Decision Control Structures

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Decision Control Structures

• Dr. Tim Margush
• University of Akron

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Goals

• Learn how to alter the flow of control using
  decision structures
• Understand how comparisons of numbers, strings,
  and objects can be used in decision structures
• Be able to use Boolean operators to create
  Boolean expressions

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If

• The basic decision structure is the if statement
• To do, or not to do that is the question
• Hamlet?
• Used to execute or skip a statement or block of
  statements depending on a Boolean expression

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Syntax of If

• if (expression) statement
• or
• if (expression)
• Statement list

true branch
agelt12?
false branch
price 0.75
Block Statement
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If Else

• The if statement allows an else clause
• Two roads diverged in a yellow wood, and sorry I
  could not travel both and be one traveler
• Frost
• If-else is used to select between two
  alternatives
• Choose exactly one of two execution paths

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Syntax of If-Else

• if (expression)
• statement
• else
• statement
• Either statement may be a block statement

true branch
false branch
student.isMale()?
student.reject()
student.accept()
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Java Selection Operator

• Java includes the selection operator
• Syntax expression ? value1 value2
• This evaluates to a single value
• Result is value1 if expression is true
• Result is value2 if expression is false
• Notice that the result is an expression
• max_xy xgty ? x y
• In this example, the value returned by the
  selection operator becomes an operand of the
  assignment operator.

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Comparisons

• Relational operators
• gt gt lt lt !
• Be careful not to confuse with
• Used to compare primitive data of the same type
• Coercion allows comparisons between compatible
  types
• Comparing objects (including strings) requires a
  method
• Comparing object variables using the above
  operators actually compares their references, not
  the objects!

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Comparing Floating Point Types

• Warning! Data of type float and double are
  approximations of real numbers
• float x10, y1if (y/x 0.1) y/x might not
  be exactly 0.1
• Rewrite these comparisons as a difference being
  smaller than a threshold value
• if (Math.abs( y/x 0.1) lt 1e-6)
• The threshold value depends on the application
  (and the magnitude of the values)

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Comparing Strings

• Object comparisons require methods
• if (aName.equals("Fred"))
• True when the String aName has value "Fred"
• Use .equalsIgnoreCase to disregard upper/lower
  case differences
• if (description.compareTo("M")lt0)
• True if String description has a value that
  starts with a letter coming before "M"
• lt0 means less than, 0 means equal, gt0 means
  greater than
• Use .compareToIgnoreCase if desired

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Lexicographic Order

• Lexicographic ordering
• A "Computer Science"
• B "Computing Concepts"
• e comes before i so A lt B
• Ordering of letters is based on the Unicode value
  for each character

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Object References

• String one "Fr""ed"
• String two "Fred"
• one two ?
• one.equals(two) ?
• one two
• one two ?
• Comparing object variables is simply comparing
  their references

one
two
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null Object References

• String x
• This object variable has not been given a String
  object to point to yet is it initialized?
• The null value is often used to indicate this
  situation
• x null //no object yet
• You may compare an object variable to null to
  determine if an object is referenced
• if (x ! null) //do something with Object x

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Options, Options, Options?

• Choosing between two fundamentally different
  selection structures
• Are the choices mutually exclusive or can more
  than one alternative occur?

• "If I find fifty righteous people in the city of
  Sodom,
• I will spare the whole place for their sake."
• "If I find forty-five there," he said,
• "I will not destroy it."
• "What if only forty are found there?" He said,
• "For the sake of forty, I will not do it."
• "What if only thirty can be found there?" He
  answered,
• "I will not do it if I find thirty there."
• "What if only twenty can be found there?" He
  said,
• "For the sake of twenty, I will not destroy it."
  
• "What if only ten can be found there?" He
  answered,
• "For the sake of ten, I will not destroy it."
• -Genesis 18

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Multiway Selection

• Some/None of Many
• if (condition1)
• action1
• if (condition2)
• action2
• if (condition3)
• action3
• A sequence of if statements

• Exactly One of Many
• if (condition1)
• action1
• else if (condition2)
• action2
• else if (condition3)
• else
• default_action
• A single if statement

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Sequence of If's

• Exponential number of execution paths
• n statements yields 2n possible paths
• Use only when logic allows multiple actions to be
  processed

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Nested If's

• Linear number of execution paths
• n nested statements yields n1 possible paths
• Use when logic requires 1 action (or none) to be
  processed
• Final else is optional

condition1?
action1
condition2?
action2
condition3?
action3
default_action
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Dangling Else

• Nesting an if inside an if-else causes the
  dangling else problem
• if (condition1)
• if (condition2)
• action1
• else
• action2

• Java follows this rule
• Each else is matched with the closest unmatched
  if above it in the code
• Thus the else matches with the nested if
  statement
• The indentation is misleading as action 2 is
  executed only if condition1 is true and
  condition2 is false

if (condition1) if (condition2)
action1 else action2
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Dangling Else

• Avoiding the dangling else
• if (condition1)
• if (condition2)
• action1
• else
• action2

• Use a block statement to properly connect the
  else with the outer if thus avoiding the
  dangling-else problem

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Multiway Selection via Switch

• Useful when the decision is based on an integer
  value in a limited range
• Notice the break statement
• A break causes the flow of control to immediately
  exit the current block structure

• switch (intValue)
• case 2 statement break
• case 5 statement break
• case 9 statement break
• default statement
• Note cases can have more than one statement
  after the case label and before the break
  statement a block statement is not required.

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Enumerated Types

• Used to represent a limited range of discrete
  data
• Rather than using integer codes such as 1, 2, 3
• public class Person
• public enum AgeGroup INFANT, CHILD, TEEN,
  ADULT
• private AgeGroup ag null
• public AgeGroup getAgeGroup return ag
• //a client might use this as follows
• if (p.getAgeGroup() Person.INFANT)

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Boolean Expressions

• Evaluate to a Boolean value
• Boolean values are true or false
• Example Boolean expressions
• Predicate methods return a boolean value
• name.equals("Fred")
• Comparisons calculate a boolean result
• 10 lt age

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Boolean Operators

• p and q
• Written p q
• True when both p and q are true
• p or q
• Written p q
• True when at least one is true
• not p
• Written !p
• True when p is false

• Examples
• agegt12 agelt20
• Character.isDigit(x) x!'0'
• name null name.equals("")
• alt0 agt9
• !fred.isHome()
• !(alt4 bgt7)

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Lazy Evaluation

• Logical expressions involving a sequence of 's
  or 's are evaluated left to right
• If the entire result can be determined before all
  of the sub-expressions are evaluated, then
  evaluation stops early
• This is also called short circuit evaluation
• agt0 alt9 short circuits to false if a is -3
• alt0 agt9 short circuits to true if a is -3

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Boolean Variables

• Data type is boolean (lowercase b)
• boolean done false
• Store result of a boolean expression
• done Character.isDigit(x)
• done x0
• Use without a comparison operator
• if (done) not if (donetrue)
• if (!done) not if (donefalse)

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DeMorgan's Law

• Negating 's results in 's
• Negating 's results in 's
• !(a xlt7) is the same as !a xgt7
• !(xlt0 xgt9) is the same as !(xlt0) !(xgt9)
  which is also xgt0 xlt9

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Summary

• The basic decision structure in Java is specified
  using if and optionally else
• The block statement is a group of statements that
  syntactically act as a single statement
• Conditions control the flow of execution
• Comparisons between primitive data (numbers,
  characters, and object references)
• Comparisons between objects (via a method)
• Boolean variables and expressions