LAB: STRUCTURES, ARRAYS,

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1 LAB: STRUCTURES, ARRAYS, AND FILES IN C++ MODULE 2 JEFFREY A. STONE and TRICIA K. CLARK COPYRIGHT VERSION 3.3

2 PALMS MODULE 2 LAB: STRUCTURES, ARRAYS, AND FILES IN C++ 2 Introduction This lab will provide students with an introduction to the use of structures, arrays, and files in the C++ programming language. The focus of this lab is on how to solve complex problems using data structures. At the end of this module, students will Be able to solve simple problems using simple data structures Understand the use of the struct keyword in C++ Understand the use C++ data structures to solve analysis problems. For those exercises which ask you to write code, structure your storage drive in the following manner. There should be a palms folder, and within that folder there should be a LabStructs folder. All coding exercises (and related folders) for this lesson should be placed in the LabStructs folder. As always, be sure to properly document your code. Consult the C++ Coding Guidelines document for proper coding standards. Use good design principles and design the solution before attempting to write code. Let s begin! Example: Line of Best Fit A common procedure in data analysis is the computation of the line of best fit for a set of (x, y) coordinate data. Suppose you have a set of data on the amount of CO 2 emissions in the United States for the years Plotting this data on an (x, y) coordinate plan would give us the following: Emissions (kt) Emissions (kt)

3 PALMS MODULE 2 LAB: STRUCTURES, ARRAYS, AND FILES IN C++ 3 Suppose we wanted to determine the line of best fit, i.e. the trend line that best fits the points on the graph (which is really a scatterplot). We could write a C++ program to assist us. This program would necessarily involve the following elements: Reading the data (years and emissions, in kilotons) from a text file; Storing the data into one or more arrays; Processing the arrays to compute the line of best fit coordinates Producing an output file with the coordinates for the line of best fit. We will begin this process by first writing some code to echo-print the emissions data from a text file using a new data type. Next, we will extend this code to perform the line of best fit calculations. Finally, we will be produce an output file of the line of best fit coordinates. Exercise 1: We could begin by creating a new C++ source file called LineOfBestFit.cpp. Make sure that you include the <fstream> library. Besides the normal program structure (a set of library includes, a main function) we would want to add the following new type definition to the area above the main function: struct CO2Record int year; the year for the data double emissions; the amount of emissions (kilotons) ; This new type CO2Record is a C++ struct ( structure ) definition. A C++ struct is a data structure, i.e. a set of distinct data elements grouped under one name. One-dimensional arrays are homogeneous data structures (all elements of the same type) whereas a struct is considered a heterogeneous data structure (all elements can be of different types). Once the new type declaration has been added, add some code to main to do the following: Create a CO2Record variable named data. Create an ifstream variable called ifs. Review the ifstream tutorial if necessary. Using the ifs variable, open a file named CO2.txt and check to see if the open worked. If it did not, inform the user. If the open worked, output a simple message to the user indicating that the file is now open. The CO2.txt will be provided by the instructor. Close the file associated with the ifs variable. Compile and run your program to verify it works as it should. Exercise 2: Add code to your LineOfBestFit.cpp file to read the CO2.txt file. You will want to read the file one token at a time. Since each line of the file contains two tokens one for the

4 PALMS MODULE 2 LAB: STRUCTURES, ARRAYS, AND FILES IN C++ 4 year, one for the emissions in kilotons you can use the data variable created in Exercise 1 to store the two pieces of data. After reading a complete line of the file, echo-print the data to the screen like so: Press any key to continue... You will need a while loop to read the entirety of the file. Inside the while loop, make two reads (one for year, one for emissions) and print out the data. Remember that you can use the >> operator to read a token from a file. For example, to read the year, you could have this code: get the next year... ifs >> data.year; Exercise 3: Change the declaration of data to be an array of 50 elements, all of type CO2Record. Modify the file read performed in Exercise 2 to fill the contents of this array. Instead of echoprinting the file as you go, remove the cout statement from inside your while loop. Add a second loop to output the contents of the data array after the file has been completely read. The output should be exactly the same as in the previous exercise, but the code will be different. Once you have verified that the program successfully echo-prints the contents of the file, remove the for loop which outputs the contents of the array we will not need it again. Making the Calculations We will use the Least Square Method to determine an equation for our line of best fit. Read over the following link before moving on to the next exercise: Line of Best Fit (Least Square Method) Hotmath.com Exercise 4: Add the following function to your LineOfBestFit.cpp file; don t forget to add a function prototype!

5 PALMS MODULE 2 LAB: STRUCTURES, ARRAYS, AND FILES IN C++ 5 / FUNCTION NAME: computesumofx PARAMETERS: CO2Record list[] -- the array of data int size -- number of elements in the array RETURN TYPE: double PURPOSE: Sums and returns the "year" values in the array. / double computesumofx( CO2Record list[], int size ) double sum = 0.0; stores the sum for (int i = 0; i < size; i++) add the year to the sum... sum += list[i].year; return sum; return the sum value Compile your program to check for any syntax errors. Exercise 5: Add the following function to your LineOfBestFit.cpp file; don t forget to add a function prototype! / FUNCTION NAME: computesumofy PARAMETERS: CO2Record list[] -- the array of data int size -- number of elements in the array RETURN TYPE: double PURPOSE: Sums and returns the "emissions" values in the array. / double computesumofy( CO2Record list[], int size ) double sum = 0.0; stores the sum for (int i = 0; i < size; i++) add the emissions to the sum... sum += list[i].emissions; return sum; return the sum value Compile your program to check for any syntax errors.

6 PALMS MODULE 2 LAB: STRUCTURES, ARRAYS, AND FILES IN C++ 6 Exercise 6: Add the following functions to your LineOfBestFit.cpp file: double computesumofxsquared( CO2Record list[], int size ); calculates the sum of the squares of the year values found in list double computesumofxtimesy( CO2Record list[], int size ); calculates the sum of the ( year * emissions ) values found in list double computeslope( double sum_xy, double sum_x_squared, double sum_x, double sum_y, int size); calculates the slope using the given values. Hint: The computesumofxsquared and computesumofxtimesy functions are very similar to the functions created in Exercise 4 and Exercise 5 (and to each other). Compile your program to check for any syntax errors. Exercise 7: Add the following functions to your LineOfBestFit.cpp file: double computexmean( CO2Record list[], int size ); calculates the mean of the year values found in list double computeymean( CO2Record list[], int size ); calculates the mean of the emissions values found in list Hint: You can use the computesumofx and computesumofy functions to simplify the calculations. You don t even need for loops! Compile your program to check for any syntax errors. Exercise 8: Add the following function to your LineOfBestFit.cpp file; don t forget to add a function prototype! / FUNCTION NAME: computeyintercept PARAMETERS: double mean_x -- the mean X value double mean_y -- the mean Y value double slope -- the slope RETURN TYPE: double PURPOSE: Computes and returns the Y-intercept. / double computeyintercept( double mean_x, double mean_y, double slope ) return the y-intercept... return mean_y - ( slope * mean_x ); Compile your program to check for any syntax errors.

7 PALMS MODULE 2 LAB: STRUCTURES, ARRAYS, AND FILES IN C++ 7 Exercise 9: Add the following code to your main function, just after the while loop which performs the file read. The purpose of this code is to test the functions you wrote in Exercises 4 through 8. Computing the relevant values... double sum_x = computesumofx( data, 50 ); sum of X values double sum_y = computesumofy( data, 50 ); sum of Y values double sum_x2 = computesumofxsquared( data, 50 ); sum of X^2 values double sum_xy = computesumofxtimesy( data, 50 ); sum of X*Y values double mean_x = computexmean( data, 50 ); mean X value double mean_y = computeymean( data, 50 ); mean Y value slope value... double m = computeslope( sum_xy, sum_x2, sum_x, sum_y, 50 ); y-intercept... double b = computeyintercept( mean_x, mean_y, m ); cout.precision(2); cout.setf(ios::fixed); two digits of precision always show the decimal point output the values... cout << "Sum of X: " << setw(20) << sum_x << endl << "Sum of Y: " << setw(20) << sum_y << endl << "Sum of X^2: " << setw(18) << sum_x2 << endl << "Sum of X*Y: " << setw(18) << sum_xy << endl << "Mean of X: " << setw(19) << mean_x << endl << "Mean of Y: " << setw(19) << mean_y << endl << "Slope: " << setw(23) << m << endl << "Y-Intercept: " << setw(17) << b << endl << endl; Once the code is typed in, compile and run your program to verify that the following output is produced. Sum of X: Sum of Y: Sum of X^2: Sum of X*Y: Mean of X: Mean of Y: Slope: Y-Intercept: Press any key to continue... If one or more of the numbers produced by your program do not match those listed above, go back to that function and check your logic. Correct any errors before moving on to the next exercise.

8 PALMS MODULE 2 LAB: STRUCTURES, ARRAYS, AND FILES IN C++ 8 Exercise 10: We will next want to add in the code to compute the line of best fit coordinates. Add a new 50-element array of type CO2Record to your main function; name it new_data_points. Next add the following function to your file; don t forget to add a function prototype! / FUNCTION NAME: computelineofbestfit PARAMETERS: CO2Record list[] -- the array of data CO2Record new_list[] -- the array of "new" data int size -- number of elements in the array double slope -- the "m" value double yint -- the "b" value RETURN TYPE: void PURPOSE: Fills the "new_list" array with the "line of best fit" values. / void computelineofbestfit( CO2Record list[], CO2Record new_list[], int size, double slope, double yint ) Begin Loop through the array... set the "x" coordinate (year) of the "new" array element to be the "old" element s year. set the "y" coordinate (emissions) of the "new" array element using the formula y = mx + b. End loop You will need to complete the code for this function. Use the comments as a guide. Once your function is complete, add a line of code to the main function to call computelineofbestfit. The function call should occur just after the code you added in Exercise 9. Creating the New File Our final step is to create a new file on disk containing the original data AND the new emissions values for the line of best fit. Exercise 11: Picking up where you left off in the previous exercise, add some code to main to do the following: Create an ofstream variable named ofs. Review the ofstream tutorial if necessary Using the ofs variable, open a file named CO2_LOBF.TXT and check to see if the open worked. If it did not, inform the user. If the open worked, create a for loop which writes the following data to the file, three parts per line, 50 lines in all: o The year (from the data array) o The original emissions value (from the data array) o The new emissions value (from the new_data_points array) Close the file associated with the ofs variable.

9 PALMS MODULE 2 LAB: STRUCTURES, ARRAYS, AND FILES IN C++ 9 Compile and run your program to verify it works as it should. Your CO2_LOBF.txt file should contain the data found on the next page. In the end, the points your program produced could be graphed like so: Emissions (kt) Emissions (kt) Trend

10 PALMS MODULE 2 LAB: STRUCTURES, ARRAYS, AND FILES IN C++ 10 CO2_LOBF.TXT Contents