Decision II
Outline
 Boolean expressions
 switch statement (section 4.8)
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Table 4.7
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English Conditions as C++
Expressions
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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);
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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
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Table 4.8
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Examples of Comparisons
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Table 4.6
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Operator Precedence
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Example a
x
y
z
flag
3.0
4.0
2.0
false
x + y / z <= 3.5
2.0
5.0
false
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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
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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.
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Additional Assignment Examples
 inRange = (n > -10) && (n < 10);
 isLetter = ((‘A’ <= ch) && (ch <= ‘Z’)) ||
((‘a’ <= ch) && (ch <= ‘z’));
 even = (n % 2 == 0);
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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
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Complements of Relational
Operators
<
>
==
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>=
<=
!=
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Examples
Expression
Flag
x>=1
x>5
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Complement
!Flag
!(x>=1) or x<1
!(x>5) or x<=5
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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
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DeMorgan’s Theorem
It explains how to complement a compound
logical expression
!(exp1 && exp2)
!(exp1 || exp2)
is the same as
is the same as
!exp1 || !exp2
!exp1 && !exp2
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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
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Table 4.7
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English Conditions as C++
Expressions
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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
A or a
B or b
C or c
D or d or F or f
anything else
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Output
Excellent
Good
Fair
Poor
Invalid grade
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Code Segments
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;
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switch selector
switch (grade)
case label
{
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;
optional
default:
cout << “Invalid grade” << endl;
}
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The switch Control Statement
switch (selector )
{
case label1: statements1;
break;
case label2: statements2;
break;
.
.
.
case label10: statements10;
break;
default: statementsd;
// optional
}
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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.
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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.
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Listing 4.5
switch statement to determine
life expectancy of a lightbulb
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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
3.80 <= GPA <= 4.00
3.60 <= GPA < 3.80
3.40 <= GPA < 3.60
2.00 <= GPA < 3.40
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Honorary Degree
Highest Honours
High Honours
Honours
Pass with no Honours
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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;
}
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Next lecture will be about
looping construct in C++
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