Notes
Outline
Flow of Control
Flow of control is the order in which the computer executes statements in a program.
Flow of control is normally sequential.
Control structures allow for non-sequential flow of control.
The first control structure we are going to look at is a selection control structure.
Selection
More Selection
The if Statement
The If-Then-Else Form
IfStatement (the If-Then-Else form)
if (Expression)
Statement1A
else
Statement1B
The if Statement
The If-Then-Else Form
IfStatement (the If-Then-Else form)
if (Expression)
Statement1A
else
Statement1B
The If-Then Form
IfStatement  (the If-Then form)
if (Expression)
Statement1A
Conditions and Logical Expressions
Asking questions in C++ means making an assertion that is either true or false.
The computer evaluates the assertion.
We have a new data type that is used in assertions, bool. It has two possible values true and false.
We declare variables of type bool in the same way we declare variables of other types.
Example bool dataOK;
Logical Expressions
Assertions take the form of logical expressions, they evaluate to either true or false.
Examples
Boolean variable or constant
An expression followed by a relational operator followed by an expression.
A logical expression followed by a logical operator followed by a logical expression.
Relational Operators
Relational Operators test the relationship between two values.
Example  bool lessThan; int i, j;
cin >> i >> j;
lessThan = (i < j);
Operators
== Equal to
!= Not equal to
> Greater than
< Less than
>= Greater than or equal to
<= Less than or equal to
Relational Operator Examples
x y Expression Result
12 2 x + 3 <= y * 10 true
Caution: The relational operator, ==, and the assignment operator, =, are not the same.
Relational Operator Examples
x y Expression Result
12 2 x + 3 <= y * 10 true
20 2 x + 3 <= y * 10 false
Caution: The relational operator, ==, and the assignment operator, =, are not the same.
Relational Operator Examples
x y Expression Result
12 2 x + 3 <= y * 10 true
20 2 x + 3 <= y * 10 false
7 1 x + 3 != y * 10 false
Caution: The relational operator, ==, and the assignment operator, =, are not the same.
Relational Operator Examples
x y Expression Result
12 2 x + 3 <= y * 10 true
20 2 x + 3 <= y * 10 false
7 1 x + 3 != y * 10 false
17 2 x + 3 == y * 10 true
Caution: The relational operator, ==, and the assignment operator, =, are not the same.
Relational Operator Examples
x y Expression Result
12 2 x + 3 <= y * 10 true
20 2 x + 3 <= y * 10 false
7 1 x + 3 != y * 10 false
17 2 x + 3 == y * 10 true
100 5 x + 3 > y * 10 true
Caution: The relational operator, ==, and the assignment operator, =, are not the same.
Logical Operators
The binary AND operator (&&) requires both relationships to be true in order for the overall result to be true.
The binary OR operator (||) has an overall result of true if either or both of two logical expressions are true.
The unary NOT operator (!) has a result of true if the logical expression is false.
Precedence of Operators Revisited
! Unary + Unary - Highest precedence
/ % *
+ -
<  <=  >  >=
==  !=
&&
||
= Lowest precedence
Warning
Do not compare floating-point numbers for equality. Remember that the binary representations of floating-point numbers are approximations. Instead, test for near equality.
Example: fabs(r – s) < 0.00001
if Statement Examples
(If-Then-Else Form)
if (hours <= 40.0)
pay = rate * hours;
else
pay = rate * (40.0 + (hours – 40.0) * 1.5);
cout << pay;
if Statement Examples
(If-Then-Else Form)
if (hours <= 40.0)
pay = rate * hours;
else
pay = rate * (40.0 + (hours – 40.0) * 1.5);
cout << pay;
if (divisor != 0)
result = dividend / divisor;
else
cout << “Division by zero is not allowed. “ << endl;
Block Statements
Anywhere one statement can be used, a series of statements can be used, they must be enclosed in braces {}.
Example:
  if (divisor != 0)
  {
    result = dividend / divisor;
    cout << “Division performed.” << endl;
  }
  else
  {
    cout << “Division by zero is not allowed.” << endl;
    result = 9999;
  }
if Statement Examples
(If-Then Form)
if (age < 18)
cout << “Not an eligible”;
cout << “voter.” << endl;
if Statement Examples
(If-Then Form)
if (age < 18)
cout << “Not an eligible”;
cout << “voter.” << endl;
if (result < 0.0)
{
cout << “Check box 24A” << endl;
result = 0.0;
}