Chapter 3 Decision Structures

transcript

Chapter 3Decision Structures

Contents1. The if Statement2. The if-else Statement3. The if-else-if Statement4. Nested if Statement5. Logical Operators6. Comparing String Objects7. More about Variable Declaration and

Contents (Cont’d)8. The Conditional Operators 9. The Switch Statement10.Creating Objects with the

DecimalFormat Class11.The printf Method

1. The if Statement Problem:

Write a program to calculate user’s average of 3 test scores. If the average is greater than 95, the program congratulates the users on obtaining a high score.

1. The if Statement (Cont’d)

1. The if Statement (Cont’d) Simple decision structure logic

1. The if Statement (Cont’d) if(BooleanExpression) statement;

The BooleanExpression must be a boolean expression

A boolean expression is either true or false If the BooleanExpression is true, the very next

statement is executed. Otherwise, it is skipped. The statement is conditionally executed

because it only executes under the condition that the expression in the parentheses is true.

Using Relational Operators to Form Conditions Typically, the boolean expression is

formed with a relational operator (binary operator.

Relational Operators(in order of precedence)

Meaning

> Greater< Less than>= Greater than or equal to<= Less than or equal to== Equal to!= Not equal to

Using Relational Operators to Form Conditions (Cont’d) Assuming that a is 4, b is 6, and c

is 4 b>=a true c<=a true a>=5 false c==6 false b!=6 false

Programming Style and the if Statement Two important style rules

The conditionally executed statement should appear on the line after the if statement.

The conditionally executed statement should be indented on level from the if statement.

if(value>32) System.out.println(“Invalid number”);

Be Careful with Semicolonsif(BooleanExpression)

statement;

int x = 0, y = 10;if(x > y);

System.out.println(x + “is greater than “ + y);

No semicolon here

Semicolon goes here

It will always execute.

Multiple Conditionally Executed Statements Enclosing a group of statements by

braces

if(sales > 5000){bonus = 500.0;commissionRate = 0.12;dayOff += 1;

These three statements are executed if sales is greater than 5000

Comparing Characters Using the relational operators to

test character data as well as number

Assuming ch is a char variable: Compare ch to the character ‘A’:

if(ch==‘A’)System.out.println(“The letter is A.”);

Compare the ch is not equal to ‘A’:if(ch!=‘A’)

System.out.println(“Not the letter is A.”);

Comparing Characters (Cont’d) In Unicode, letters are arranged in

alphabetic order: ‘A’ comes before ‘B’, the numeric

code of ‘A’ (65) is less than the code of ‘B’ (66).

‘A’ < ‘B’ true In Unicode, the uppercase letters

come before the lowercase letter.

2. The if-else statement Problem

Write a program to get two numbers and divide the first number by the second number.

2. The if-else statement

Logic of the if-else Statement

Logic of the if-else Statement (Cont’d) The if-else statement will

execute one group of statement if its boolean expression is true, or another group if its boolean expression is false.if(BooleanExpression)

statement or blockelse

statement or block

3. The if-else-if Statement Problem

Write a program to ask the user to enter a numeric test score. Display a letter grade (A, B, C, D, or F) for the score.

score < 60 F 60 <= score < 70 D 70 <= score < 80 C 80 <= score < 90 B 90 <= score <= 100 A score > 100 Invalid score

Logic of the if-else-if StatementLogic of the if-

else-if Statement

3. The if-else-if Statement The if-else-if statement is a chain of if-else

statements. Each statement in the chain performs its test until one of the tests is found to be true.

if(BooleanExpression)statement or block

else if(BooleanExpression)statement or block

////Put as many else if statement as needed here//elsestatement or block

4. Nested if Statement Problem

Write a Java program to determine whether a bank customer qualifies for a loan. To qualify, a customer must earn at least $30,000 per year, and must have been on his or her current job for at least two years.

4. Nested if Statement Input

User’s annual salary Number of years at the current job

4. Nested if Statement (Cont’d) An if statement appears inside

another if statement, it is considered nested.

The rule for matching else clauses with if clauses is this:

An else clause goes with the closet previous if clause that doesn’t already have its own else clause.

Alignment of if and else clauses

5. Logical Operators Logical operators connect two or

more relational expressions into one or reverse the logic of an expression.

Java provides two binary logical operators

&& : AND || : OR

one unary logical operator ! : NOT

5. Logical Operators (Cont’d)Operator Meaning Effect&& AND Connects two boolean expression into

one. Both expressions must be true for the overall expression to be true.

|| OR Connects two boolean expression into one. One or both expressions must be true for the overall expression to be true. It is only necessary for one to be true, and it does not matter which one.

! NOT Reverses the truth of a boolean expression. If it is applied to an expression that is true, the operator returns false. If it is applied to an expression that is false, the operator return true.

5. Logical Operators (Cont’d)

Expression Meaningx > y && a < b Is x greater than y AND is a less than b?x == y || x == z Is x equal to y OR is x equal to z?!(x > y) Is the expression x > y NOT true?

The && Operator

The && performs short-circuit evaluation

If the expression on the left side of the && operator is false, the expression on the right side will not be checked.

x y x && ytrue true true

true false false

false true false

false false false

The && Operator (Cont’d) A different version of the

LoanQualifier program

The || Operator

The || performs short-circuit evaluation

If the expression on the left side of the || operator is true, the expression on the right side will not be checked.

x y X || ytrue true true

true false true

false true true

false false false

The || Operator Problem

Write a Java program to determine whether a bank customer qualifies for a loan. To qualify, a customer must earn at least $30,000 per year, OR must have been on his or her current job for at least two years.

The || Operator (Cont’d) Input

User’s annual salary Number of years at the current job

A Better Solution

The ! Operator The ! Operator performs a logical

NOT operation

!(x > 1000) x <= 1000

x !xtrue false

false true

The Precedence and Associativity of Logical Operators The logical operators have orders of

precedence and associativity. The precedence of the logical operators, from

highest to lowest!&&||

!(x > 2) Applies the ! operator to the expression x > 2

!x > 2 Causes a compiler error

The Precedence and Associativity of Logical Operators (Cont’d) The && and || operators rank lower in precedence

than the relational operators(a > b) && (x < y) a > b && x < y(x == y) || (b > a) x == y || b > a

The logical operators evaluate their expression from left to right

a < b || y == z a < b is evaluated before y == z

a < b || y == z && m > j y == z is evaluated first because the && operator has higher

precedence than || Is equivalent to the following(a < b) || ((y == z) && (m > j))

The Precedence and Associativity of Logical Operators (Cont’d)

Order of Precedence

Operators

1 - (unary negation) !2 * / %

4 < > <= >=

5 == !=

8 = += -= *= /= %=

Checking Numeric Ranges with Logical Operators Determining whether a numeric

value is within a specific range of values

Using the && operator x >= 20 && x <= 40

Determining whether a numeric value is outside a specific range of values

Using the || operator x < 20 || x > 40

20,40x

6. Comparing String Objects We cannot use relational

operators to compare String objects. Instead we must use a String method.String name1 = “Marks”;String name2 = “Mary”;

name1 == name2 will be false because the variables name1 and name2 reference different objects.

6. Comparing String Objectsif(name1 == name2)

6. Comparing String Objects To compare the contents of two

String objects correctly using the method equals of String class

if(name1.equal(name2)) The equals method returns true if

they are the same, or false if they are not the same.

6. Comparing String Objects Comparing String objects to

string literals Pass the string literal as the argument

to the equals method

if(name1.equals(“Mark”))

6. Comparing String Objects The compareTo method of the

String class To determine whether one string is

greater than, equal to, or less than another string.

StringReference.compareTo(OtherString) StringReference is a variable that

references a String object, OtherString is either another variable that references a String object or a string literal.

6. Comparing String Objects If the method’s return value

< 0 : the string referenced by StringReference is less than the OtherString argument

0 : The two strings are equal. > 0 : the string referenced by StringReference is greater than the OtherString argument

6. Comparing String Objects

6. Comparing String Objects String comparison of “Mary” and

“Mark” The character ‘y’ is greater than ‘k’,

so “Mary” is greater than “Mark”.

Ignore Case in String Comparisons The equals and compareTo methods

perform case sensitive comparisons. In other words, “A” is not the same as “a”.

The String class provides the equalsIgnoreCase and compareToIgnoreCase methods

The case of characters in strings is ignored.

7. More about Variable Declaration and Scope The scope of a variable is limited to the

block in which it is declared. It is a common practice to declare all of a

method’s local variables at the beginning of the method, it is possible to declare them at later points.

Sometimes programmers declare certain variables near the part of the program where they are used in order to make their purpose more evident.

7. More about Variable Declaration and Scope A local variable’s scope always

starts at the variable’s declaration ends at the closing brace of the block

of code in which it is declared.

7. More about Variable Declaration and Scope

The Conditional Operator The conditional operator (a ternary

operator) is used to create short expressions that work like if-else statements.

Expression1 ? Expression2 : Expression3 Exprerssion1 is a boolean

expression. If Expression1 is true, then Expression2 is executed. Otherwise Expression3 is executed.

The Conditional Operator (Cont’d)

x < 0 ? y = 10 : z = 20;

if(x < 0) y = 10;

elsez = 20;

The Conditional Operator (Cont’d) We can put parentheses around

the subexpressions in a conditional expression

(x < 0) ? (y = 10) : (z = 20);

Using the Value of a Conditional Expression The conditional expression also

returns a value. If Expression1 is true, the value of

the conditional expression is the value of Expression2. Otherwise it is the value of Expression3.

number = x > 100 ? 20 : 50;

Using the Value of a Conditional Expression (Cont’d)

number = x > 100 ? 20 : 50;

if(x > 100) number = 20;

elsenumber = 50;

Using the Value of a Conditional Expression (Cont’d)System.out.println(“Your grade is: “ +

(score < 60 ? “Fail” : “Pass”));

if(score < 60)Systym.out.println(“Your grade is: Fail.“);

elseSystym.out.println(“Your grade is: Pass.“);

7. The switch Statement The switch statement lets the value

of a variable or expression determine where the program will branch to.

The if-else-if statement allows the program to branch into one of several possible paths. It tests a series of boolean expressions, and branches if one of those expression is true.

7. The switch Statement (Cont’d) The switch statement tests the

value of an integer or character expression and then uses that value to determine which set of statements to branch to.

7. The switch Statement (Cont’d)switch(SwitchExpression){

case CaseExpression1:// place one or more statements herebreak;case CaseExpression2:// place one or more statements herebreak;

// case statements may be repeated as many // times as necessary

default:// place one or more statements here

7. The switch Statement (Cont’d) The SwitchExpression is an expression that must

result in a value of one of these types: char, byte, short, or int.

The CaseExpression is a literal or a final variable which must be of the char, byte, short, or int types.

The CaseExpressions of each case statement must be unique.

The default section is optional. If we leave it out, the program will have nowhere to branch to if the SwitchExpression doesn’t match any of CaseExpressions.

7. The switch Statement (Cont’d)if(SwitchExpression == CaseExpression1)

// place statement hereelse if(SwitchExpression == CaseExpression2)

// place statement here

// else if statements may be repeated// as many times as necessary

else// place statement here

7. The switch Statement (Cont’d)

7. The switch Statement (Cont’d) The case statements show where to start

executing in the block and the break statements show the program where to stop.

Without break statements, the program would execute all of the lines from the matching case statement to the end of the block.

The default section (or the last case section if there is no default) does not need a break statement.

Without break Statement Without break statement, the

program falls through all the statements below the one with the matching case expression.

Without break Statement

Without break Statement ! Write a program that asks the

user to select a grade of pet food. The available choices are A, B, and C. The program will recognize either uppercase or lowercase letters.

Without break Statement

10. Creating Objects with the DecimalFormat class DecimalFormat class can be used to

format the appearance of floating-point numbers rounded to a specified number of decimal places.

In Java, a value of the double type can be displayed with as many as 15 decimal places, and a value of float type can be displayed with up to 6 decimal places.

10. Creating Objects with the DecimalFormat class (Cont’d)double number;number = 10.0/6;System.out.println(number);

1.666666666666667 How to control the number of

decimal places that are displayed ?

10. Creating Objects with the DecimalFormat class (Cont’d) Using the DecimalFormat class

import java.text.DecimalFormat; Create an object of the DecimalFormat class

DecimalFormat formatter;formatter = new DecimalFormat(“#0.00”);

Constructor Is automatically executed To initialize the object’s attributes with appropriate data and

perform any necessary setup operations.

constructor

format pattern

10. Creating Objects with the DecimalFormat class (Cont’d)

#: specifies that a digit should be displayed if it is present. If there is no digit in this position, no digit should be displayed.

0: specifies that a digit should be displayed in this position if it is present. If there is no digit in this position, a 0 should be displayed.

“#0.00”

“000.00”

“#,##0.00”

“#0%” Formatting numbers as

percentages Writing the % character at the last

position in the format pattern. This causes a number to be multiplied by 100, and the % character is appended to its end.

“#0%”

The printf Method The System.out.printf method

allows you to format output in a variety of ways.

System.out.printf(FormatString, ArgumentList)

contains text and/or special

formatting specifiers.

is a list of zero or more

additional arguments.

Format Specifiers %d : For a decimal integer

int hours = 40;System.out.printf(“I worked %d hours this week.\n”, hours);

I worked 40 hours this week.

int dogs = 2;int cats = 4;System.out.printf(“We have %d dogs and %d cats.\n”, dogs, cats);

We have 4 dogs and 2 cats.

Format Specifiers %nd : the number should be printed in

a field that is n places wide.int nunber = 9;System.out.printf(“The value is %6d.\n”, number);

The value is 9. If the field is wider than the specified width,

the field width will be expanded to accommodate the value.

int nunber = 97654;System.out.printf(“The value is %2d.\n”, number);

The value is97654.

Format Specifiers %f : to print a floating-point numberdouble nunber = 1278.92;System.out.printf(“The value is %f.\n”, number);

The value is 1279.920000.

double nunber = 1278.92;System.out.printf(“The value is %18f.\n”, number);

The value is 1279.920000;

double nunber = 1278.92714;System.out.printf(“The value is %8.2f.\n”, number);

The value is 1278.93.

Format Specifiersdouble nunber = 1253874.92714;System.out.printf(“The value is %,.2f.\n”, number);

The value is 1,253,874.93.

%s : To print a string argumentString name = “Ringo”;System.out.printf(“My name is %s.\n”, name);

My name is Ringo.

String name = “Ringo”;System.out.printf(“My name is %10s.\n”, name);

My name is Ringo.

Chapter 3 Decision Structures

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