Conditional Statements

Introduction to Conditional Statements

There are three entities that participate on a traffic light: the lights, the human beings who interact with the light, and the law. The road provides a platform on which these components come together.

The Traffic Light
Everything taken into consideration, a traffic light is made of three light colors: Green – Yellow/Orange – Red. When the light is green, the road is clear for moving in. The red light signals to stop and wait. A yellow light means, “Be careful, it is not safe to proceed right now. Maybe you should wait.” When it is not blinking, the yellow light usually serves as a transition period from green to red. There is no transition from red to green.

The Drivers
There are two main categories of people who deal with the traffic light: the drivers and the walkers. To make our discussion a little simpler, we will consider only the driver. When the light is green, a driver can drive through. When the light is red, the driver is required to stop and wait.

The Law
Rules and regulations dictate that when a driver does not obey the law by stopping to a red light, he is considered to have broken the law and there is a consequence.

The most independent of the three entities is the traffic light. It does not think, therefore it does not make mistakes. It is programmed with a timer or counter that directs it when to act, that is, when to change lights. The second entity, the driver, is a human being who can think and make decisions based on circumstances that are beyond human understanding. A driver can decide to stop at a green light or drive through a red light…

A driver who proceeds through a red light can get a ticket depending on one of two circumstances: either a police officer caught him “hand-in-the-basket” or a special camera took a picture. Worse, if an accident happens, this becomes another story.

The traffic light is sometimes equipped with a timer or counter. We will call it Timer T. It is equipped with three lights: Green, Yellow, and Red. Let’s suppose that the light stays green for 45 seconds, then its turns and stays yellow for 5 seconds, and finally it turns and stays red for 1 minute = 60 seconds. At one moment in the day, the timer is set at the beginning or is reset and the light is green: T = 0. Since the timer is working fine, it starts counting the seconds 1, 2, 3, 4, … 45. The light will stay green from T = 0 to T = 45. When the timer reaches 45, the timer is reset to 0 and starts counting from 0 until it reaches 5; meanwhile, Color = Yellow.

if a Condition is True

In C++, comparisons are made from a statement. Examples of statements are:

• "You are 12 years old"
• "It is raining outside"
• You live in Sydney"

When a driver comes to a traffic light, the first thing he does is to examine the light's color. There are two values the driver would put together: The current light of the traffic and the desired light of the traffic.

Upon coming to the traffic light, the driver would have to compare the traffic light variable with a color he desires the traffic light to have, namely the green light (because if the light is green, then the driver can drive through). The comparison is performed by the driver making a statement such as "The light is green".

After making a statement, the driver evaluates it and compares it to what must be true.

When a driver comes to a traffic light, he would likely expect the light to be green. Therefore, if the light is green (because that is what he is expecting), the result of his examination would receive the Boolean value of TRUE. This produces the following table:

Color Statement Boolean Value The light is green true

One of the comparisons the computer performs is to find out if a statement is true (in reality, programmers (like you) write these statements and the computer only follows your logic). If a statement is true, the computer acts on a subsequent instruction.

The comparison using the if statement is used to check whether a condition is true or false. The syntax to use it is:

if(Condition) Statement;

If the Condition is true, then the compiler would execute the Statement. The compiler ignores anything else:

If the statement to execute is (very) short, you can write it on the same line with the condition that is being checked.

Consider a program that is asking a user to answer Yes or No to a question such as "Are you ready to provide your credit card number?". A source file of such a program could look like this:

#include <iostream>
using namespace std;

int main()
{
	char Answer;

	// Request the availability of a credit card from the user
	cout << "Are you ready to provide your credit card number(1=Yes/0=No)? ";
	cin >> Answer;

	// Since the user is ready, let's process the credit card transaction
	if(Answer == '1') cout << "\nNow we will get your credit card information.\n";

	cout << "\n";
	return 0;
}

You can write the if condition and the statement on different lines; this makes your program easier to read. The above code could be written as follows:

#include <iostream>
using namespace std;

int main()
{
	char Answer;

	// Request the availability of a credit card from the user
	cout << "Are you ready to provide your credit card number(1=Yes/0=No)? ";
	cin >> Answer;

	// Since the user is ready, let's process the credit card transaction
	if(Answer == '1')
		cout << "\nNow we will get your credit card information.\n";

	cout << "\n";
	return 0;

}

You can also write the statement on its own line if the statement is too long to fit on the same line with the condition.

Although the (simple) if statement is used to check one condition, it can lead to executing multiple dependent statements. If that is the case, enclose the group of statements between an opening curly bracket “{“ and a closing curly bracket “}”. Here is an example:

#include <iostream>
using namespace std;

int main()
{
	char Answer;
	char CreditCardNumber[40];

	// Request the availability of a credit card from the user
	cout << "Are you ready to provide your credit card number(1=Yes/0=No)? ";
	cin >> Answer;

	// Since the user is ready, let's process the credit card transaction
	if(Answer == '1')
	{
		cout << "\nNow we will continue processing the transaction.";
		cout << "\nPlease enter your credit card number without spaces: ";
		cin >> CreditCardNumber;
	}

	cout << "\n";
	return 0;
}

If you omit the brackets, only the statement that immediately follows the condition would be executed.

When studying logical operators, we found out that if a comparison produces a true result, it in fact produces a non zero integral result. When a comparison leads to false, its result is equivalent to 0. You can use this property of logical operations and omit the comparison if or when you expect the result of the comparison to be true, that is, to bear a valid value. This is illustrated in the following program:

#include <iostream>
using namespace std;

int main()
{
	int Number;

	cout << "Enter a non zero number: ";
	cin >> Number;

	if(Number)
		cout << "\nYou entered " << Number << endl;

	cout << endl;
	return 0;
}

Using the Logical Not

When a driver comes to a light that he expects to be green, we saw that he would use a statement such as, "The light is green". If in fact the light is green, we saw that the statement would lead to a true result. If the light is not green, the "The light is green" statement produces a false result. This is shown in the following table:

Color Statement Boolean Value The light is green true The light is green false

 
As you may realize already, in Boolean algebra, the result of performing a comparison depends on how the Condition is formulated. If the driver is approaching a light that he is expecting to display any color other than green, he would start from a statement such as "The light is not green".  If the light IS NOT green, the expression "The light is not green" is true (very important). This is illustrated in the following table:
 

Color Statement Boolean Value The light is green true The light is not green true

 
The "The light is not green" statement is expressed in Boolean algebra as “Not the light is green”. Instead of writing “Not the light is green", in C++, using the logical Not operator , you would formulate the statement as, !"The light is green". Therefore, if P means “The light is green”, you can express the negativity of P as !P. The Boolean table produced is:

Color Statement Boolean Value Symbol The light is green true P The light is not green false !P

When a statement is true, its Boolean value is equivalent to a non-zero integer such as 1. Otherwise, if a statement produces a false result, it is given a 0 value. Therefore, our table would be:

Color Statement Boolean Value Integer Value The light is green true 1 The light is not green false 0

Otherwise: if…else

The if condition is used to check one possibility and ignore anything else. Usually, other conditions should be considered. In this case, you can use more than one if statement. For example, on a program that asks a user to answer Yes or No, although the positive answer is the most expected, it is important to offer an alternate statement in case the user provides another answer. Here is an example:

#include <iostream>
using namespace std;

int main()
{
	char Answer;	

	cout << "Do you consider yourself a hot-tempered individual(y=Yes/n=No)? ";
	cin >> Answer;

	if( Answer == 'y' ) // First Condition
	{
		cout << "\nThis job involves a high level of self-control.";
		cout << "\nWe will get back to you.\n";
	}

	if( Answer == 'n' ) // Second Condition
		cout << "\nYou are hired!\n";

	return 0;
}

The problem with the above program is that the second if is not an alternative to the first, it is just another condition that the program has to check and execute after executing the first. On that program, if the user provides y as the answer to the question, the compiler would execute the content of its statement and the compiler would execute the second if condition.

You can also ask the compiler to check a condition; if that condition is true, the compiler will execute the intended statement. Otherwise, the compiler would execute alternate statement. This is performed using the syntax:

if(Condition)
    Statement1;
else
    Statement2;

The above program would better be written as:

#include <iostream>
using namespace std;

int main()
{
	char Answer;	

	cout << "Do you consider yourself a hot-tempered individual(y=Yes/n=No)? ";
	cin >> Answer;

	if( Answer == 'y' ) // One answer
	{
		cout << "\nThis job involves a high level of self-control.";
		cout << "\nWe will get back to you.\n";
	}

	else // Any other answer
		cout << "\nYou are hired!\n";

	return 0;
}

The Ternary Operator (?:)

The conditional operator behaves like a simple if…else statement. Its syntax is:

Condition ? Statement1 : Statement2;

The compiler would first test the Condition. If the Condition is true, then it would execute Statement1, otherwise it would execute Statement2. When you request two numbers from the user and would like to compare them, the following program would do find out which one of both numbers is higher. The comparison is performed using the conditional operator:

#include <iostream>
using namespace std;

int main()
{
	signed Num1, Num2, Max;

	cout << "Enter two numbers: ";
	cin >> Num1 >> Num2;

	Max = (Num1 < Num2) ? Num2 : Num1;

	cout << "\nThe maximum of " << Num1 
	     << " and " << Num2 << " is " << Max;

	return 0;
}

Conditional Statements: if…else if and if…else if…else

The previous conditional formula is used to execute one of two alternatives. Sometimes, your program will need to check many more than that. The syntax for such a situation is:

if(Condition1)
    Statement1;
else if(Condition2)
    Statement2;

An alternative syntax would add the last else as follows:

The compiler will check the first condition. If Condition1 is true, it will execute Statement1. If Condition1 is false, then the compiler will check the second condition. If Condition2 is true, it will execute Statement2. When the compiler finds a Condition-n to be true, it will execute its corresponding statement. It that Condition-n is false, the compiler will check the subsequent condition. This means you can include as many conditions as you see fit using the else if statement. If after examining all the known possible conditions you still think that there might be an unexpected condition, you can use the optional single else.

A program we previously wrote was considering that any answer other than y was negative. It would be more professional to consider a negative answer because the program anticipated one. Therefore, here is a better version of the program:

#include <iostream>
using namespace std;

int main()
{
	char Answer;	

	cout << "Do you consider yourself a hot-tempered individual(y=Yes/n=No)? ";
	cin >> Answer;

	if( Answer == 'y' ) // Unique Condition
	{
		cout << "\nThis job involves a high level of self-control.";
		cout << "\nWe will get back to you.\n";
	}
	else if( Answer == 'n' ) // Alternative
		cout << "\nYou are hired!\n";
	else
		cout << "\nThat's not a valid answer!\n";

	return 0;
}

The switch Statement

When defining an expression whose result would lead to a specific program execution, the switch statement considers that result and executes a statement based on the possible outcome of that expression, this possible outcome is called a case. The different outcomes are listed in the body of the switch statement and each case has its own execution, if necessary. The body of a switch statement is delimited from an opening to a closing curly brackets: “{“ to “}”. The syntax of the switch statement is:

The expression to examine is an integer. Since an enumeration (enum) and the character (char) data types are just other forms of integers, they can be used too. Here is an example of using the switch statement:

#include <iostream>
using namespace std;

int main()
{
	int Number;	

	cout << "Type a number between 1 and 3: ";
	cin >> Number;

	switch (Number)
	{
	case 1:
		cout << "\nYou typed 1.";
	case 2:
		cout << "\nYou typed 2.";
	case 3:
		cout << "\nYou typed 3.";
	}

	return 0;
}

The program above would request a number from the user. If the user types 1, it would execute the first, the second, and the third cases. If she types 2, the program would execute the second and third cases. If she supplies 3, only the third case would be considered. If the user types any other number, no case would execute.

When establishing the possible outcomes that the switch statement should consider, at times there will be other possibilities other than those listed and you will be likely to consider them. This special case is handled by the default keyword. The default case would be considered if none of the listed cases matches the supplied answer. The syntax of the switch statement that considers the default case would be:
 

#include <iostream>
using namespace std;

int main()
{
	int Number;

	cout << "Type a number between 1 and 3: ";
	cin >> Number;

	switch (Number)
	{
	case 1:
		cout << "\nYou typed 1.";
	case 2:
		cout << "\nYou typed 2.";
	case 3:
		cout << "\nYou typed 3.";
	default:
		cout << endl << Number << " is out of the requested range.";
	}

	return 0;
}

Counting and Looping

The C++ language provides a set of control statements that allows you to conditionally control data input and output. These controls are referred to as loops.

The while Statement

The while statement examines or evaluates a condition. The syntax of the while statement is:

while(Condition) Statement;

To execute this expression, the compiler first examines the Condition. If the Condition is true, then it executes the Statement. After executing the Statement, the Condition is checked again. AS LONG AS the Condition is true, it will keep executing the Statement. When or once the Condition becomes false, it exits the loop.

Here is an example:

int Number;

while( Number <= 12 )
{
	cout << "Number " << Number << endl;
	Number++;
}

To effectively execute a while condition, you should make sure you provide a mechanism for the compiler to use a get a reference value for the condition, variable, or expression being checked. This is sometimes in the form of a variable being initialized although it could be some other expression. Such a while condition could be illustrated as follows:

An example would be:

#include <iostream>
using namespace std;

int main()
{
	int Number;// = 0;	

	while( Number <= 12 )
	{
		cout << "Number " << Number << endl;
		Number++;
	}

	return 0;
}

The do...while Statement

The do…while statement uses the following syntax:

do Statement while (Condition);

The do…while condition executes a Statement first. After the first execution of the Statement, it examines the Condition. If the Condition is true, then it executes the Statement again. It will keep executing the Statement AS LONG AS the Condition is true. Once the Condition becomes false, the looping (the execution of the Statement) would stop.

If the Statement is a short one, such as made of one line, simply write it after the do keyword. Like the if and the while statements, the Condition being checked must be included between parentheses. The whole do…while statement must end with a semicolon.

Another version of the counting program seen previously would be:

#include <iostream>
using namespace std;

int main()
{
	int Number = 0;	

	do
		cout << "Number " << Number++ << endl;
	while( Number <= 12 );

	return 0;
}

If the Statement is long and should span more than one line, start it with an opening curly braket and end it with a closing curly bracket.

The do…while statement can be used to insist on getting a specific value from the user. For example, since our ergonomic program would like the user to sit down for the subsequent exercise, you can modify your program to continue only once she is sitting down. Here is an example on how you would accomplish that:

#include <iostream>
using namespace std;

int main()
{
	char SittingDown;	

	cout << "For the next exercise, you need to be sitting down\n";
	do {
		cout << "Are you sitting down now(y/n)? ";
		cin >> SittingDown;
	}
	while( !(SittingDown == 'y') );

	cout << "\nWonderful!!!";
	return 0;
}

The for Statement

The for statement is typically used to count a number of items. At its regular structure, it is divided in three parts. The first section specifies the starting point for the count. The second section sets the counting limit. The last section determines the counting frequency. The syntax of the for statement is:

for( Start; End; Frequency) Statement;

The Start expression is a variable assigned the starting value. This could be Count = 0;
The End expression sets the criteria for ending the counting. An example would be Count < 24; this means the counting would continue as long as the Count variable is less than 24. When the count is about to rich 24, because in this case 24 is excluded, the counting would stop. To include the counting limit, use the <= or >= comparison operators depending on how you are counting. The Frequency expression would let the compiler know how many numbers to add or subtract before continuing with the loop. This expression could be an increment operation such as ++Count.

Here is an example that applies the for statement:

#include <iostream>
using namespace std;

int main()
{
	for(int Count = 0; Count <= 12; Count++)
		cout << "Number " << Count << endl;

	return 0;
}

The C++ compiler recognizes that a variable declared as the counter of a for loop is available only in that for loop. This means the scope of the counting variable is confined only to the for loop. This allows different for loops to use the same counter variable. Here is an example:

#include <iostream>
using namespace std;

int main()
{
	for(int Count = 0; Count <= 12; Count++)
		cout << "Number " << Count << endl;
	cout << endl;

	for(int Count = 10; Count >= 2; Count--)
		cout << "Number " << Count << endl;

	return 0;
}

Some compilers do not allow the same counter variable in more than one for loop. The counter variable’s scope spans beyond the for loop. With such a compiler, you must use a different counter variable for each for loop. An alternative to using the same counter variable in different for loops is to declare the counter variable outside of the first for loop and call the variable in the needed for loops. Here is an example:

#include <iostream>
using namespace std;

int main()
{
	int Count;

	for(Count = 0; Count <= 12; Count++)
		cout << "Number " << Count << endl;
	cout << endl;

	for(Count = 10; Count >= 2; Count--)
		cout << "Number " << Count << endl;

	return 0;
}