Introduction to Control Statements JavaScript – Part 3 George Mason University June 3, 2010.

Introduction to Control Statements

JavaScript – Part 3

George Mason University

June 3, 2010

Selection Statements

if Statements

switch Statements

Conditional Operators

Simple if Statements

Boolean Expression

true

Statement(s)

false (radius >= 0)

true

area = radius * radius * PI; document.write("The area for the circle of " + "radius " + radius + " is " + area);

false

(A) (B)

if (booleanExpression) { statement(s);}

if (radius >= 0)

{ area = radius * radius * PI;

document.write("The area"

+ " for the circle of radius "

+ radius + " is " + area);

}

Note

if ((i > 0) && (i < 10)) { document.write("i is an " + + "integer between 0 and 10"); }

(a)

Equivalent

(b)

if ((i > 0) && (i < 10)) document.write("i is an " + + "integer between 0 and 10");

Outer parentheses required Braces can be omitted if the block contains a single statement

CautionCommon Logic Error

if (radius >= 0);

{

area = radius*radius*PI;

document.write(

"The area for the circle of radius " +

radius + " is " + area);

}

Wrong

The if...else Statementif (booleanExpression) { statement(s)-for-the-true-case;}else { statement(s)-for-the-false-case;}

Boolean Expression

false true

Statement(s) for the false case Statement(s) for the true case

if...else Example

if (radius >= 0) { area = radius * radius * 3.14159;

document.write(“The area for the “ + “circle of radius “ + radius + “ is “ + area);}else { document.write(“Negative input”);}

Multiple Alternative if Statements

if (score >= 90.0) grade = "A"; else if (score >= 80.0) grade = "B"; else if (score >= 70.0) grade = "C"; else if (score >= 60.0) grade = "D"; else grade = "F";

Equivalent

if (score >= 90.0) grade = "A"; else if (score >= 80.0) grade = "B"; else if (score >= 70.0) grade = "C"; else if (score >= 60.0) grade = "D"; else grade = "F";

Trace if-else statement

if (score >= 90.0) grade = "A";else if (score >= 80.0) grade = "B";else if (score >= 70.0) grade = "C";else if (score >= 60.0) grade = "D";else grade = "F";

Suppose score is 70.0 The condition is false



Suppose score is 70.0 The condition is true



Suppose score is 70.0 grade is C



Suppose score is 70.0 Exit the if statement

Note

The else clause matches the most recent if clause in the same block.

int i = 1; int j = 2; int k = 3; if (i > j) if (i > k) document.write("A"); else document.write("B");

(a)

Equivalent

(b)

int i = 1; int j = 2; int k = 3; if (i > j) if (i > k) document.write("A"); else document.write("B");

Note, cont.Nothing is printed from the preceding statement. To force the else clause to match the first if clause, you must add a pair of braces: i = 1;

j = 2;

k = 3;

if (i > j) {

if (i > k)

document.write("A");

}

else

document.write("B");

This statement prints B.

TIP

if (number % 2 == 0) even = true; else even = false;

(a)

Equivalent even = number % 2 == 0;

(b)

Note

if (even == true) document.write( "It is even.");

(a)

Equivalent if (even) document.write( "It is even.");

(b)

Example 3.1 Computing TaxesThe US federal personal income tax is calculated based on the filing status and taxable income. There are four filing statuses: single filers, married filing jointly, married filing separately, and head of household. The tax rates for 2002 are shown in Table 3.1.

Example 3.1 Computing Taxes, cont.if (status == 0) { // Compute tax for single filers}else if (status == 1) { // Compute tax for married file jointly}else if (status == 2) { // Compute tax for married file separately}else if (status == 3) { // Compute tax for head of household}else { // Display wrong status}

switch Statement Flow Chart

status is 0 Compute tax for single filers break

Compute tax for married file jointly break status is 1

Compute tax for married file separatly break status is 2

Compute tax for head of household break status is 3

Default actions default

Next Statement

switch Statementsswitch (status) { case 0: compute taxes for single filers; break; case 1: compute taxes for married file jointly; break; case 2: compute taxes for married file separately; break; case 3: compute taxes for head of household; break; default: document.write("Errors: invalid status");}

switch Statement Rules

switch (switch-expression) {

case value1: statement(s)1;

break;


break;

…

case valueN: statement(s)N;

break;

default: statement(s)-for-default;

}

The switch-expression must always be enclosed in parentheses.

switch Statement Rules

The keyword break is optional, but it should be used at the end of each case in order to terminate the remainder of the switch statement. If the break statement is not present, the next case statement will be executed.

switch (switch-expression) {


break;


break;

…

case valueN: statement(s)N;

break;

default: statement(s)-for-default;

}

The default case, which is optional, can be used to perform actions when none of the specified cases matches the switch-expression. The case statements are executed in sequential

order, but the order of the cases (including the default case) does not matter. However, it is good programming style to follow the logical sequence of the cases and place the default case at the end.

Trace switch statement

switch (ch) { case "a": document.write(ch); case "b": document.write(ch); case "c": document.write(ch);}

Suppose ch is "a":


switch (ch) { case 'a': document.write(ch); case "b": document.write(ch); case "c": document.write(ch);}

ch is "a":



Execute this line



Next statement;

Execute next statement


switch (ch) { case "a": document.write(ch); break; case "b": document.write(ch); break; case "c": document.write(ch);}

Suppose ch is "a":


switch (ch) { case 'a': document.write(ch); break; case "b": document.write(ch); break; case "c": document.write(ch);}

ch is 'a':


switch (ch) { case "a": document.write(ch); break; case "b": document.write(ch); break; case "c": document.write(ch);}

Execute this line


switch (ch) { case 'a': docuent.write(ch); break; case 'b': document.write(ch); break; case 'c': document.write(ch);}

Next statement;

Execute next statement

Example using Else If structure

exam1 = parseFloat(prompt("Enter exam 1 grade (0-100)")); // convert input to numberexam2 = parseFloat(prompt("Enter exam 2 grade (0-100)")); // convert input to number

average = (exam1 + exam2) /2; // need to have converted to number, otherwise string concatenation // here

if (average > 90) letterGrade = "A";else if (average > 80) letterGrade = "B";else if (average >70) letterGrade = "C";else if (average >60) letterGrade = "D";else letterGrade ="F";

document.write("Average: " + average + "Grade: " + letterGrade);}}

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Switch statement only tests for equality and thus one would have to do calculations to use a switch statement for the above problem

Conditional Operator

if (x > 0) y = 1else y = -1;

is equivalent to

y = (x > 0) ? 1 : -1;(booleanExpression) ? expression1 : expression2

Ternary operatorBinary operatorUnary operator

Conditional Operator

if (num % 2 == 0)

document.write(num + “ is even”);else document.write(num + “ is odd”);

document.write( (num % 2 == 0)? num + “ is even” : num + “ is odd”);

Conditional Operator, cont.

(booleanExp) ? exp1 : exp2

39

Arithmetic Expressions

)94

(9))(5(10

5

43

y

x

xx

cbayx

is translated to

(3+4*x)/5 – 10*(y-5)*(a+b+c)/x + 9*(4/x + (9+x)/y)

Introduction to Control Statements JavaScript – Part 3 George Mason University June 3, 2010.

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