CS101 - Introduction to Computing - Lecture Handout 23

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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

FLOW CONTROL

  • if … else
  • switch

FLOW CONTROL 1

Compound Statements

  • At times, we need to put multiple statements at places where JavaScript expects only oneFor those situations, JavaScript provides a way of grouping a number of statements into a

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” ;
}

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

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 ;

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 … else: Example 6

Example 6

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 … else: Example 7

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

LOOPS

}
Switch Example 2
LOOPS
Loop through a set of statements as long as a condition is true

LOOPS

Decimal to Binary Conversion in JavaScript

Decimal to Binary Conversion in JavaScript

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

Example 1

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 ) ;
}

for: Example 3

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

for: Example 4

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

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