CS101 Introduction to Computing 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 value Variables 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

JavaScript Operators JavaScript 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

comments let the code speak for itself!

Decimal to Binary Conversion in JavaScript 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 ) ;

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

Flow Control

Select between alternate courses of action depending upon the evaluation of a condition True False statement block 1 statement block 2

JavaScript Flow Control Structures if … else switch

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

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?

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

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

Compound Statements 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” ; }

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 … else: Example 2 if ( GPA >= 1.0 ) { bhola = “Pass” ; } else bhola = “Fail” ;

if … else: Example 3 if ( GPA >= 1.0 ) { bhola = “Pass” ; mood = “Great” ; } else bhola = “Fail” ;

if … else: Example 4 if ( GPA >= 1.0 ) { bhola = “Pass” ; mood = “Great” ; } else { bhola = “Fail” ; mood = “Terrible” ;

if … else: Example 5 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 ; This piece of code is correct, but not very efficient! What can we do to improve it?

Nested if … else Structures

if … else: Example 6 if ( grade == “A” ) points = 4.0 ; else { if ( grade == “B” ) points = 3.0 ; if ( grade == “C” ) points = 2.0 ; if ( grade == “D” ) points = 1.0 ; else points = 0.0 ; } if … else: Example 6

JavaScript Flow Control Structures if … else switch

switch: Example 1 switch ( grade ) { case “A” : points = 4.0 ; break ; case “B” : points = 3.0 ; case “C” : points = 2.0 ; case “D” : points = 1.0 ; default : points = 0.0 ; } switch: Example 1 A colon following the case label is required The expression enclosed in parentheses is evaluated and matched with case labels This is a case label This ‘break’ statement is the exit point The ‘default’ statement acts like the ‘else’ clause in the ‘if…else’ structure

switch: Example 2 switch ( inquiry ) { case “apple” : document.write( “Apples are Rs 50/kg” ) ; break ; case “mangos” : document.write( “Mangos are Rs 90/kg” ) ; case “grapes” : document.write( “Grapes are Rs 60/kg” ) ; default : document.write( inquiry + “? Please retry!” ) ; }

? if … else switch

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 … else: Example 7 if ( ( GPA >= 1.0 ) && ( attendance >= 40 ) ) bhola = “Pass” ; else { if ( ( GPA >= 2.0 ) && ( attendance >= 36 ) ) bhola = “Probation” ; else bhola = “Fail” ; }

Loops

Loop through a set of statements as long as a condition is true block True False

JavaScript’s Looping Structures while for …

Decimal to Binary Conversion in JavaScript The condition enclosed in parentheses 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 ) ;

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 ; }

JavaScript’s Looping Structures while for …

for: Example 1 x = 1 ; while ( x < 6000 ) { document.write ( x ) ; x = x + 1 ; } Initial count Condition Operation for ( x = 1 ; x < 6000 ; x = x + 1 ) { document.write ( x ) ; }

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

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?

for: Example 4 for ( x = 6000 ; x < 0 ; x = x - 1 ) { document.write ( x ) ; } How many iterations would this ‘for’ loop run for? None?

? for while

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

‘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