Tuesday, February 19 Week 6

Chapter 5

Student Notes

Objectives

Discuss project 4

Boolean Expressions and IF statements

Swapping

Multiple IF statements

1. Why use an IF statement?

2. Ex, finding yesterday and tomorrow

· Ex. program 5.1

· Why use IF Today = Days’First THEN

Yesterday := Days’Last ;

ELSE

Yesterday := Days’Pred(Today);

END IF;

When we could just Monday and Sunday?

3. Boolean Expressions and Conditions

· There are only two possible outcomes of a Boolean expression:

·

· Some Boolean expressions are called

·

· We use Conditions to compare one thing to another. All of conditions follow one of two patterns:

variable relational operator variable

variable relational operator constant

· Relational operators are familiar logical operators:

< (less than)

<= (less than or equal to)

> (greater than)

>= (greater than or equal to)

= (equal to)

/= (not equal to)

· Notice that greater than or equal to is >= and not =>

5. Enumeration types and Conditional statements

· When using conditionals with enumeration types, decisions are made based on the order of the definition so for the following two enumeration types:

TYPE Days IS (Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday);

TYPE Colors IS (Red, Orange, Yellow, Green, Blue, Purple);

All of these statements evaluate to true:

Monday < Tuesday

Wednesday /= Thursday

Wednesday = Wednesday

Wednesday >= Tuesday

Purple > Red

Yellow < Green

Green >= Yellow

· The following statements would cause compilation errors because the two values in each comparison are of different types:

Purple > Friday

3 <= 4.5

Green > 2

6. Two ways to use an IF statement

· IF condition THEN

statement sequence T

ELSE

Statement sequence F

END IF;

·

·

·

· IF condition THEN

statement sequence T

END IF;

7. Swapping

· IF X > Y THEN

Temp := X; -- Store old X in a temp variable

X := Y; -- Store old Y in X

Y := Temp; -- Y now holds the old value of X

END IF;

8. The Multiple-Alternative IF statement

IF Response = Strawberry THEN

Straw := Straw + 1;

ELSIF Response = Chocolate THEN

Choc := Choc +1;

ELSIF Response = Vanilla THEN

Van := Van +1;

ELSE

Other := Other +1;

END IF;

· Multiple IF statements evaluate from the top down. You can use this to your advantage at times.

· All Multiple IF statements should end with an ELSE statement, not an ELSIF. Sometimes this takes a little clever thinking on the part of the programmer.