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Conditions, Logical Expressions,and Selection Control Structures

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Topics Using Relational and Logical Operators to

Construct and Evaluate Logical Expressions If-Then-Else Statements If-Then Statements Nested If Statements for Multi-way Branching

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Flow of Control

the order in which program statements are executed

WHAT ARE THE POSSIBILITIES. . .

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Flow of Control

is Sequential unless a “control structure” is used to change that

there are 2 general types of control structures:

Selection (also called branching)

Repetition (also called looping)

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C++ control structures

Selectionifif . . . else

Repetitionfor loopwhile loopdo . . . while loop

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Control Structuresuse logical expressions which may include:

6 Relational Operators

< <= > >=

=== !==

3 Logical Operators

! && ||

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=== vs ==The identity (===) operator behaves identically to the equality (==) operator except no type conversion is done, and the types must be the same to be considered equal.Reference: Javascript Tutorial: Comparison OperatorsThe == operator will compare for equality after doing any necessary type conversions. The ===operator will not do the conversion, so if two values are not the same type === will simply return false. It's this case where === will be faster, and may return a different result than ==. In all other cases performance will be the same.To quote Douglas Crockford's excellent JavaScript: The Good Parts,JavaScript has two sets of equality operators: === and !==, and their evil twins == and !=. The good ones work the way you would expect. If the two operands are of the same type and have the same value, then === produces true and !== produces false. The evil twins do the right thing when the operands are of the same type, but if they are of different types, they attempt to coerce the values. the rules by which they do that are complicated and unmemorable. These are some of the interesting cases:'' == '0' // false0 == '' // true0 == '0' // true false == 'false' // falsefalse == '0' // true false == undefined // falsefalse == null // falsenull == undefined // true ' \t\r\n ' == 0 // trueThe lack of transitivity is alarming. My advice is to never use the evil twins. Instead, always use ===and !==. All of the comparisons just shown produce false with the === operator.

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are used in expressions of form:

ExpressionA Operator ExpressionB

temperature > humidity

B * B - 4.0 * A * C > 0.0

abs (number ) === 35

initial !== ‘Q’

6 Relational Operators

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var x, y ;x = 4;y = 6;

EXPRESSION VALUEx < y

truex + 2 < y

falsex !== y

truex + 3 >= y

truey === x

falsey === x+2

true

IF / ELSE Statement

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if ( Expression ) {StatementA

}else {

StatementB }

NOTE: StatementA and StatementB each can be a single statement, a null statement, or a block.

If-Then-Else Syntax

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if ... else provides two-way selection

between executing one of 2 clauses (the if clause or the else clause)

TRUE FALSE

if clause else clause

expression

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if ( i < 0 ) {println( i + “ is negative”);

}else {

println( i + “ is non-negative”);}

If-Then-Else Example

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Use of blocks required

if ( Expression ) {

}else{

}

“if clause”

“else clause”

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If-Then-Else for a mail order

Assign value .25 to discountRate and assign value 10.00 to shipCost if purchase is over 100.00

Otherwise, assign value .15 to discountRate and assign value 5.00 to shipCost

Either way, calculate totalBill

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Solution

if ( purchase > 100.00 ) { discountRate = 0.25 ;

shipCost = 10.00 ;}else {

discountRate = .15 ; shipCost = 5.00 ;

}

totalBill = purchase * (1.0 - discountRate) + shipCost ;

IF / THEN Statement

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If-Then statement is a selection

of whether or not to execute a statement (which can be a single statement or an entire block)

TRUE

FALSEstatement

expression

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if ( Expression ) {

Statement}

If-Then Syntax

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Write If-Then or If-Then-Else for each

If taxCode is ‘T’, increase price by adding taxRate times price to it.

If code has value 1, calculate and display taxDue as their product.

If A is strictly between 0 and 5, set B equal to 1/A, otherwise set B equal to A.

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Some Answers

if (taxCode === ‘T’) {price = price + taxRate * price;

}

if ( code === 1){

taxDue = income * taxRate;println( taxDue);

}

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Remaining Answer

if ( ( A > 0 ) && (A < 5) ) {B = 1/A;

}else {

B = A;}

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Both the if clause and the else clause

of an if...else statement can contain any kind of statement, including another selection statement.

Nested IF Statements

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Multi-alternative Selection

is also called multi-way branching, andcan be accomplished by using NESTED if statements.

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if ( Expression1 ) { Statement1

}else if ( Expression2 ) {

Statement2}

.

.else if ( ExpressionN ) {

StatementN}else { Statement N+1

}

EXACTLY 1 of these statements will be executed.

Nested if Statements

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Nested if StatementsEach Expression is evaluated in sequence, until

some Expression is found that is true.

Only the specific Statement following that particular true Expression is executed.

If no Expression is true, the Statement following the final else is executed.

Actually, the final else and final Statement are optional. If omitted, and no Expression is true, then no Statement is executed.

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if ( creditsEarned >= 90 ) {

println(“SENIOR STATUS “); }else if ( creditsEarned >= 60 ) {

println(“JUNIOR STATUS “);

}else if ( creditsEarned >= 30 ) {

println(“SOPHOMORE STATUS “);

}else {

println(“FRESHMAN STATUS “); }

Multi-way Branching

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Writing Nested if Statements

Display one word to describe the int value of number as “Positive”, “Negative”, or “Zero”

Your city classifies a pollution index less than 35 as “Pleasant”, 35 through 60 as “Unpleasant”, and above 60 as “Health Hazard.” Display the correct description of thepollution index value.

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One Answer

if (number > 0){ println( “Positive”);}else if (number < 0) {

println( “Negative”);}else {

println( “Zero”);}

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Other Answerif ( index < 35 ) {

println( “Pleasant”);}else if ( index <= 60 ) {

println( “Unpleasant”);}else {

println( “Health Hazard”);}

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Operator Meaning Associativity

! NOT Right*, / , % Multiplication, Division, Modulus Left+ , - Addition, Subtraction Left

< Less than Left<= Less than or equal to Left> Greater than Left>= Greater than or equal to Left=== Is equal toLeft!== Is not equal to Left&& AND Left|| OR Left= Assignment Right

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LOGICAL EXPRESSION MEANING DESCRIPTION

! p NOT p ! p is false if p is true! p is true if p is false

p && q p AND q p && q is true ifboth p and q are

true. It is false otherwise.

p || q p OR q p || q is true if eitherp or q or both are

true. It is false otherwise.

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var age ; var isSenior, hasFever ;var temperature ;

age = 20;temperature = 102.0 ;isSenior = (age >= 55) ; // isSenior is falsehasFever = (temperature > 98.6) ; // hasFever is true

EXPRESSION VALUE

isSenior && hasFever false

isSenior || hasFever true

! isSenior true

! hasFever false

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What is the value?

var age, height;

age = 25;height = 70;

EXPRESSION VALUE

!(age < 10) ?

!(height > 60) ?

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Write an expression for each

taxRate is over 25% and income is less than $20000

temperature is less than or equal to 75 or humidity is less than 70%

age is over 21 and age is less than 60

age is 21 or 22

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Some Answers

(taxRate > .25) && (income < 20000)

(temperature <= 75) || (humidity < .70)

(age > 21) && (age < 60) (age == 21) || (age == 22)

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Use Precedence Chart var number ;var x ;

number !== 0 && x < 1 / number

/ has highest priority< next priority!== next priority&& next priority

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What went wrong?This is only supposed to display “HEALTHY AIR” if

the air quality index is between 50 and 80.

But when you tested it, it displayed “HEALTHY AIR” when the index was 35.

int AQIndex ;AQIndex = 35 ;if (50 < AQIndex < 80) {

println( “HEALTHY AIR“ ;}

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Analysis of Situation

AQIndex = 35;

According to the precedence chart, the expression

(50 < AQIndex < 80) means

(50 < AQIndex) < 80 because < is Left Associative

(50 < AQIndex) is false (has value 0)

(0 < 80) is true.

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Corrected Version

var AQIndex ;AQIndex = 35 ;

if ( (50 < AQIndex) && (AQIndex < 80) ) {

println( “HEALTHY AIR“) ; }

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Mouse Functions mouseIsPressed

boolean function (use in if)

mouseButton (LEFT/RIGHT/CENTER) if (mouseIsPressed) { if (mouseButton === LEFT) { stroke(255); } else { stroke(0); } }

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Key Functions

keyIsPressed

key char inputted

keyCode ASCII code of char

inputted UP/DOWN LEFT/RIGHT

if (keyIsPressed) {if (keyCode === UP) {

fill(255, 0, 0); } else { fill(255, 255, 255); }}//==============================fill(255, 0, 0);textSize(450);

var draw = function() { background(255); if (keyIsPressed) { text(key, 75, 325); }};