chapter 4: decision making with control structures and statements javascript - introductory
TRANSCRIPT
Chapter 4: Decision Making with Control Structures and Statements
JavaScript - Introductory
Previewing CartoonQuiz.html File
Section A:
Decision Making
Objectives
In this section, students will learn about:
• if statements
• if …else statements
• Nested if statements
• Switch statements
If Statements
• The process of determining the order in which statements execute in a program is called decision making or flow control
• The if statement is used to execute specific programming code if the evaluation of a conditional expression returns a value of true
• Syntax for the if statement is:if (conditional expression) {
statement(s);}
If Statements
• The if statement contains three parts: key word if, a conditional expression enclosed within parentheses, and executable statements
• The statement immediately following the if statement in Figure 4-2 can be written on the same line as the if statement itself
If Statements
• Command block refers to multiple statements contained within a set of braces
• When an if statement contains a command block, the statements in the block execute when the if statement’s condition evaluates to true
• Now, if the condition evaluates to false, both statements will be bypassed, since they are contained within a command block
If Statements
Comparison and Logical Operators with the If Statement
Output of CartoonQuiz1.html
If … Else Statements
• When using an if statement, include an else clause to run an alternate set of code if the conditional expression evaluated by the if statement returns a value of false
• An if statement that includes an else clause is called an if … else statement
• Consider the else clause a backup plan
• An if statement can be constructed without the else clause. However, the else clause can only be used with an if statement
Nested If and If…Else Statements
• An if statement contained within an if or if… else statement is called a nested if statement
• Similarly, an if…else statement contained within an if or if… else statement is called a nested if … else statement
• Use nested if and if…else statements to perform conditional evaluations in addition to the original conditional evaluation (for example:var numbr = 7; if (number >5)
if (number < 10) documnt.writeIn (
“The number is between t and 10.”);
Greeting Program with Nested If Statements
Modified Greeting Program with Nested If Statements
Switch Statements
• A switch statement controls program flow by executing a specific set of statements, depending on value of an expression
• The switch statement compares the value of an expression to a label contained within a switch statement
• If the value matches a particular label, then the statements associated with the label execute
• The labels within a switch statement are called case labels and mark specific code segments
Examples of Case Labels
Switch Statements
• A Case label consists of keyword case, followed by a literal value or variable name, followed by a colon
• A case label can be followed by a single statement or multiple statements
• Default label contains statements that execute when the value returned by a switch statement’s conditional expression does not match a case label; keyword is default followed by a colon
• Break statement is used to exit switch statements and other program control statements
Function Containing a Switch Statement
Greeting Program Using a Switch Statement
Section A: Chapter Summary
• Flow control is the process of determining the order in which statements are executed in a program
• The if statement is used to execute specific programming code if the evaluation of a conditional expression returns true
• A command block refers to multiple statements contained within a set of braces
• After an if statement’s condition evaluates true, either first statement following condition or command block following condition executes
Section A: Chapter Summary
• Statements following an if statement’s command or command block execute regardless of whether if statement’s conditional expression evaluates true or false
• The else clause runs an alternate set of code if the conditional expression evaluated by an if statement returns a value of false
• In an if…else construct, only one set of statements executes: either statement following if statement or statements following the else clause
Section A: Chapter Summary
• An if statement contained within another if statement is called a nested if statement
• The switch statement controls program flow by executing a specific set of statements; depending on the value returned by an expression
• Case labels within a switch statement mark specific code segments
Section A: Chapter Summary
• A Default label contains statements that execute when the value returned by switch statement’s conditional expression does not match a case label
• When a switch statement executes, the value returned by the conditional expression is compared to each case label in the order in which it is encountered
• A break statement is used to exit a switch statement
Section B:
Repetition
Objectives
In this section, students will learn about:
• while Statements
• do … while Statements
• for Statements
• for … in Statements
• with Statements
• continue Statements
While Statements
• Loop statement repeatedly executes a statement or a series of statements while a specific condition is true or until a specific condition becomes true
• The simplest type of loop statement is the while statement used for repeating a statement or series of statements as long as a given conditional expression evaluates true
• Each repetition of a looping statement is called an iteration
While Statements
• A Counter is a variable that increments or decrements with each iteration of a loop statement
• Often name counter variables count, counter, or something similar
• The following code is an example of a while statement: var count = 1; while (count <= 5) { document.writeIn(count);
++count; }
Output of a While Statement Using an Increment Operator
Output of a While Statement Using a Decrement Operator
While Statements
• while loop using decrementing counter variables var count = 10; while (count > 0) {
document.writeIn (count);--count; }
• Infinite loop is a situation in which a loop statement never ends because its conditional expression is never updated or false
var count = 1;while (count <= 10) {
alert (“The number is “ + count);}
While Statements
• The user must force a web browser that is caught in an infinite loop to close by pressing Ctrl+Alt+Delete to access Task List or Manager
Output of SpeedLimit.html
Do… While Statements
• Do … while statement executes a statement or statements once, then repeats execution as long as given conditional expression evaluates to true
• The syntax for do…while statement is as follows:do {
statement(s);} while (conditional expression);
• As with the while statement, include code that changes some part of conditional expression in order to prevent an infinite loop from occurring
Examples of a Do … While Statement
Days of Week Program in a Web Browser
For Statements
• Use the for statement to loop through code
• A for statement is used for repeating a statement or series of statements as long as a given conditional expression evaluates true
• The for statement performs essentially the same function as the while statement: if a conditional expression in for statement evaluates true, the for statement executes repeatedly until it evaluates false
For Statements
• When the JavaScript interpreter encounters a for loop, the following steps occur:– The initialization expression is started– The for loop’s condition is evaluated– If the condition evaluation in Step 2 returns a value of
true, then the for loop’s statements execute, Step 4 occurs
– The update statement in the for statement’s constructor is executed
• You can omit any of the three parts of for statement constructor, but must include semicolons to separate each section
A For Statement that Displays the Contents of an Array
Output of Fast Foods Program
Examples of a For Statement
Output of CartoonQuizFinal.html
For …In Statements
• for … in statement is a looping statement that executes the same statement or command block for all the properties within an object
• The syntax for the for… in statement is:for (variable in object) {
statement (s);}
• Unlike other loop statements, the for…in does not require a counter or any other type of code to control how the loop functions
A For … In Statement Printing the Names of Properties Within an Object
For …In Statements
• There is no set order or way to control how the properties in an object are assigned to a for…in statement’s variable
• One benefit of for…in statement is that it enumerates, or assigns an index to, each property in an object
Output of SportsCar.html
With Statements
• The with statement eliminates need to retype the name of an object when properties of same object are being referenced in a series
• The syntax for the with statement is as follows:with (object) {
statement(s);}
• The name of object you want to reference is placed within parentheses following the with keyword
Example of a With Statement Assigning Values to Object Properties
Continue Statements
• A Continue statement halts looping statement and restarts the loop with a new iteration
• Use the continue statement when you want to stop loop for current iteration, but want loop to continue with a new iteration
A For Loop with a Continue Statement
Output of SportsCar3.html
Section B: Chapter Summary
• A loop statement repeatedly executes statement as long as a specific condition is true or until a specific condition becomes true
• The while statement is used for repeating a statement or series of statements as long as given conditional expression evaluates true
• Each repetition of a looping statement is called an iteration
• A counter is a variable that increments with each iteration of a loop statement
Section B: Chapter Summary
• You must include code that tracks the progress of the while statement and changes the value produced by conditional expression once desired tasks done
• If a counter variable is beyond range of a while statement’s conditional expression, the while statement is bypassed completely
• In an infinite loop, a loop statement never ends because its conditional expression is never updated
Section B: Chapter Summary
• The do…while statement executes a statement or statements once, then repeats execution as long as given conditional expression is true The for statement is used for repeating a statement or series as long as a given conditional expression is true
• You can omit any of the three parts of the for statement constructor, but you must include the semicolon that separates each section
• The variable name in the for…in statement constructor holds an individual object property
Section B: Chapter Summary
• The for…in statement executes the same statement or command block for all the properties within an object
• The for…in statement enumerates, or assigns an index to, each property in an object
• The with statement eliminates the need to retype the name of an object when properties of same object are being referenced in a series
• The continue statement halts a looping statement and restarts the loop with a new iteration