CS 303E Lecture 6 Selection / Conditionals: if and if-else statements

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CS 303E Lecture 6Selection / Conditionalsif
and if-else statements
As soon as questions of will or decision or
reason or choice of action arise, human science
is at a loss.Noam Chomsky
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Linear flow of control

• In method button clicked, all we have seen so far
  is one statement after another
• The program must execute each statement in the
  order written. It cant decide what to do next.
• radius radiusField.getNumber()
• area 3.14 radius radius
• areaField.setNumber (area)
• areaField.setPrecision(2)

From CircleArea
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Branching making decisions

• If the condition is true, execute statement_1
• otherwise, execute statement_2
• The condition is an expression that can be true
  or false, such as x gt 0

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Branching example

• An example for a checking account
• The account gets interest or a penalty depending
  on whether the balance is over 0.

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Empty false branch

• If the condition is true, execute statement
• otherwise, do nothing
• Condition is an expression that can be true or
  false.

false
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Syntax -- if Statement

• if (condition) // one-way branch
• statement1
• statement2
• statement3
• if (condition) // two-way branch
• statement
• else
• statement

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if Statement Examples

• if (sales gt 5000)
• commission commission 1.1
• if (balance gt 0)
• interest balance INT_RATE
• balance balance interest
• else
• balance balance - penalty

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Compound Statements

• Often, one statement isnt enough.
• So use a compound statement
• statement
• . . . zero or more statements
• statement between braces

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if with Compound Statements

• if (condition) if (condition)
• statement statement
• . . . . . .
• statement statement
• else
• statement
• . . .

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Example

• // Pay at payRate with time and one half for
• // overtime
• pay hoursWorked payRate
• if (hoursWorked gt 40)
• overtime hoursWorked - 40
• pay pay overtime (payRate / 2)

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Conditions orBoolean Expressions

• Condition is another name for boolean expression
  -- an expression that evaluates to true or false.
• One form of condition is
• expression relop expression
• where relop is a relational operator.

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Boolean Relational Operators

• Math Name Java Java Example
• equal balance 0
• ? not equal ! answer ! y
• gt greater than gt expenses gt income
• ? greater than or equal gt points gt 60
• lt less than lt pressure lt max
• ? less than or equal lt expenses lt income

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Boolean Expression Examples

• count 1 gt a b
• far - near gt 10
• Relational operators have lower precedence
• than arithmetic operators. The expressions are
  evaluated before the comparison.
• Invalid
• a lt b lt c // syntax error
• a b c // syntax error

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Indentation Helps Programmers but Java Ignores it.

• if (hours gt 40)
• over hours - 40
• pay pay over (rate / 2)

All simple statements within another statement
should be indented further to the right than the
outer statement. The two assignment statements
are within the if. All statements in a sequence
of statements in a compound statement should be
indented the same amount, as the two assignment
statements are.
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What Does This Program Do?

• import java.awt.import BreezyGUI.import
  java.math.public class
• Class extends GBFrameIntegerField z
  addIntegerField
• (0,1,1,2,1)Button publiCaddButton("Sure",3,2,1,1
  )DoubleField Z
• addDoubleField(0,2,1,2,1)Button
  jaddButton("Yea, whatever",3,1
• ,1,1)int buttondouble integerFielddouble gint
  kpublic void
• buttonClicked(Button buttonObj)buttonz.getNumber
  ()if(buttonObj
• j)integerFieldMath.pow(0.5,
  button)Z.setNumber(integerField)
• else if(buttonObjpubliC)integerField(double)
  1/buttonZ.
• setNumber(integerField)public
• static void main(String args)Frame frmnew
  Class()frm.setSize(
• 300,200)frm.setVisible(true)
• A little hard to tell, isnt it?

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Multi-way Decisions

• Several possible paths

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Multi-way Decisions

• Assume outField is a TextField for display
• if (balance gt 0)
• outField.setText (Positive)
• else if (balance lt 0)
• outField.setText (Negative)
• else
• outField.setText (Zero)
• Compound statements can be used here too.

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Nested conditionals CircleAreaAndRadius (p. 76)
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• CircleAreaAndRadius -- Text p. 76.
• import java.awt.
• import BreezyGUI.
• public class CircleAreaAndRadius extends GBFrame
• Label radiusLabel addLabel
  ("Radius",1,1,1,1)
• DoubleField radiusField addDoubleField
  (0,1,2,1,1)
• Label areaLabel addLabel ("Area",2,1,1,1)
• DoubleField areaField addDoubleField
  (0,2,2,1,1)
• Button radiusButton
• addButton ("Compute Radius",3,1,1,1)
• Button areaButton addButton ("Compute
  Area",3,2,1,1)
• double radius, area

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• public void buttonClicked (Button buttonObj)
• if (buttonObj areaButton)
• radius radiusField.getNumber()
• area Math.PI radius radius
• areaField.setNumber (area)
• else
• area areaField.getNumber()
• if (area gt 0)
• radius Math.sqrt (area / Math.PI)
• radiusField.setNumber (radius)
• else
• messageBox ("Error The area must not
  be\n"
• "a negative number.")

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• public static void main (String args)
• Frame frm new CircleAreaAndRadius()
• frm.setSize (200, 150)
• frm.setVisible (true)

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CS 303E Lecture 7Selection II (and a little
design)
You know you've achieved perfection in design,
not when you have nothing more to add,but when
you have nothing more to take away.-Antoine de
Saint Exupery.
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Designing a Program

• 1. Design interface
• A Label and Field for each datum.
• One or more Buttons for computation.
• 2. Design computation
• Get input.
• Compute result.
• Display result.
• (There are many more complex patterns.)

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Step-Wise Refinement

• Or top-down implementation
• Start with a general outline.
• Refine each piece by adding detail.
• Refine each detail with more detail, as
  necessary.
• Keep the pieces separate -- dont mix them up.
• Book mentions waterfall. Not used anymore!!

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More on stepwise Refinement

• Dont follow the Analysis-Design-Implement-Integra
  te(Test) cycle blindly.
• Better to
• Design a little
• Code a little
• Test a little
• Repeat
• may not need to do this for small projects, but
  dont get use to it.

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Logical Operators --

• (and)
• boolean_expression may be
• boolean_expression boolean_expression
• Truth Table for the boolean and operator
• p and q are both boolean expressions
• p q p q
• false false false
• false true false
• true false false
• true true true

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Logical Operators --

• (or)
• boolean_expression may be
• boolean_expression boolean_expression
• Truth Table for the boolean and operator
• p and q are both boolean expressions
• p q p q
• false false false
• false true true
• true false true
• true true true

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Logical Operators -- !

• ! (not)
• boolean_expression may be
• ! boolean_expression
• Truth Table for the boolean and operator
• p is a boolean expression.
• p !p
• false true
• true false

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Type Boolean

• boolean b1, b2, b3
• int i 3, j 4, k 5
• b1 true
• b2 i lt j // value is true or false
• b3 j lt k
• private boolean isPrime (int number)
• . . .
• return false
• . . .
• return true

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Type Boolean

• Used in loops and if statements to control
  computation.
• if (b1)
• outField.setText(Yes!)
• Used with logical operators
• (AND) (OR) ! (NOT)

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Boolean Variables
Examples boolean chk a gt 0 (b lt 0 c lt
0) boolean dataOk false . . . dataOk
name.equals(inputName)
id.equals(inputId) numberField.isValid
() numberField.getNumber() gt 0
.
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Logical Operators Truth Tables
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Evaluation
exp1 exp2 . . . expn Evaluate left to
right, stop and return true when first exp is
true short circuit evaluation. if all are
false, return false. exp1 exp2 . . .
expn Evaluate left to right, stop and return
false when first exp is false short circuit
evaluation. if all are true, return true.
.
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Nested ifs and
if (cond1) if (cond2) . if
(condn) statement // Is equivalent
to if (cond1 cond2 . . condn)
statement The correct style depends on the
situation. Sometimes nested if better, sometime
compound boolean statement
.
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The switch Statement

• Another way to do some multiway selection is via
  the switch statement
• switch(expression)
• case literal 1
• group of statements
• break
• case literal 2
• group of statements
• break
• case literal n
• group of statements
• break
• default
• group of statements
• break

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The switch Statement

• Can be used in some cases as an alternative to
  multiple if - else if - else statements.
• Limited in its usefulness
• the controlling expresion must evaluate to an
  integer or a character, (another primitive data
  type.)
• The cases must be constant expressions. They may
  only be constants or literals.
• Missing break evaluation continues looking for a
  match

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The Assignment that Could Have Been

• Request
• Write a program to allow a user to play
  rock-paper-scissors against the computer. Keep a
  track of how many games each player wins and the
  number of ties
• Okay, Go.

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Analysis

• Questions for the requestor?

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Design

• GUI
• Program

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Implementation

• public void buttonClicked(Button buttonObj)