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CS101 Introduction to Computing

Lecture 23Flow Control & Loops

(Web Development Lecture 8)

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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

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JavaScript Data Types

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

– Numbers

– Booleans

– Strings

– Undefined

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Variables

• Variables give us the ability to manipulate data through reference instead of actual value

• Variables are containers that hold values

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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!)

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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

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JavaScript OperatorsJavaScript has numerous operators, classified in many categories. We will look at only a few of them belonging to the following categories:

– Assignment operators– Arithmetic operators– Comparison operators– Logical operators– String operators

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comments let the code speak

for itself!

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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

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Today’s Lecture:Flow Control & Loops

• We’ll try to understand the concept of flow control using the “if” and “switch” structures

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

• We will solve simple problems using flow control and loop structures

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Flow Control

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Select between alternate courses of action depending upon the evaluation of a condition

statementblock 1

conditionTrue False

statementblock 2

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JavaScript Flow Control Structures

if … else

switch

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if: Example 1

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

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

The condition enclosed in parentheses semicolon

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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 condition is true?

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if: Example 2

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

}

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

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

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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

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Compound Statements

1. At times, we need to put multiple statements at places where JavaScript expects only one

2. For those situations, JavaScript provides a way of grouping a number of statements into a single statement, called a “statement block”

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Compound Statements

3. This is done simply by enclosing any number of statements within curly braces, { }

4. NOTE: Although the statements within the block end in semicolons, the block itself doesn’t

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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”?

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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

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if … else: Example 1

if ( GPA >= 1.0 )

bhola = “Pass” ;

else

bhola = “Fail” ;

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if … else: Example 2

if ( GPA >= 1.0 ) {

bhola = “Pass” ;

}

else

bhola = “Fail” ;

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if … else: Example 3

if ( GPA >= 1.0 ) {

bhola = “Pass” ;

mood = “Great” ;

}

else

bhola = “Fail” ;

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if … else: Example 4

if ( GPA >= 1.0 ) {

bhola = “Pass” ;

mood = “Great” ;

}

else {

bhola = “Fail” ;

mood = “Terrible” ;

}

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if … else: Example 5if ( 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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Nestedif … else

Structures

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if … else: Example 6

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 ;}

}}

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JavaScript Flow Control Structures

if … else

switch

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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 2switch ( 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!” ) ;}

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?if … elseif … else

switchswitch

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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

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if … else: Example 7

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

bhola = “Pass” ;

else {

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

else

bhola = “Fail” ;

}

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Loops

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Loop through a set of statements as long as a condition is true

conditionTrue

False

statementblock

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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

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while: Example 2

while ( tankIsFull == false ) {

tank = tank + bucket ;

}

document.write ( “Tank is full now” ) ;

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while: Example 3

x = 1 ;

while ( x < 6000 ) {

document.write ( x ) ;

x = x + 1 ;

}

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JavaScript’s Looping Structures

while

for

…

…

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for: Example 1

x = 1 ;while ( x < 6000 ) {

document.write ( x ) ;x = x + 1 ;

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

document.write ( x ) ;

}

Initial count Condition Operation

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for: Description (1)

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

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

3. 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)

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

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

6. Otherwise, the ‘for’ loop goes through the loop once again

7. Repeat from step 4

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for: Example 2

for ( x = 99 ; x < 6000 ; x = x + 1 ) {

document.write ( x ) ;

}

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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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for: Example 4

for ( x = 6000 ; x < 0 ; x = x - 1 ) {

document.write ( x ) ;

}

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

None?

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?for for

whilewhile

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for --?-- while

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

• When the number of iterations depend upon a condition being met, use the ‘while’ loop

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‘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

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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

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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