Conditionals

if(some condition is true) then
	do this
else
	do this instead

The bool data type

bool isDataOK;
bool done;

Logical Expressions

• bool isDataOK; – Boolean expression which can be true or false.
• (x < y) – Is x less than y? If x and y are int variables this condition can be true or false.
• (a < b) AND (c > d) – Two logical expressions. If a, b, c and d are int variables this condition can be true (if a is less than b and c is greater than d) or false.

Relational Operators

	int x = 10;
	int y = 15;
	char ch1 = 'm';
	char ch2 = 'n';
	float f1 = 1.2345;
	float f2 = 1.2345;
	string s1 = "This is a string";
	string s2 = "This is a string, too";

1. Don’t try to compare floats or doubles as the internal representation of these can make two seemingly equal values appear not equal.
2. If you compare a Boolean with an int (example; (isDataOK < 5)) the boolean value true is type coerced to the integer value 1 and the boolean value false is type coerced to the integer value 0. That means this conditional will always be false since both 0 and 1 are less than 5.

The if statement

The if-else statement

Warning

	int n;
	cin >> n;
	if(n = 3)
		cout << "n equals 3" << endl;
	else
		cout << "n not equal 3" << endl;

1. At the statement cin >> n; the user will input an integer value.
2. The statement in parentheses is NOT a conditional statement. It actualls sets the variable n equal to 3.
3. Now with n equal to 3 the actual conditional that is tested is if (n). But, If n is zero the conditional is evaluated as false. If n is not zero the conditional is evaluated as true.

Nested if statements

	if (x < y)
		cout << "x is less than y" << endl;
	else if (x == y)
		cout << "x is equal to y"  << endl;
	else
		cout << "x is greater than y" << endl;

Warning: The dangling else

	if (average >= 60)
		if(average < 70)
			cout << "Passing but marginal";
	else
		cout << "Failing";

	if (average >= 60)
	{
		if(average < 70)
			cout << "Passing but marginal";
	}
	else
		cout << "Failing";

Logical Operators

	If (this is true) AND (that is true)
	If (this is true) OR (that is true)
	If NOT (this is false)

• && - Logical AND
• || - Logical OR
• ! - Logical NOT

	if ((x == 5) && (y < 10))
		cout << "both conditions OK" << endl;

if ((x == 5) || (y < 10))
	cout << "At least one condition is OK" << endl;

if (!(x == y))
	cout << "x not equal to y" << endl;

	if(condition 1) && (condition 2)

	if(condition 1) || (condition 2)

Precedence with Conditional Operators

Testing the state of an I/O Stream

	#include <iostream>
	#include <fstream>
	#include <string>
	using namespace std;

	int main()
	{
		string inFileName, outFileName;
		ifstream inFile;
		ofstream outFile;
		int year, month, day;
		string str1, str2;
		// Prompt user for input
   		cout << "Enter the name of the input data file." << endl;
   		cin >> inFileName;
   		cout << "Enter the name of the output file." << endl;
   		cin >> outFileName;
   		// Let user know what is happening
   		cout << "Opening data file " << inFileName << " for input." << endl;
   		inFile.open(inFileName.c_str());
		
		if(!inFile.good())
		{
			cout << "Failed to open data file. Program terminating." << endl;
			return 0;
		}
		
		cout << "Opening data file " << outFileName << " for output." << endl;
		outFile.open(outFileName.c_str());
		
		if(!outFile.good())
		{
			cout << "Failed to ouput data file. Program terminating." << endl;
			return 0;
		}
		// The rest of the input code section of this example is here.
	}

Conditional expressions

In some situations a simple if-else statement can be replaced with a conditional expression. The syntax of a conditional expression consists of a conditional statement in parentheses. This can be just like the conditionals used in an if-else statement. This is followed by a question mark, a value to be set if the condition is true, a colon, and finally a value to be set if the condition is false.



Suppose you have this if-else statement:

	if(a < b)
		max = a;
	else
		max = b;

This if-else expression can also be written using the conditional expression syntax:

	max = (a < b) ? a : b;

The conditional expression is handy short-hand syntax for certain situations, but be careful, you cannot substitued a conditional expression for every if-else statement.

The Switch statement

In some situations where you have a nested if it may be more efficient to use a switch statement. The general syntax for a switch statement is:



This consists of the keyword switch followed by an interger expression (usually just an integer variable) in parentheses. Then enclosed in braces will be any number of case blocks consisting of the keyword case, a value which the interger expression could take and a colon. This is followed by any number of statements to be executed if the interger expression has this value. The list of statements to be excuted is then terminated using the break keyword. You may also end the list of case blocks with an optional default block containing statements to be executed if the integer expression holds none of the given case values.

Suppose you have the following nexted if-else statement:

	int dNum;
	cout << "Which doctor do you want to see?\n"
	cout << "(1)Smith, (2)Jones, (3)Quack\n";
	cin >> dNum;

	if(dNum == 1)
     		cout << "Down the hall to room 123\n";
	else if(dNum == 2)
		cout << "Up stairs to room 234\n";
	else if(dNum == 3)
		cout << "Take elevator to room 1313\n";
	else
		cout << "Sorry, no such doc here.\n';

This can be rewritten in the form of a switch statement:

	int dNum;
	cout << "Which doctor do you want to see?\n"
	cout << "(1)Smith, (2)Jones, (3)Quack\n";
	cin >> dNum;

	switch(dNum)
	{
		case 1 : cout << "Down the hall to room 123\n";
			break;
		case 2 : cout << "Up stairs to room 234\n";
			break;
		case 3 : cout << "Take elevator to room 1313\n";
			break;
		default: cout << "Sorry, no such doc here.\n";
	}

Here is an example of using a swith statement with a char variable as the interger expression.

	char grade;
	// Variable grade set to 'A', 'B', 'C', 'D', or 'F'

	switch(grade)
	{
		case 'A' : cout << "Excellent work\n";
			break;
		case 'B' : cout << "Good work\n";
			break;
		case 'C' : cout << "Average work\n";
			break;
		case 'D' : cout << "Poor work\n";
			break;
		case 'F' : cout << "Change your major!\n";
			break;
	}

If you fail to include the break statement at the end of the list of statements to be executed for a case block then execution will "fall through" to the next case block and execute the statements there. Sometimes this may be what you want. Suppose in the example above the char variable grade could hold either an upper or lower case letter grade. The following switch statement would give the appropriate message regardless of the case of the leter grade.

	char grade;
	// Variable grade set to 'a', 'A', 'b', 'B', 'c', 'C', 'd', 'D', 'f', or 'F'

	switch(grade)
	{
		case 'a' : // No break so execution falls through to the next case
		case 'A' : cout << "Excellent work\n";
			break;
		case 'b' :
		case 'B' : cout << "Good work\n";
			break;
		case 'c' :
		case 'C' : cout << "Average work\n";
			break;
		case 'd' :
		case 'D' : cout << "Poor work\n";
			break;
		case 'f' :
		case 'F' : cout << "Change your major!\n";
			break;
	}