CS303E Selection:if and if-else statements 1

CS 303E

Lecture 6

Selection / Conditionals:

if and if-else statementsAs soon as questions of will or decisionor reason or choice of action arise, human science is at a loss.Noam Chomsky

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Linear flow of controlIn 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 can’t decide what to do next.

radius = radiusField.getNumber(); area = 3.14 * radius * radius; areaField.setNumber (area); areaField.setPrecision(2);

Statement_1 Statement_2 Statement_3

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 > 0

Statement_1

Statement_2

Condition

true

false

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Branching exampleAn example for a checking account:

The account gets interest or a penalty depending on whether the balance is over 0.

balance = balance + interest;

balance = balance – penalty;

balance > 0

true

false

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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

Statement

Condition

true

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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 Examplesif (sales > 5000)

commission = commission * 1.1;

if (balance > 0){

interest = balance * INT_RATE;balance = balance + interest;

}else{

balance = balance - penalty;}

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Compound StatementsOften, one statement; isn’t 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 > 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’

> greater than > expenses > income

greater than or equal >= points >= 60

< less than < pressure < max

less than or equal <= expenses <= income

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Boolean Expression Examples

count + 1 > a * b

far - near >= 10

Relational operators have lower precedencethan arithmetic operators. The expressions are evaluated before the comparison.

Invalid:

a < b < c // syntax error

a == b == c // syntax error

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Indentation Helps Programmers but Java Ignores it.

if (hours > 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 GBFrame{IntegerField z = addIntegerField

(0,1,1,2,1);Button publiC=addButton("Sure",3,2,1,1);DoubleField Z

=addDoubleField(0,2,1,2,1);Button j=addButton("Yea, whatever",3,1

,1,1);int button;double integerField;double g;int k;public void

buttonClicked(Button buttonObj){button=z.getNumber();if(buttonObj

==j){integerField=Math.pow(0.5, button);;;Z.setNumber(integerField)

;}else if(buttonObj==publiC){integerField=(double)1/button;Z.

setNumber(integerField);;}}public

static void main(String[] args){Frame frm=new Class();frm.setSize(

300,200);frm.setVisible(true);}}

A little hard to tell, isn’t it?

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Multi-way Decisions

Several possible paths:

display “Positive”

display “Negative”

balance > 0

true

false

balance < 0

true

display “Zero”

false

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Multi-way Decisions

Assume outField is a TextField for display:

if (balance > 0)outField.setText (“Positive”);

else if (balance < 0)outField.setText (“Negative”);

elseoutField.setText (“Zero”);

Compound statements can be used here too.

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Nested conditionals CircleAreaAndRadius (p. 76)

Compute area or radius?

Compute area from radius;Display area;area

radius > 0?

radiusCompute radius from area;Display radius;

Display error message;

yes

no

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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 >= 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 7:

Selection 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 -- don’t mix them up. Book mentions waterfall. Not used anymore!!

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More on stepwise Refinement Don’t follow the Analysis-Design-Implement-

Integrate(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 don’t get use to it.

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Logical Operators -- &&&& (and):

boolean_expression may be:

boolean_expression && boolean_expressionTruth Table for the boolean and operatorp and q are both boolean expressions

p q p && qfalse false falsefalse true falsetruefalse falsetruetrue true

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Logical Operators -- |||| (or):

boolean_expression may be:

boolean_expression || boolean_expressionTruth Table for the boolean and operatorp and q are both boolean expressions

p q p || qfalse false falsefalse true truetruefalse truetruetrue true

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Logical Operators -- !

! (not):

boolean_expression may be:

! boolean_expressionTruth Table for the boolean and operatorp is a boolean expression.

p !pfalse truetruefalse

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Type Boolean

boolean b1, b2, b3;int i = 3, j = 4, k = 5;b1 = true;b2 = i < j; // value is true or falseb3 = j <= 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 > 0 && (b <= 0 || c <= 0);

boolean dataOk = false;. . .dataOk = name.equals(inputName) && id.equals(inputId) && numberField.isValid() && numberField.getNumber() > 0;

.

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Logical Operators Truth TablesA B A || B A && B

true true true truetrue false true falsefalse true true falsefalse false false false

A ! Atrue falsefalse true

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Evaluationexp1 || 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)

{