Chapter 3 Selections

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Chapter 3:Selections

Introduction to J ava Programming, Daniel Liang, 8th Edition

Spring 2012 - 2013


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Introduction

Lets recall the ComputeAreaprogram.

What if the user enters a negative value?

How to prevent that? Using Selection Statements

Fatima Kanj, Spring 2012 - 20132


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Comparisons and boolean Expressions We can perform comparison in java.

The result of the comparison is a booleanvalue: trueor false Example: System.out.println(1 < 2); //Output: false

Comparison Operators (Relational Operators)



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Boolean Variables

Declaration of booleanvariables:

booleanvariableName;

Range of values of booleantype:

Two values: true or false

Example:

booleanisEquilateral = true;

booleanflag = false;



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Types of Selection Statements

Java has several types of selection statements:

if Statements:

One-way ifStatements

Two-way ifStatements

Nested ifStatements switch Statements

Conditional Expressions



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One-Way if Statement

A one-way ifstatement executes an action if and only ifthe condition is true.

Example:



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Example:


Write a program that prompts the user to enter an integer. If the number is amultiple of5, printHiFive. If the number is divisible by2, printHiEven.


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Two-Way if statement

The actions that a two-way ifstatement specifies differ basedon whether the condition istrueor false.



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Example:

To check whether an integer is even or odd:



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Nested if Statement The statement in an ifor if ... elsestatement can be any legal Java

statement, including another ifor if ... elsestatement.

The inner ifstatement is said to benestedinside the outer ifstatement. The nested ifstatement can contain another ifstatement; in fact, there is no limit to the

depth of the nesting.

The nested ifstatement can be used to implement multiplealternatives.



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Example:

Finding the letter grade of a given score:



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Common Errors in Selection Statements

Common Error 1: Forgetting Necessary Braces

Common Error 2: Semicolon at the if Line



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Common Error 3: Redundant Testing of Boolean Values

Common Error 4: Dangling else Ambiguity (Theelseclause alwaysmatches the most recent unmatched ifclause in the same block).



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Common Error 5: Assignment operator (=); if the value of even wasfalse, this will assign true to even and prints It is even. to the console.



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switch Statements

To replace nested if statements to handle multiple conditionsefficiently.

The syntax is as follows:



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Rules of the switch Statement The switch-expression must yield a value of char, byte, short, or int type The value1, and valueN must have the same data type as the value of the

switch-expression.

The keyword break is optional: It ends the switch statement.

The default case is optional: It can be used to perform actions when none of

the specified cases matches the switch-expression.



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switch Example

A simple program which prints on the console a differentmessage when the user enters an integer from 1 to 4.



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Conditional Expressions

boolean-expression ? expression1 : expression2;

The result of this conditional expression isexpression1 ifboolean-expression is true; otherwise the result isexpression2.



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Logical Operators or Boolean Operators

Used to combine several conditions.



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Truth Table for Operator !



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Truth Table for Operator &&



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Truth Table for Operator ||



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Truth Table for Operator



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Boolean Operators: Example 1 Write a program that checks whether a number is divisible by2and3, by2

or 3, and by 2or 3but not both:



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Boolean Operators: Example 2

Write a the program that lets the user enter a year andchecks it is leap.



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Unconditional vs. Conditional Boolean Operators

Unconditional Operators (& , | ) They work exactly the same as the&&and || operators with

one exception: the&and | operators always evaluate bothoperands.

Conditional (&&, ||)

When evaluatingp1 && p2, Java first evaluatesp1and thenevaluatesp2 ifp1 istrue; ifp1 is false, it does not evaluatep2.



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Formatting Console Output

If you wish to display only two digits after the decimal point ina floating-point value, you may write the code like this:

A better way: Use theprintfmethod.

where format is a string that may consist of substrings and formatspecifiers. A format specifier specifies how an item should bedisplayed.



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Frequently Used Specifiers



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Formatting Output



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Example:



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Remark

By default, the output is right justified. You can put theminus sign (-) in the specifier to specify that the item isleft justified in the output within the specified field.



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Evaluation of Expressions

The expressions in parentheses are evaluated first.

When evaluating an expression without parentheses, Javaoperators are evaluated according to:

Precedence (check table 3.10) Associativity: If operators with the same precedence are next to each

other, their associativity determines the order of evaluation.

Binary operators are left associative

Assignment operators are right associative



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Operator Precedence



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Example

Applying the operator precedence and associativity rule, theexpression 3 + 4 * 4 > 5 * (4 + 3) - 1 is evaluated as follows:

3 + 4 * 4 > 5 * ( 4 + 3) - 1

3 + 4 * 4 > 5 * 7 1

3 + 16 > 5 * 7 1

3 + 16 > 35 1

19 > 35 1

19 > 34

f al se

(1) inside arentheses firs

(2) multiplication

(3) multiplication

(4) addition

(5) subtraction

(6) greater than


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Chapter 3 Selections