Using Control Structures

Goals Understand the three basic forms of control structures Understand how to create and manage conditionals Understand how to change program execution flow based on conditionals Bring it! (Practice with conditionals)

Programming and Control Structures All programming languages support Control Structures Essentially, control structures give programmers a toolset for “controlling” what code gets executed, and how many times. There are three basic control structures: ◦ Sequential ◦ Branching ◦ Looping

Control Structures: Sequential Sequential Code – Unless otherwise indicated, programming instructions are executed in order… the first line of code, then the next, then the third. Sequential processing is the default control structure (ie, as a programmer, you get it without asking!)

Control Structures: Branching Branching structures allow the programmer to structure mutually exclusive blocks of code Based on the value of some conditional, either one block of code executes or another…. For example: If it’s raining, take an umbrella. Else, leave the umbrella at home.

Control Structures: Looping Looping structures allow the programmer to create repeating blocks of code. The code can repeat zero or many times, based on the value of some conditional. For example: Would you like to play a game? Game is played and ends, and the user is asked: Would you like to play again? Game is played and ends, and the user is asked: Would you like to play again…

Understanding Conditionals In a later presentation, we will look at branching and looping in detail, but for now, let’s flesh out the toolset of the conditional A conditional is a Boolean expresssion that can be used to control program execution. (And btw, a Boolean expression is one that can be evaluated as True or False) As it turns out, you already know quite a bit about conditionals!

“Concepts” to “Conditions” Consider the following statements: ◦ “It’s raining.” ◦ “Your shirt is blue.” ◦ “There are no more records in the file.” ◦ “The user clicked ‘YES’” All of these statements are Boolean expressions – we could figure out if each is a True or False statement On the other hand, an expression such as “x + 3” isn’t a Boolean. Even if you told me what x was, ‘x + 3’ doesn’t result in a True or False. Programmers construct conditionals to act as Guards in front of a section of code… if the conditional is TRUE, the guard let’s you pass into a section of code. If the conditional is FALSE, the guard blocks you from entering.

The Conditional as Sentry So… consider the following conditional: If (x > 3) {Do a bunch of fun stuff; Including more fun stuff; and even more fun stuff; and yati, yati, yati!!!} Else {Poke yourself in the eye with a sharp stick} The conditional (x>3) stands guard in front of the fun lines of code. If the expression turns out to be True (ie, x IS greater than 3), the sentry (ie, soldier) lets you pass into the fun code. This sentry analogy is so compelling that the variable in a conditional is referred to as the “sentinel” value. If the conditional is False, the sentinel blocks you from entering the fun code and yikes, you gotta get that stick…. Given how important the conditional is, it’s worth understanding in detail the power it offers a programmer.

Using the Relational Operator The relational (comparison) operator compares two values of THE SAME TYPE (i.e. – Strings are compared to strings, integers are compared to integers, etc.) There are several different types of comparison operators (see next slide) Comparison operators HAVE NO PRECEDENCE among themselves and are evaluated LEFT to RIGHT (however, parentheses can be added to alter precedence)

Available Relational Operators Comparison JavaScript Relational Operator Is equal to == Is not equal to != Is less than < Is less than or equal to <= Is greater than > Is greater than or equal to >=

Multiple Conditions Sometimes, it’s necessary to combine multiple conditions to form a single, more complex text. It is a good idea to first establish a table (on paper) mapping the multiple conditions, the possible responses and the program’s reaction to responses.

Boolean Operators Sometimes, conditions can be joined together in the same test using Boolean operators: ◦ Logical AND is represented by && Example: if (A = B && C = D) … ◦ Logical OR is represented by || Example: if(A = B || C = D) … ◦ Logical NOT is represented by ! (bang symbol) Example: if( !(A = B) ) … Boolean Precedence: NOT, followed by AND, followed by OR

More on Boolean Operators From Boolean expressions, we can develop Truth Tables: Cond A Cond. B A && B A || B !(A) TTTTF TFFTF FTFTT FFFFT

Multiple Conditions Cond.QuestionYesNoResponse I Weekend? X- Response 1 First day? X- Response 2 II Weekend? X- Response 3 First day? -X Response 4 III Weekend? -X Response 5 First day? X- Response 6 IV Weekend? -X Response 7 First day? -X Response 8

Operator Precedence OrderDescriptionOperator 1 Dot Operator. 2 Parenthesis (Work from inside out) () 3 Instance Operator new 4 Multiplication, Division, Modulus * / % 5 Addition, Subtraction, Concatenation + -

Operator Precedence (continued) OrderDescriptionOperator 6Assignment = += -= *= /= %= 7 Comparison Operators, =, = 8Inequality!= 9 Boolean AND && 10 Boolean OR ||

Boolean Values When testing against Boolean values, you can use shortcuts (True Example): if(myVar == true) … is the same thing as if(myVar) … False Example: if(myVar == false) … is the same thing as if(!(myVar)) …

window.confirm() Method Boolean values are returned by the window.confirm() method. window.confirm() takes input from the user, based on which button they choose (OK=TRUE & CANCEL=FALSE) It looks and feels a lot like window.alert(), except that it has both an OK and a CANCEL button

window.confirm() & Boolean Example Code Example

Questions?

Resources JavaScript: The Definitive Guide by David Flanagan (O’Reilly, 2002) JavaScript Concepts & Techniques: Programming Interactive Web Sites by Tina Spain McDuffie (Franklin, Beedle & Associates, 2003) Extended Prelude to Programming: Concepts and Design by Stewart Venit (Scott/Jones, Inc., 2002)