Marketing Chapter 22

Virtual University Introduction to Computing

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© Copyright Virtual University of Pakistan

Lecture 23 Flow Control & Loops

(Web Development Lecture 8) During the last lecture we had a discussion on Data Types, Variables & Operators

• We found out about JavaScript data types

• About variables and literals

• We also discussed several operators supported by JavaScript

JavaScript Data Types

• JavaScript recognizes & distinguishes among the following types of values:

• Numbers

• Booleans

• Strings

• Undefined

Variables

• Variables give us the ability to manipulate data through reference instead of actual valueVariables are containers that hold values

Declaring Variables Although JavaScript allows variable declaration, it does not require it - except in the case when we want to declare a variable being local (more on local variables later in the course!)

JavaScript Variables are Dynamically Typed Any variable in JavaScript can hold any type of value, and the that type can change midway through the program

FLOW CONTROL Select between alternate courses of action depending upon the

evaluation of a condition

statement block 1

condition

True False

statement block 2

JavaScript Flow Control Structures

• if … else

• switch


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if: Example 1

if ( day == “Sunday” ) bhola = “Cool” ;

The condition enclosed in

semicolon

Set the value of the variable ‘bhola to ‘Cool’ if the ‘day’ is equal to ‘Sunday’

This was the case if we want to execute a single statement given that the condition is true What if we want to execute multiple statements in case the

if ( day == “Sunday” ) { bhola = “Cool” ; mood = “Great” ; clothing = “Casual” ; }

if: Example 2

These curly braces group the multiple statements into a single compound statement

Set the value of the variable ‘bhola to ‘Cool’, ‘mood’ to ‘Great’, and ‘clothing’ to ‘casual’ if the

if: Example 2

if ( day == “Sunday” ) { bhola = “Cool” ; mood = “Great” ; clothing = “Casual” ; } Set the value of the variable ‘status’ to ‘Cool’, ‘mood’ to ‘Great’, and ‘clothing’ to ‘casual’ if the ‘day’ is equal to ‘Sunday’

Note: No semicolon after the closing curly brace

Compound Statements

• At times, we need to put multiple statements at places where JavaScript expects only oneFor those situations, JavaScript

2. This is done simply by enclosing any number of statements within curly braces, { }NOTE: Although the statements within the block end in semicolons, the block itself doesn’t

if: Example 3

if ( (day == “Sunday”) || (day == “Saturday”) ) { bhola = “Cool” ; mood = “Great” ; clothing = “Casual” ; }


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if: Example 4

weekend = ( day == “Sunday” ) || ( day == “Saturday” ) ; if ( weekend ) { bhola = “Cool” ; mood = “Great” ; clothing = “Casual” ; }

What is the data type of the variable “weekend”?

We now know how to execute a statement or a block of statements given that the condition is true What if we want to include an alternate action as well, i.e. a statement or a block of statements to be executed in case the condition in not true

if … else: Example 1

if ( GPA >= 1.0 )

bhola = “Pass” ;

else

bhola = “Fail” ;


if ( GPA >= 1.0 ) {


}

Else



if ( GPA >= 1.0 ) {


mood = “Great” ;

} else


if ( GPA >= 1.0 ) {


mood = “Great” ;

} else {


mood = “Terrible” ; }


if ( grade == “A” ) points = 4.0 ;

if ( grade == “B” ) points = 3.0 ;

if ( grade == “C” ) points = 2.0 ;

if ( grade == “D” ) points = 1.0 ;

if ( grade == “F” ) points = 0.0 ;

What can we do to improve it?

This piece of code is correct, but not very efficient!


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if ( grade == “A” )

points = 4.0 ;

if ( grade == “B” )

points = 3.0 ;

if ( grade == “C” )

points = 2.0 ;

if ( grade == “D” )

points = 1.0 ;

if ( grade == “F” )

points = 0.0 ;

What can we do to improve it?

This piece of code is correct, but not very efficient!


if ( grade == “A” ) points = 4.0 ; else { if ( grade == “B” ) points = 3.0 ; else { if ( grade == “C” ) points = 2.0 ; else { if ( grade == “D” ) points = 1.0 ; else points = 0.0 ; } } }

switch: Example 1

switch ( grade ) { case “A” : points = 4.0 ; break ; case “B” : points = 3.0 ; break ; case “C” : points = 2.0 ; break ; case “D” : points = 1.0 ; break ; default : points = 0.0 ; }

The expression enclosed in parentheses is evaluated and matched with case labels

This is a case label

A colon following the case label is required

This ‘break’ statement is the exit point

The ‘default’ statement acts like the ‘else’ clause in the ‘if…else’ structure


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switch: Example 2

switch ( inquiry ) { case “apple” : document.write( “Apples are Rs 50/kg” ) ; break ; case “mangos” : document.write( “Mangos are Rs 90/kg” ) ; break ; case “grapes” : document.write( “Grapes are Rs 60/kg” ) ; break ; default : document.write( inquiry + “? Please retry!” ) ; }

if…else --?-- switch

• If the action to be taken of the value of a single variable (or a single expression), use ‘switch’

• When the action depends on the values of multiple variables (or expressions), use the ‘if...else’ structure


if ( ( GPA >= 1.0 ) && ( attendance >= 40 ) )


else {

if ( ( GPA >= 2.0 ) && ( attendance >= 36 ) ) bhola = “Probation” ;

else


}

Switch Example 2

LOOPS Loop through a set of statements as long as a condition

is true

condition

True

False

statement block

JavaScript’s Looping Structures

while

for

…

…


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x = 75 ; // x is the decimal number y = “” ; // y is the binary equivalent while ( x > 0 ) { remainder = x % 2 ; quotient = Math.floor( x / 2 ) ; y = remainder + y ; x = quotient ; } document.write( “y = ” + y ) ;

Decimal to Binary Conversion in JavaScript

The condition enclosed in parentheses

while: Example 2

while ( tankIsFull == false ) { tank = tank + bucket ; } document.write ( “Tank is full now” ) ;

while: Example 3

x = 1 ; while ( x < 6000 ) { document.write ( x ) ; x = x + 1 ; }

for: Example 1

for ( x = 1 ; x < 6000 ; x = x + 1 ) { document.write ( x ) ; }

Initial count Condition

Operation

for: Description (1)

• The ‘for’ loop starts by initializing the counter variable (which in this case is x)

• The initial value in this case is ‘1’, but can be any other positive or negative number as well

• Next the ‘for’ loop checks the condition. If the condition evaluates to a ‘true’ value, the ‘for’ loop goes through the loop once


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for: Description (2)

• After reaching the end of that iteration, the ‘for’ loop goes to the top once again, performs the operation, checks the condition

• If the condition evaluates to a ‘false’ value, the ‘for’ loop finishes looping

• Otherwise, the ‘for’ loop goes through the loop once again

• Repeat from step 4

for: Example 2

for ( x = 99 ; x < 6000 ; x = x + 1 ) { document.write ( x ) ; }

for: Example 3

for ( x = 6000 ; x > 0 ; x = x - 1 ) { document.write ( x ) ; }

How many iterations would this ‘for’ loop run for?

6000?


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During Today’s Lecture …

• We discussed the concept of flow control using the “if” and “switch” structures

• And also the concept behind the “while” and “for” looping structures

• We also solved simple problems using flow control and loop structures

Next (the 9th) Web Dev Lecture: Arrays

• We will find out why we need arrays

• We will become able to use arrays for solving simple problems

for: Example 3

for ( x = 6000 ; x > 0 ; x = x - 1 ) { document.write ( x ) ; }


6000? for: Example 4

for ( x = 6000 ; x < 0 ; x = x - 1 ) { document.write ( x ) ; }


None? for --?-- while

• When the exact number of iterations is known, use the ‘for’ loop

• ‘for’ loops become especially useful when used in conjunction with arrays We’ll find out about arrays next time, and we’ll probe their usefulness as part of ‘for’ loop structures

During Today’s Lecture …

• We discussed the concept of flow control using the “if” and “switch” structures

• And also the concept behind the “while” and “for” looping structures

• We also solved simple problems using flow control and loop structures

Next (the 9th) Web Dev Lecture: Arrays

• We will find out why we need arrays

• We will become able to use arrays for solving simple problems


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