2) { System.out.println ("apple"); } System.out.println ("orange"); System.out.println ("pear"); int num = 87, max = 25; if (num >= max*2) System.out.println ("apple"); System.out.println ("orange"); System.out.println ("pear");

if Statement Gotcha if (score >= 90); grade = "A"; No matter the result of the condition, grade will be assigned "A". The semicolon after the if statement is a semantic error.

In-Class Exercises What is printed by the following code fragment? int num = 87, max = 25; if (num >= max*2) System.out.println ("apple"); System.out.println ("orange"); System.out.println ("pear"); Write a code fragment that will print the value of val if val is less than MAX. apple orange pear if (val < MAX) { System.out.println (val); }

Two-Way Selection

The if-else Statement if ( condition ) statement1; else statement2; If the condition is true, statement1 is executed; if the condition is false, statement2 is executed One or the other will be executed, but not both

The if-else Statement if (height <= MAX) { adjustment = 0; } else { adjustment = MAX-height; }

In-Class Exercise What is the value of adjustment given the following values of height and MAX ? if (height <= MAX) { adjustment = 0; } else { adjustment = MAX-height; } height = 60, MAX = 80 height = 100, MAX = 75 height = 45, MAX =

Nested if Statements The statement (or block) executed as a result of an if statement can be another if statement (nested if). An else clause is always matched to the closest unmatched if. Closing curly brace signals the end of an if statement.

Nested if Statements The general syntax of a nested if statement is: if (condition1) { block1 } else if (condition2) { block2 } else { block3 }

Nested if Statements if (hasFourLegs) { if (playsFetch) { System.out.println ("DOG"); } else { System.out.println ("CAT"); } else if (hasWings) { System.out.println ("BIRD"); } else { System.out.println ("FISH"); } has four legs does not have four legs plays fetch doesn't play fetch has wings doesn't have wings

Min.java Example

The switch Statement

Provides another means to decide which statement to execute next Evaluates an expression, then attempts to match the result to one of several possible cases Each case contains a value and a list of statements The flow of control transfers to statement associated with the first value that matches

The switch Statement The general syntax of a switch statement is: switch ( expression ) { case value1 : statement-list1 case value2 : statement-list2 case value3 : statement-list3 case... } switch and case are reserved words If expression matches value2, control jumps to here

The switch Statement Expression evaluated must be integral type –integer or character, not boolean or floating point Case values must be constant (literal), not variable Can be implemented with nested if statements, but is clearer with switch statements

The switch Statement default –no case value matches the expression –if no default exists and no case value matches the expression, no statements in the switch statement are executed break –processing jumps to statement following the switch statement –usually at the end of each case

The switch Statement System.out.print ("Enter prize code: "); int prize = Integer.parseInt(keyboard.readLine()); switch (prize) { case 1: System.out.println (``A Brand New Car!’’); break; case 2: System.out.println (``A Trip to Hawaii!’’); break; default: System.out.println (``Sorry, Try Again’’); break; }

GradeReport.java Example

Questions Assume movieRating is an int and movieName is a String switch (movieRating) { case 1: System.out.println ("Run away!"); if (movieName.equals("Gigli")) { System.out.println ("Quickly!"); } case 2: System.out.println ("Save your money"); break; case 3: System.out.println ("It's OK"); break; } Run away! Quickly! Save your money [Nothing] 1. What is printed if movieRating is 1 and movieName is "Gigli"? 2. What is printed if movieRating is 5 ?

Questions if ((x > 0) && (y < 0)) { z = x+y; System.out.println (“z = ” + z); } System.out.print (“The sum is “); if (x+y < 0) { System.out.println (“negative.”) } else { System.out.println (“positive.”); } z=1 The sum is positive. The sum is negative. The sum is positive. What is printed with the following values of x and y: 1.x = 5, y = -4 2.x = -9, y = 5 3.x = 5, y = 5

Next Time in CSCI 51 Iteration or Loops: for loops, while loops. Reading Assignment: Chapter 6