CSCI 1110 - C++ Lecture Notes
Chapter 5
Conditions, Logical Expressions and Selection Control Structures
Lecture Notes

Flow of Control

Conditions and Logical Expressions

The If Statement

Nested If Statements

Testing the State of an I/O Stream

Flow of Control

• Flow of Control - The order in which the computer executes statements in a program.
  
  
• Control Structure - A statement used to alter the normal sequential flow of control.
  
  
• There are four types of control structures used in C++:
  
  
1. Sequence - statements are executed one after the other.

 
2. Selection - one of two paths are executed based on a condition (true/false expression).


 
3. Loop - one or more statements are executed while (or until) a condition tests true.



4. Function call - execution of statements is suspended while another function is executed. After the function returns, execution continues immediately after the function call.

Conditions and Logical Expressions

• Logical expressions evaluate to true or false.
  
  
• In C++, an integer expression that evaluates to 0 is false. Any other value is true.
  
  
• A typedef statement is used to create C++ data types based on previously defined types.
  
  


typedef int Boolean;
const Boolean TRUE = 1;
const Boolean FALSE = 0;
.
.
.
Boolean OK;
.
.
.
OK = TRUE;
.
.
.
OK = FALSE;
• Relational operators are used to compare two values:
  
  
= =    Equal
! =     Not equal
>       Greater than
>=     Greater than or equal
<       Less than
<=     Less than or equal


Examples:
x != y
x > y
y == 0
ch >= 'A'
x < y - z
 
• Logical operators consist of && (and), ||(or) and ! (not).
  
  
 

x	y	x && y	x || y	!x
0	0	0	0	1
0	1	0	1	1
1	0	0	1	0
1	1	1	1	0



Examples:
finalScore > 90 && midtermScore > 70
midtermGrade == 'A' || finalGrade == 'A'
!(hours > 40)is equivalent to hours <= 40
 
• Short-circuit evaluation. Evaluation of logical expressions stops as soon as the result can be determined.
  
  
c <= d || e == f

if the first term is true, then the second term is not evaluated.



• Precedence of Operators:
  
  
!                                            Highest precedence
*  /   %  
+  -
<  <=  >  >=
== !=
&&
||
=                                             Lowest precedence
 
Examples:
dataInvalid = inputVal == 0;
dataInvalid = (inputVal == 0);
OK = x * 5 < y + 6;
done = !OK && x < y;


• Relational operators and floating point types. Consider the following:
  
  
float oneThird = 1.0 / 3.0;
float one = oneThird + oneThird + oneThird;
cout << one << endl;
 
outputs
 
0.9999999
 
instead of 1.0. Why? Because 1.0 / 3.0 is not exactly one third.
 
To determine if two floating values are equal, you should test to see if their difference is sufficiently near zero.
 
isEqual = fabs(one - 1.0) < 0.00001;

The If Statement

• The if statement is used to select between two paths in a C++ program.
  
  
• Syntax template
  
  

Example:
if (hours <= 40.0)
pay = rate * hours;
else
pay = rate * (40.0 + (hours - 40.0) * 1.5);

• A Compound Statement consists of a Block of simple statements enclosed in braces { and }.
  
  

Example:

if (n >= 2)
{
alpha = 5;
beta = 15;
}
else
{
alpha = 23;
beta = 199;
}
•  An alternate form of the if statement omits the else clause.
  
  

Example:
if (n >= 2)
{
alpha = 5;
beta = 15;
}
 
• A common mistake can occur when the == (equality) operator is confused with the = (assignment) operator.
  
  

Example:
cin >> n;
if (n = 3)
cout << "n equals 3";
else
cout << "n doesn't equal 3";
 
What actually happens in this example?
 
In the if statement, n is assigned the value 3. The assignment operator also results in a value (3) which is then tested for true/false. Since 3 is not 0, the result is true, and the message
n equals 3
is output.

To avoid such problems, you can write any test that includes a constant as follows:
 
cin >> n;
if (3 == n)

cout << "n equals 3";
else
cout << "n doesn't equal 3";

Nested If Statements

• There are no restrictions on what the statements in an if statement can be. For example an if statement can contain another if statement.
  
  
if (x < 0)
if (y != 4)
z = y * x;
else
z = y / x;
else
if (y > 4)
z = y + x;
else
z = y - x;
 
• Consider the following code:
 
if (day == 1)
cout << "Sunday";
if (day == 2)
cout << "Monday";
if (day == 3)
cout << "Tuesday";
if (day == 4)
cout << "Wednesday";
if (day == 5)
cout << "Thursday";
if (day == 6)
cout << "Friday";
 if (day == 7)
cout << "Saturday";
 
It could be written more efficiently as:
 
 if (day == 1)
cout << "Sunday";
else
if (day == 2)
cout << "Monday";
else
if (day == 3)
cout << "Tuesday";
else
if (day == 4)
cout << "Wednesday";
else
if (day == 5)
cout << "Thursday";
else
if (day == 6)
cout << "Friday";
 else
if (day == 7)
cout << "Saturday";


• Eventhough the previous example conforms to the style guidelines for if statements, it isn't very easy to read. The following alternative style is often used for a compound if like this:
  
  

if (day == 1)
cout << "Sunday";
else if (day == 2)
cout << "Monday";
else if (day == 3)
cout << "Tuesday";
else if (day == 4)
cout << "Wednesday";
else if (day == 5)
cout << "Thursday";
else if (day == 6)
cout << "Friday";
 else if (day == 7)
cout << "Saturday";
 
• Remember that the layout of the program doesn't matter to the compiler:
  
  

if (temp > 32)
if (temp <= 40)
cout << "Freeze Watch";
else
cout << "Freeze Warning";
 
The above code won't have the desired effect. It is equivalent to:
 
if (temp > 32)
if (temp <= 40)
cout << "Freeze Watch";
else
cout << "Freeze Warning";
 
There are two ways to correct the problem. The following solution uses what is known as the null else which is the word else followed by a semicolon. It means to do nothing.
 
if (temp > 32)
if (temp <= 40)
cout << "Freeze Watch";
else;
else
cout << "Freeze Warning";
 
The sceond solution uses braces to delimit the embedded if statement.
 
if (temp > 32)
{
if (temp <= 40)
cout << "Freeze Watch";
}
else
cout << "Freeze Warning";
 

Testing the State of an I/O Stream

• A C++ stream variable can be used in a logical expression as if it were a Boolean variable. If the result of the last I/O operation on that stream succeeded, the result is nonzero (TRUE). If the last I/O operation failed, the result is zero (FALSE).
  
  

#include <iostream.h>
#include <fstream.h>

int main()
{

int height;
int width;
ifstream inFile;

inFile.open("mydata.dat");
if (!inFile)
{

cout << "Can't open input file.";
return 1;
}

inFile >> height >> width;
.
.
.

return 0;

}