Variables: Local and Global Variables

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Introduction to Computing CS101

document.write( parseInt( string1 ) ) ;

document.write( " " ) ;

NaN

string2 = "$100.00" ;

document.write( parseInt( string2 ) ) ;

document.write( " " ) ;

string3 = " 23 " ;

document.write( parseInt( string3 ) ) ;

1. Parses the string argument; returns an integer

2. If it encounters a non-numeral - anything other than (+,-) or (0-9) - it ignores it and all succeeding

characters, and returns the integer value parsed up to that point

3. If the first character cannot be converted to a number, parseInt returns NaN

4. parseInt truncates numbers to integer values

5. Leading and trailing spaces are ignored

parseFloat( )

Syntax: parseFloat ( string )

string1 = "3.14159" ;

document.write( parseFloat( string1 ) ) ;

document.write( " " ) ;

string2 = "$100.00" ;

document.write( parseFloat( string2 ) ) ;

document.write( " " ) ;

string3 = " 23E-15 " ;

document.write( parseFloat( string3 ) ) ;

1. Parses the string argument; returns a FP number

2. If it encounters a character other than

A sign (+,-)

A numeral (0-9)

A decimal point

An exponent

it returns the value up to that point, ignoring that and all succeeding characters

3. If the first character cannot be converted to a number, parseFloat returns NaN

4. Leading and trailing spaces are ignored

29.6 Scope of Variable

Defining the space in which a variable is effective is known as

defining the scope of a variable. A variable can be either local or global in scope.

Local and Global Variables

Local or Function-level Variable

Effective only in the function in which they are declared

Global Variables

Visible everywhere on the Web page

Example

function factorial( n ) {

product = 1 ;

for ( k = 1 ; k <= n ; k = k + 1 ) {

product = product * k

}

return product ;

}

n = window.prompt( "Enter a number ", "" ) ;

document.write( "k = ", k ) ;

document.write( " " ) ;

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Introduction to Computing CS101

document.write(n, "! = ", factorial( n ) ) ;

function factorial( n ) {

product = 1 ;

for ( k = 1 ; k <= n ; k = k + 1 ) {

product = product * k

}

return product ;

}

n = window.prompt( "Enter a number ", "" ) ;

document.write( "k = ", k ) ;

document.write( " " ) ;

document.write(n, "! = ", factorial( n ) ) ;

function factorial( n ) {

product = 1 ;

10! = 3628800

for ( k = 1 ; k <= n ; k = k + 1 ) {

k = 11

product = product * k

}

return product ;

}

n = window.prompt( "Enter a number ", "" ) ;

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Introduction to Computing CS101

document.write(n, "! = ", factorial( n ) ) ;

document.write( " " ) ;

document.write( "k = ", k ) ;

function factorial( n ) {

var k ;

product = 1 ;

for ( k = 1 ; k <= n ; k = k + 1 ) {

product = product * k

}

return product ;

}

n = window.prompt( "Enter a number ", "" ) ;

document.write(n, "! = ", factorial( n ) ) ;

document.write( " " ) ;

document.write( "k = ", k ) ;

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Introduction to Computing CS101

`k' is a Local Variable

`k' is not declared or used in the main code

Instead, it is declared within the function `factorial' only

`k' i local to the `factorial' function, and does not hold any meaning outside that function

function factorial( n ) {

Here `product' has been made a local

var k, product ;

variable as well

product = 1 ;

for ( k = 1 ; k <= n ; k = k + 1 ) {

product = product * k

}

return product ;

}

n = window.prompt( "Enter a number ", "" ) ;

document.write(n, "! = ", factorial( n ) ) ;

document.write( " " ) ;

document.write( product ) ;

Local Variables

Declaring variables (using the var keyword) within a function, makes them local·They are available only

within the function and hold no meaning outside of it

Global Variables

All other variables used in a Web page (or window) are global

They can be manipulated from the main code as well as from any of the functions

They include:

All variables declared in the main code

All variables used but not declared in the main code

All variables used but not declared in any of the functions defined on the Web page (or window)

function writeList( heading, words ) {

document.write( heading + " " ) ;

for ( k = 0 ; k < words.length ; k = k + 1 ) {

Would the functionality

document.write( words[ k ] + " " ) ;

change if we delete the

argument `words' from these

}

4 places?

}

words = new Array ( 10 ) ;

for ( k = 0 ; k < words.length ; k = k + 1 ) {

words[ k ] = window.prompt( "Enter word # " + k, "" ) ;

}

writeList( "Unsorted Words:", words ) ;

words.sort( ) ;

writeList( "Sorted List:", words ) ;

words.reverse( ) ;

writeList( "Reverse-Sorted List:", words ) ;

Local vs- Global

Global variables can make the logic of a Web page difficult to understand

Global variables also make the reuse and maintenance of your code much more difficult

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Introduction to Computing CS101

Glob

Local

var a, b ;

p=q+2;

var u ;

document.write( m ) ;

r=s;

var c, d ;

x=y*y;

Variables declared within functions are local; all others global

HEURISTIC:

If it's possible to define a variable as local, do it!

During Today's Lecture ...

We looked at functions and their use for solving simple problems

We became familiar with a couple of JavaScript's built-in functions

We became familiar with the concept of local and global variables

Next Web Dev Lecture:

Event Handling

We'll learn to appreciate the concept of event driven programming

We will produce solutions for simple problems using various event handlers

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