Nested if Statements

	Example (in pseudocode)
	IF today is Saturday or Sunday
	IF it is raining
	Sleep late
	ELSE
	Get up and go outside
	ELSE
	Go to work

if Alternatives

	if (month == 1) if (month == 1)
		cout << “January”;   cout << “January”;
	if (month == 2) else
	cout << “February”;   if (month == 2)
	if (month == 3)     cout << “February”;
	cout << “March”;   else
	.     if (month == 3)
	.       cout << “March”;
	.     else
	if (month == 12)       if (month == 4)
	cout << “December”;

Multiple Alternatives

	cout << “The recommended activity is ”;
	if (temperature > 85)
	cout << “swimming.” << endl;
	else if (temperature > 70)
	cout << “tennis.” << endl;
	else if (temperature > 32)
	cout << “golf.” << endl;
	else if (temperature > 0)
	cout << “skiing.” << endl;
	else
	cout << “dancing.” << endl;

Beware of the dangling else

	if (average >= 60.0)
	if (average < 70.0)
	cout << “Passing but marginal”;
	else
	cout << “Failing”;

Beware of the dangling else

	if (average >= 60.0) // Incorrect
	if (average < 70.0)
	cout << “Passing but marginal”;
	else
	cout << “Failing”;

Beware of the dangling else

	if (average >= 60.0) // Incorrect
	if (average < 70.0)
	cout << “Passing but marginal”;
	else
	cout << “Failing”;
	if (average >= 60.0) // Correct
	{
	if (average < 70.0)
	cout << “Passing but marginal”;
	}
	else
	cout << “Failing”;

Testing the State of an
I/O Stream

	C++ provides a way to check whether a stream is in the fail state. You simply use the name of the stream object as if it were a Boolean variable.

I/O Stream State Test Example

	int main()
	{
	int height, width;
	ifstream inFile;
	inFile.open(“mydata.dat”);
	if ( !inFile )
	{
	cout << “Can’t open the input file.”;
	return 1;
	}
	inFile >> height >> width;
	return 0;
	}

Problem-Solving Case Study-
Warning Notices

	Problem: In order to warn freshmen who are in danger of failing a class, your program should calculate the average of three tests. Then, the program prints out the student id, average, and either pass, fail, or marginal.
	Input: Student ID number (long) followed by three test grades.
	Output: A prompt for input, echo print input, id, average, message with possible error message for invalid data (<0).
	Discussion: Passing is >= 70.0, marginal is < 70.0 and >= 60.0, failing is < 60.0.

Main Module (Level 0)

	Get data
	Test data
	IF data OK
		Calculate average
		Print message indicating status
	ELSE
		Print “Invalid Data: Score(s) less than zero.”

Print Message Indicating Status
(Level 1)

	Print average
	IF average >= 60.0
		Print “Passing”
		IF average < 70.0
			Print “but marginal”
		Print ‘.’
	ELSE
		Print “Failing”

Notices Program

	//************************************************************
	// Notices program
	// This program determines (1) a student's average based on
	// three test scores and (2) the student's passing/failing
	// status
	//************************************************************
	#include <iostream>
	#include <iomanip>    // For setprecision()
	using namespace std;
	int main()
	{
	float average;        // Average of three test scores
	long  studentID;      // Student's identification number
	int   test1;          // Score for first test
	int   test2;          // Score for second test

More Notices Program

	int   test3;          // Score for third test
	bool  dataOK;         // True if data is correct
	cout << fixed << showpoint;         // Set up floating pt.
	//   output format
	// Get data
	cout << "Enter a Student ID number and three test scores:"
	<< endl;
	cin >> studentID >> test1 >> test2 >> test3;
	cout << "Student number: " << studentID << "  Test Scores: "
	<< test1 << ", " << test2 << ", " << test3 << endl;
	// Test data
	if (test1 < 0 || test2 < 0 || test3 < 0)
	dataOK = false;
	else
	dataOK = true;

Notices Program (continued)

	if (dataOK)
	{
	// Calculate average
	average = float(test1 + test2 + test3) / 3.0;
	// Print message
	cout << "Average score is " << setprecision(2)
	<< average << "--";
	if (average >= 60.0)
	{
	cout << "Passing";            // Student is passing
	if (average < 70.0)
	cout << " but marginal";  // But marginal
	cout << '.' << endl;
	}

Notices Program (The End)

	else                        // Student is failing
	cout << "Failing." << endl;
	}
	else                                 // Invalid data
	cout << "Invalid Data:  Score(s) less than zero."
	<< endl;
	return 0;
	}

Testing and Debugging Phases

	Problem-Solving Phase
		Algorithm Walk-Through
			Precondition - an assertion which must be true before a module is executed in order for the module to execute correctly.
			Postcondition - an assertion which will be true after the module has executed, if it has done its job properly.

Testing and Debugging Phases

	Implementation Phase
		Code Walk-Through - make sure that you’ve faithfully reproduced the algorithm.
		Execution Trace - play computer and execute each statement of a program by hand.
		Testing Selection Control Structures - you need to execute each branch at least once and verify the results.

Testing Selection
Control Structures

Test Set 1
test1 = 100, test2 =100, test3 = 100

Test Set 2
test1 = 60, test2 = 60, test3 = 63

Test Set 3
test1 = 50, test2 = 50, test3 = 50

Test Set 4
test1 = -50, test2 = 50, test3 = 50