Decision II

CSCE 106 2

Outline Boolean expressions switch statement (section 4.8)

CSCE 106 3

Table 4.7 English Conditions as C++ Expressions

CSCE 106 4

Boolean Assignment

bool type values are true and false Assignment statements have general form

variable = expression; E.g.: (for variable called same of type

bool)

same = true;

same = (x == y);

CSCE 106 5

Comparing Characters and Strings

Letters are in typical alphabetical order Upper and lower case significant Digit characters are also ordered as

expected String objects require string library

Compares corresponding pairs of characters

CSCE 106 6

Table 4.8 Examples of Comparisons

CSCE 106 7

Table 4.6 Operator Precedence

CSCE 106 8

Example aflagx y z

3.0 4.0 2.0 false

x + y / z <= 3.5

2.0

5.0

false

CSCE 106 9

Example bflagx y z

3.0 4.0 2.0 false

! flag || (y + z >= x - z)

6.0 1.0

true

true

true

CSCE 106 10

Short Circuit Evaluation

(single == ‘y’ && gender == ‘m’ && age >= 18) If single condition is false, gender and age

conditions are not evaluated.

(single == ‘y’ || gender == ‘m’ || age >= 18) If single condition is true, gender and age

conditions are not evaluated.

CSCE 106 11

Additional Assignment Examples

inRange = (n > -10) && (n < 10); isLetter = ((‘A’ <= ch) && (ch <= ‘Z’)) ||

((‘a’ <= ch) && (ch <= ‘z’)); even = (n % 2 == 0);

CSCE 106 12

Writing bool Values

Boolean values are represented by integer values in C++ 0 for false non-zero (typically 1) for true

Outputting (or inputting) bool type values is done with integers

CSCE 106 13

Complements of Relational Operators

< >=

> <=

= = !=

CSCE 106 14

Examples

Expression Complement

Flag !Flag

x>=1 !(x>=1) or x<1

x>5 !(x>5) or x<=5

CSCE 106 15

Complements of Boolean Expressions (cont’d)

Complementing (or getting opposite of) boolean/logical expressions can be done in 2 ways: using logical operator ! (not) using DeMorgan’s Theorem

CSCE 106 16

DeMorgan’s Theorem

!(exp1 && exp2)

is the same as

!exp1 || !exp2

!(exp1 || exp2)

is the same as

!exp1 && !exp2

It explains how to complement a compound logical expression

CSCE 106 17

ExampleExpression

z <= x && x <= yComplement

!(z <= x && x <= y)the equivalent using DeMorgan’s theorem

!(z <= x) || !(x <= y)which is equivalent to (without the not “!”)

z > x || x > y

CSCE 106 18

Table 4.7 English Conditions as C++ Expressions

CSCE 106 19

Multiple Decision ExercisePlease write a multiple decision C++ segment to evaluate a letter grade, input by the user, and output the corresponding phrase according to the following table:

Grade OutputA or a ExcellentB or b GoodC or c FairD or d or F or f Pooranything else Invalid grade

CSCE 106 20

if (grade == ‘A’ || grade == ‘a’)

cout << “Excellent” << endl;

else if (grade == ‘B’ || grade == ‘b’)

cout << “Good” << endl;

else if (grade == ‘C’ || grade == ‘c’)

cout << “Fair” << endl;

else if (grade == ‘D’ || grade == ‘d’ || grade == ‘F’ || grade == ‘f’)

cout << “Poor” << endl;

else cout << “Invalid grade” << endl;

switch (grade) { case ‘A’: case ‘a’:

cout << “Excellent” << endl;

break;

case ‘B’: case ‘b’:

cout << “Good” << endl;

break;case ‘C’: case ‘c’:

cout << “Fair” << endl;

break;case ‘D’: case ‘d’:case ‘F’: case ‘f’:

cout << “Fair” << endl;

break;

default:

cout << “Invalid grade” << endl;}

Code Segmentsswitch selector

case label

optional

CSCE 106 21

The switch Control Statementswitch (selector ) { case label1: statements1;

break;

case label2: statements2;

break;.

.

.

case label10: statements10;

break;

default: statementsd; // optional}

CSCE 106 22

switch Control Statement (cont’d) Used in C++ to select one of several

alternatives. Alternative to multiple if statements in some

cases. Especially useful when the selection (called

the switch selector) is based on the value of a single variable or a simple expression.

switch selector must be of type int, char, or bool only.

CSCE 106 23

switch Control Statement (cont’d)

switch selector value compared to each case label. When there is an exact match, statements for that case are executed.

If no case label matches the selector, the entire switch body is skipped unless it contains a default case label.

break is typically used between cases to avoid fall through.

CSCE 106 24

Listing 4.5 switch statement to determine life expectancy of a lightbulb

CSCE 106 25

switch Control Statement (cont’d) You have to use a formula to convert ranges to

an integer, so as to be able to use a switch statement.

For example, if you are asked to use a switch statement to give the following corresponding outputs for some given ranges:

Final GPA Honorary Degree3.80 <= GPA <= 4.00 Highest Honours3.60 <= GPA < 3.80 High Honours3.40 <= GPA < 3.60 Honours2.00 <= GPA < 3.40 Pass with no Honours

CSCE 106 26

switch Control Statement (cont’d)Here is one example for a formula that works, together with its corresponding case statements, provided that you check the range of GPA when input before:

int x = GPA *10;

switch (x){ case 40: case 39: case 38: cout<<“Highest Honours”; break; case 37: case 36: cout<<“High Honours”; break; case 35: case 34: cout<<“Honours”; break; default: cout<<“Pass with no Honours”; break;}

CSCE 106 27

Next lecture will be aboutlooping construct in C++