Transcript Selection in C++

Selection in C++
If statements
Control Structures
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Sequence
Selection
Repetition
Module
Selection
If or if / else statement
 choosing between mutually
exclusive possibilities
 Two forms
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if (logical expression) statement;
 if (logical expression) statement;
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else statement;
Logical Expressions
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Relational Operators
==, !=, <=, >=, >, <
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They are binary operators – have two
operands
work on any type of data, be careful about
matching types
watch out for = and ==
produce a bool result
Syntax of if statement
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if (logical expression)
statement;
Don't put semicolon after condition
Indentation helps humans, doesn't matter to
compiler
Syntax of if statement
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if (x > 0)
y = sqrt(x);
if (x > y)
t = x; x = y; y = t; // what goes with the if?
If-Then Statement
Determine whether or not to execute a statement (which can be
a single statement or an entire block)
TRUE
statement
expression
FALSE
Examples
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output larger of two numbers
don't allow sqrt of negative number
don't allow overflow
determining even or odd
give student another chance at a question
Visual Aid
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Decision tree
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“if person is 18 or over and state is KY, they can
have regular license”
“if person is 16 or over and under 18 and state is
TN or VA, they can have learner’s permit”
“if person is under 16, in any state, they can’t
have license”
“if person is over 65 in VA, they have modified
license”
Example
What is output? Why?
int age;
age = 20;
if (age = 16)
{
cout << “Did you get driver’s license?”;
}
Example
What is output? Why?
int age;
age = 30;
if (age < 18)
cout << “Do you drive?”;
cout << “Too young to vote”;
Example
What is output? Why?
int number;
number = 0;
if (number = 0)
cout << “Zero value”;
else
cout << “Non-zero value”;
Logical Operators
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And (&&) Or (||) Not (!)
&& and || are binary operators, ! is unary
used to combine bool values
produce a bool result
truth tables
operator precedence
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! then && then ||
! is very high, above most operators
&& and || are very low, below other operators
Operator
!
*, / , %
+,<
<=
>
>=
==
!=
&&
||
=
Meaning
Associativity
NOT
Multiplication, Division, Modulus
Addition, Subtraction
Less than
Less than or equal to
Greater than
Greater than or equal to
Is equal to
Is not equal to
AND
OR
Assignment
Right
Left
Left
Left
Left
Left
Left
Left
Left
Left
Left
Right
14
Logical
Expression
Meaning
Description
!p
NOT p
! p is false if p is true
! p is true if p is false
p && q
p AND q
p && q is true if
both p and q are true.
It is false otherwise.
p || q
p OR q
p || q is true if either
p or q or both are true.
It is false otherwise.
Converting English to logic
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"0 is less than x is less than 5"
"x is 5 or 6"
"x is bigger than 5 and less than 10"
impossible situations "dead code"
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always true – “x < x + 1” “x > 5 || x < 8”
always false – “x < 5 && x > 10”
Write an expression for each
taxRate is over 25% and income is less than
$20000
temperature is less than or equal to 75 or
humidity is less than 70%
age is over 21 and age is less than 60
age is 21 or 22
Some Answers
(taxRate > .25) && (income < 20000)
(temperature <= 75) || (humidity < .70)
(age > 21) && (age < 60)
(age == 21) || (age == 22)
What went wrong?
This is only supposed to display “HEALTHY AIR” if
the air quality index is between 50 and 80.
But when you tested it, it displayed “HEALTHY AIR”
when the index was 35.
int AQIndex;
AQIndex = 35;
if (50 < AQIndex < 80)
cout << “HEALTHY AIR“;
Analysis of Situation
AQIndex = 35;
According to the precedence chart, the expression
(50 < AQIndex < 80)
means
(50 < AQIndex) < 80
because < is Left Associative
(50 < AQIndex) is false
(0 < 80) is true.
(has value 0)
Corrected Version
int AQIndex;
AQIndex = 35;
if ((50 < AQIndex) && (AQIndex < 80))
cout << “HEALTHY AIR“;
Nested if's
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the statement to be executed in an if
statement can be another if
an else branch goes with the nearest if
"dangling else"
Example
float average;
average = 100.0;
if (average >= 60.0)
if (average < 70.0)
cout << “Marginal PASS”;
else
cout << “FAIL”;
100.0
average
FAIL is printed; WHY? The compiler ignores indentation and pairs
the else with the second if
if .. else provides two-way selection
between executing one of 2 clauses (the if
clause or the else clause)
TRUE
if clause
expression
FALSE
else clause
What happens if you omit braces?
if ((carDoors == 4) && (driverAge > 24))
premium = 650.00;
cout << “ LOW RISK “;
else
premium = 1200.00;
cout << “ HIGH RISK ”;
monthlyPayment = premium / 12.0 + 5.00;
Compile error occurs: The “if clause” is the single
statement following the if
Example
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Given x, y on Cartesian plane, which quadrant is it
in? (I, II, III, IV)
if (x > 0)
if (y > 0)
quadrant = 1;
else
quadrant = 4;
else
if (y > 0) // NOT a redundant test!
quadrant = 2;
else
quadrant = 3;
Another form of nested if's
if (condition)
statement;
else if (condition)
statement;
else if (condition)
statement;
else
statement;
// good for mutually exclusive and exhaustive
conditions
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Comparing Strings
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Two objects of type string (or a string object and a C string)
can be compared using the relational operators
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A character-by-character comparison is made using the
ASCII character set values
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If all the characters are equal, then the 2 strings are equal.
Otherwise, the string with the character with smaller ASCII
value is the “lesser” string
string myState;
string yourState;
myState = “Texas”;
yourState = “Maryland”;
Expression
Value
myState == yourState
false
myState > yourState
true
myState == “Texas”
true
myState < “texas”
true
Testing
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code needs to be tested in every possible
branch
takes multiple runs
test plans
Case
Input
Output
Output
(expected) (actual)
Test Plan
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if (x + y > 0)
cout << "safe";
else if (x > 3)
cout << "high";
else cout << "low";
Test Plan
Case
x+y>0
x + y <=0,
x>3
x+y <=0, x
<= 3
Input
Output
Output
(expected) (actual)
x = 5, y = 7 "safe"
"safe"
x = 5, y=-7 "high"
"high"
x = 2, y=-3 "low"
"low"
Comparing Real Values
Do not compare floating point values for equality,
compare them for near-equality.
float myNumber;
float yourNumber;
cin >> myNumber;
cin >> yourNumber;
if (fabs (myNumber - yourNumber) < 0.00001)
cout << “They are close enough!” << endl;