Using Decision Structures

1. Using Decision Structures

2. Goals • Understand why we use decision-making structures in programming • Understand how to create If/Else structures • Understand how to test multiple conditions • Understand how to use Boolean operators • Understand how to create switch structures

3. Computers as “Decision Makers” • Modern computers seem to “think”. • Early computers, however, did not have such an ability. • Early programs used sequential logic. That is, Step A happened before Step B. Step B happened before Step C, etc. • Sequential programs are easy to write, but lack flexibility. • One of the ways we can simulate a computer’s ability to think is to program using decision structures ...

4. Computing Structures • In modern programming, all executable statements fall into one of three categories: • Sequential Structures – those structures where instructions happen in sequence. That is “A before B”, “B before C”, “C before D”, etc. • Looping (Iterative) Structures – those structures where instructions repeat until the program meets a given condition (test). • Decision (Selection) Structures – Today’s unit.

5. Introducing Decision Structures • Decision structures consist “of a test condition together with one or more groups (or blocks) of statements. The result of the ‘test’ determines which of these blocks” the program will execute (Venit) • In all decision structures, ONE AND ONLY ONE block executes. • The program will then execute the next sequential section of code, after it evaluates a decision structure and executes the appropriate code block.

6. Types of Decision Structures • JavaScript gives us three distinct types of Decision Structures: • Single-Alternative (a.k.a – If-Then) Structure • Contains a single block of executable code that will execute IF AND ONLY IF the associated condition (test) evaluates to TRUE (we don’t care if it evaluates to false). • Dual-Alternative (a.k.a – If-Then-Else) Structure • Contains exactly two blocks of code. One block will execute IF AND ONLY IF the associated condition (test) evaluates to TRUE; the other block will execute IF AND ONLY IF the associated condition (test) evaluates to FALSE.

7. Types of Decision Structures • JavaScript gives us three distinct types of Decision Structures: • Multiple-Alternative (Select Case) Structure • Contains multiple blocks (called cases), each of which will execute IF AND ONLY IF the program matches ONLY one of multiple condition (test) results. ONLY ONE BLOCK (case) will execute.

8. Humans as “Decision Makers” • Human beings are very good at understanding concepts. • We understand multiple types of syntax, too. • Additionally, we understand meaning. • This allows for some “gray area” in our language.

9. “Concepts” to “Conditions” Consider the following statements: • “It’s cool outside.” • “The weather is cool outside.” • “It’s somewhat chilly out there!” • “The temperature is expected to be around 65°”

10. “Concepts” to “Conditions” What does “cool” mean? Its meaning is arbitrary … • Humans can understand multiple types of syntax and are able to extrapolate meaning from different syntax. • However, computers don’t “think” on that level. • As programmers, we need to refine human language to a more restrictive language that computers understand. • One way to do this is to translate concepts to conditions which can be tested …

11. 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)

12. Available Relational Operators

13. Translating the Concept • Remember, that computers don’t understand arbitrary concepts • Instead, we need to define the meaning of the concept for the computer • Once we define what “cool” means, we can write our conditional statement

14. The “If” Conditional • A single-alternative (If-Then) structure, allows us to specify an action IF AND ONLY IF a condition tests to TRUE. • The parts of the If-Then structure: if (condition is true) { perform this action}

15. What if the “if” condition is not met? • It is a good idea to provide an alternative plan if a condition is not met (it evaluates to FALSE). • To do this, we use an “If-Then-Else” structure: if (condition is true) { perform action 1}else{ perform action 2}

16. The Conditional Operator • Sometimes, it may be convenient for you to use a shortcut for dual alternative (If-Then-Else) structures. • The shortcut is called the conditional operator. Its general form looks like this: Action IfFalse Action IfTrue ? : Condition

17. The Conditional Operator • We can rewrite the following code using a conditional operator: strGreeting = “Hello ”; if(strUserName != null){ strGreeting += strUserName; }else{ strGreeting += “there!”; } Example adapted from JavaScript: The Definitive Guide (Flanagan)

18. The Conditional Operator • The previous code using a conditional operator: strGreeting = “Hello ” + (strUserName != null ? strUserName : “there.”); Example adapted from JavaScript: The Definitive Guide (Flanagan)

19. What about multiple conditions? • Sometimes, we need to consider the possibility that we need to evaluate more than value as a response to user input • We can do this using nested branching.

20. Nested Branching • Consider the following statements: • “If the temperature outside is greater than 65°, I’ll wear a short-sleeved shirt.” • “If the temperature outside is less than 65°, I’ll wear a sweater.” • “If the temperature outside is less than 65° and is less than 45°, I’ll need to wear a coat.” • These statements present us with one condition to test (the temperature), but more than two (three) possible actions!

21. Nested Branching • We could write the code using a bunch of “if” statements, each independent of the other • However, a much more efficient (and cleaner, code-wise) way is to use a nested structure

22. 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

23. 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)) …

24. Multiple Conditions

25. Operator Precedence

26. Operator Precedence (continued)

27. Questions?

28. 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)