Boolean Expressions and Switch Statement in C++

Decision II

Outline • Boolean expressions • switch statement (section 4.8) CSCE 106

Table 4.7English Conditions as C++ Expressions CSCE 106

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

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

Table 4.8Examples of Comparisons CSCE 106

Table 4.6Operator Precedence CSCE 106

Example a x y z flag 3.0 4.0 2.0 false x + y / z <= 3.5 2.0 5.0 false CSCE 106

Example b x y z flag 3.0 4.0 2.0 false ! flag || (y + z >= x - z) true 6.0 1.0 true true CSCE 106

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

Additional Assignment Examples • inRange = (n > -10) && (n < 10); • isLetter = ((‘A’ <= ch) && (ch <= ‘Z’)) || ((‘a’ <= ch) && (ch <= ‘z’)); • even = (n % 2 == 0); CSCE 106

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

Complements of Relational Operators < >= > <= = = != CSCE 106

Examples Expression Complement Flag !Flag x>=1 !(x>=1) or x<1 x>5 !(x>5) or x<=5 CSCE 106

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

DeMorgan’s Theorem It explains how to complement a compound logical expression !(exp1 && exp2) is the same as !exp1 || !exp2 !(exp1 || exp2) is the same as !exp1 && !exp2 CSCE 106

Example Expression z <= x && x <= y Complement !(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

Table 4.7English Conditions as C++ Expressions CSCE 106

Multiple Decision Exercise Please 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 Output A or a Excellent B or b Good C or c Fair D or d or F or f Poor anything else Invalid grade CSCE 106

switch selector Code Segments 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; } case label 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; optional CSCE 106

The switch Control Statement switch (selector ) { caselabel1: statements1; break; caselabel2:statements2; break; . . . caselabel10:statements10; break; default: statementsd; // optional } CSCE 106

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

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

Listing 4.5switch statement to determine life expectancy of a lightbulb CSCE 106

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 Degree 3.80 <= GPA <= 4.00 Highest Honours 3.60 <= GPA < 3.80 High Honours 3.40 <= GPA < 3.60 Honours 2.00 <= GPA < 3.40 Pass with no Honours CSCE 106

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

Next lecture will be about looping construct in C++ CSCE 106

Boolean Expressions and Switch Statement in C++