Chapter 3: Flow of Control Branching Statements

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Chapter 3 Flow of ControlBranching Statements

• Dr. Hairong Zhao
• hairong_at_calumet.purdue.edu
• http//ems.calumet.purdue.edu/mcss/zhao
• Purdue University Calumet

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Review of Chapter 2 What Have You Learned?

• Java primitive types (numbers, characters, etc.)
• assignment statements and expressions
• String
• classes, methods, and objects
• simple keyboard input and screen output

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Chapter 3 Flow of Control

• Flow of control is the order in which a program
  performs actions.
• So far, all our programs are sequential they
  execute the same statements for every run of the
  program
• public class MyIncome
• public static void main(String args)
• int monthlyPay, annualPay, expense, netIncome
• monthlyPay 9999
• expense 8888
• annualPay monthlyPay 12
• netIncome annualPay - expense
• System.out.println(My monthly pay is "
  monthlyPay)
• System.out.println(My annual pay is "
  annualPay)
• System.out.println(My net income is "
  netIncome)

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Chapter 3 Flow of Control

• For general problem solving we need more
  capabilities
• The ability to control which statements are
  executed
• The ability to control how often a statement is
  executed
• A branching statement chooses between two or more
  possible actions.
• A loop statement repeats an action until a
  stopping condition occurs.

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This Lecture

• We will concentrate first on controlling which
  statements are executed
• Java provides the if and switch conditional
  constructs to control whether a statement list is
  executed
• The if constructs use boolean expressions to
  determine their course of action

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This Lecture Branching Statements

• Boolean expressions
• The if Statement
• if
• if-else
• if-else-if
• The switch Statement
• Conditional Operator

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This Lecture Branching Statements

• Boolean expressions
• The if Statement
• if
• if-else
• if-else-if
• The switch Statement
• Conditional Operator

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Boolean expressions

• A boolean expression has either the value true or
  false. It can be a
• boolean variable
• variables or expression connected by comparison
  operators
• boolean variables or expressions connected by
  logical operators

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Boolean expressions Comparison Operators
Note there is no space between the two
component operators of , !, gt, lt
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Boolean expressions Logical Operators

• not operator !
• !(Boolean_Expression)
• !(x lt 0)
• and operator
• (Sub_Expression_1) (Sub_Expression_2)
• ((score gt 90) (grade lt 100))
• Parentheses often are used to enhance
  readability.
• The whole expression is true only when both of
  the smaller expressions are true.
• or operator
• (Sub_Expression_1) (Sub_Expression_2)
• ((quantity gt 5) (cost lt 10))
• The whole expression is true if at least one of
  the smaller expressions is true.

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Boolean expressions Evaluation

• Suppose
• boolean p true
• boolean q false
• boolean r true
• What is the value of
• p q p q
• !r p r
• q ! r p r

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Boolean expressions Evaluation

• Suppose
• int i 1
• int j 2
• int k 2
• char c ''
• char d ''
• char e ''
• What is the value of
• j k i ! k
• i lt j i lt k
• c e d ! e

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Boolean expressions Character comparisons

• Character comparisons are based on their Unicode
  values
• Characters 0, 1, 9 have expected order
• Character 0 has the encoding 48
• Character 1 has the encoding 49, and so on.
• Upper case letters A, B, Z have expected
  order
• Character A has the encoding 65, character B
  has the encoding 66, and so on.
• Lower case letters a, b, z have expected
  order
• Character a has the encoding 97
• Character b has the encoding 98, and so on.

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Boolean expressions Evaluation

• Suppose
• char c '2'
• char d '3'
• char e '2'
• What is the value of
• c lt d
• c lt e
• c lt e
• d gt e

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Operator precedence revisited

• Highest to lowest
• Parentheses
• Unary operators , -, , --, !
• Multiplicative operators , /,
• Additive operators -
• Relational ordering lt, lt, gt, gt
• Relational equality , !
• Logical and
• Logical or
• Assignment

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Branching Statements Outline

• Boolean expressions
• The if Statement
• if
• if-else
• if-else-if
• The switch Statement
• Conditional Operator

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Basic if Statement

• Syntax
• if (Expression)
• Action
• If the Expression is true then execute Action
• Action is either a single statement or a group of
  statements within braces
• For us, it will always be a group of statements
  within braces

can be any statement, in particular it can be
another if statement
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Example
Is our number negative?

• if (value lt 0)
• value -value

value lt 0
true
false
value -value
Our number is now definitely nonnegative.
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Why we always use braces

• What is the output?
• int m 5
• int n 10
• if (m lt n)
• m
• n
• System.out.println(" m " m " n " n)
• m 6 n 11

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Example Sorting Two Values

• // rearrange numbers if necessary
• if (value2 lt value1)
• // values are not in sorted order
• int rememberValue1 value1
• value1 value2
• value2 rememberValue1
• // display values
• System.out.println("The numbers in sorted order
  are "
• value1 " and then " value2)

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Example Sort two values
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This Lecture Branching Statements

• Boolean expressions
• The if Statement
• if
• if-else
• if-else-if
• The switch Statement
• Conditional Operator

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if-else Statement

• Syntax
• if (Expression)
• Action1else Action2
• If Expression is true then execute Action1,
• otherwise execute Action2
• The actions are either a single statement or a
  list of statements within braces

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Example Finding the Maximum of Two Values

• int maximum
• if (value1 lt value2) // is value2 larger?
• maximum value2 // yes value2 is larger
• else // (value1 gt value2)
• maximum value1 // no value2 is not
  larger
• System.out.println("The maximum of " value1
• " and " value2 " is " maximum)

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Example Finding the Maximum of Two Values
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Example Using for Floating-point Values

• Consider
• double a 1
• double b 0.1 0.1 0.1 0.1 0.1 0.1
  0.1 0.1 0.1 0.1
• if (a b)
• System.out.println("a b")
• else
• System.out.println("a ! b")

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Example Using for Floating-point Values

• a ! b
• double a 1
• double b 0.1 0.1 0.1 0.1 0.1 0.1 0.1
  0.1 0.1 0.1
• Problem lies with the finite precision of the
  floating-point types
• Instead, test for closeness with floating-point
  values
• Math.abs(a b) lt smallNumber

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Example Using for objects

• Consider
• System.out.print("Enter a string ")
• String s1 keyboard.next()
• System.out.print("Enter another string ")
• String s2 keyboard.next()
• if (s1 s2)
• System.out.println("Same")
• else
• System.out.println("Different")

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Example Using for objects

• When it is executed
• System.out.print("Enter a string ")
• String s1 keyboard.next()
• System.out.print("Enter another string ")
• String s2 keyboard.next()
• Memory looks like
• As a result no matter what is entered s1 and s2
  are not the same
• They refer to different objects

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Example Using for objects

• Question how do we test the equality of two
  strings?

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Using

• is appropriate for determining if two integers
  or characters have the same value.
• if (a 3)
• where a is an integer type
• is not appropriate for determining if two
  floating points values are equal. Use lt and
  some appropriate tolerance instead.
• if (abs(a - b) lt epsilon)
• where a, b, and epsilon are floating point types

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Using , cont.

• is not appropriate for determining if two
  objects have the same value.
• To test the equality of objects of class String,
  use method equals, or equalsIgnoreCase
• s1.equals(s2)
• s2.equals(s1)
• (Hello.equalsIgnoreCase(hello))

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Example Compare Strings

• Strings can be compared based on their
  lexicographic order
• Lexicographic order is similar to alphabetical
  order, but is it based on the order of the
  characters in the ASCII (and Unicode) character
  set.
• All the digits come before all the letters.
• All the uppercase letters come before all the
  lower case letters.

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Example Compare Strings

• Strings comparison
• lt, gt not allowed
• using method compareTo
• String_1.compareTo(String_2)
• Method compareTo returns
• a negative number if String_1 precedes String_2
• zero if the two strings are equal
• a positive number of String_2 precedes String_1.
• String s1 Hello
• String s2 hello
• System.out.println(s1.compareTo(s2))

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This Lecture Branching Statements

• Boolean expressions
• The if Statement
• if
• if-else
• if-else-if
• The switch Statement
• Conditional Operator

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If-else-if

• Consider
• if (number 0)
• System.out.println("zero")
• else
• if (number gt 0)
• System.out.println("positive")
• else
• System.out.println("negative")

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If-else-if

• Better
• if (number 0)
• System.out.println("zero")
• else if (number gt 0)
• System.out.println("positive")
• else
• System.out.println("negative")

Same results as previous segment but this
segment better expresses the meaning of what is
going on
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Example Sorting Three Values

• For sorting values n1, n2, and n3 there are six
  possible orderings
• n1 n2 n3
• n1 n3 n2
• n2 n1 n3
• n2 n3 n1
• n3 n1 n2
• n3 n2 n1
• Suppose s1, s2, s3 are the sorted version of n1,
  n2, and n3

Try this yourself
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This Lecture Branching Statements

• Boolean expressions
• The if Statement
• if
• if-else
• if-else-if
• The switch Statement
• Conditional Operator

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Switch statement

• Software engineers often confronted with
  programming tasks where required action depends
  on the values of integer expressions
• The if-else-if construct can be used
• Separately compare the desired expression to a
  particular value
• If the expression and value are equal, then
  perform the appropriate action
• Because such programming tasks occur frequently
• Java includes a switch statement
• The task is often more readable with the switch
  then with the if-else-if
• The switch statement is a mutltiway branch that
  makes a decision based on an integral (integer or
  character) expression.

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Switch statement
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The switch Statement, cont.

• The action for each case typically ends with the
  word break.
• The optional break statement prevents the
  consideration of other cases.
• The switch expression can be anything that
  evaluates to an integral type.

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Example Processing a request

• Scanner keyboard new Scanner(System.in)
• System.out.print("Enter a number ")
• int n1 keyboard.nextInt()
• System.out.print("Enter another number ")
• int n2 keyboard.nextInt()
• System.out.print("Enter desired operator ")
• char operator keyboard.next().charAt(0)
• switch (operator)
• case '' System.out.println(n1 n2) break
• case '-' System.out.println(n1 - n2) break
• case '' System.out.println(n1 n2) break
• case '/' System.out.println(n1 / n2) break
• default System.out.println(Illegal request)

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Example Testing for Vowel-ness

• switch (ch)
• case 'a'
• case 'A'
• case 'e'
• case 'E'
• case 'i'
• case 'I'
• case 'o'
• case 'O'
• case 'u'
• case 'U' System.out.println("vowel)
• break
• default System.out.println("not a vowel)

Try to complete this as a program and run it
yourself!
The break causes an exiting of the switch
Default Handles all of the other cases
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This Lecture Branching Statements

• Boolean expressions
• The if Statement
• if
• if-else
• if-else-if
• The switch Statement
• Conditional Operator

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The Conditional Operator

• if (n1 gt n2)
• max n1
• else
• max n2
• can be written as
• max (n1 gt n2) ? n1 n2
• The ? and together are call the conditional
  operator or ternary operator.
• Not recommended

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The Conditional Operator, cont.

• The conditional operator is useful with print and
  println statements.
• System.out.print(You worked
• ((hours gt 1) ? hours hour))