Session 14 March 31, 2018

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1 Notes: Session 13 (2/28) and Session 15 (3/7) are catch up Sessions. This document contains documentation for the next group of Core Competencies, including practice problems that you should work. We will go over these practice problems during Session 16, and the next Checkpoint for the first 8 Core Competencies will be during Session 18. Preliminary Work for Session 14 You should have read Sections 5.5, 5.7, and 5.9 in the zybook and done the Participation Activities before this session. And, as usual, you should be working on the Challenge Activities from the assigned sections of Chapter 5. You should read Sections 5.6 and 5.8 sometime, but none of the Activities in these sections are assigned. Preparatory Work for Session 16 Read Sections and 6.7 and do the Participation Activities to prepare for Session 16. Sections 6.5 and 6.6 are not assigned, but you might want to skim them. Work the practice problems for Core Competencies EBL, WML, TAPT, TAPM, and TAPS. In the Previews folder there is a preview for Session 16, preview16.pdf, that you should read. Core Lab 5.4 [4 lab points] Create a class LabCh05SumSubRect that will read information from a data file into a 2D array, as detailed below, and will then ask the user for four numbers specifying a subrectangle in the array, and will then compute and report the total of all the numbers in that region. Open an input data file named Data5-4 and read the number of rows and number of columns from the first line. Create a 2D array of double that has the required size (for the number of rows and columns given in the file) Read all the numbers from the input file into the 2D array. Ask the user for the first row, last row, first column, and last column of the sub-rectangle in the 2D array that they want to total. For example, they might enter meaning they want to total the rectangular sub-region with rows 3 through 6 and columns 4 through 8. If the values entered do not form a valid sub-rectangle, display an error message and halt. CS 1050 Spring 2018 Page 42

2 Here is a picture of the situation, with reasonable names for the various variables: 0 firstcol lastcol numcols 0 firstrow lastrow numrows Compute the total of all the values in the sub-region and report it. Enhanced Lab 5.5 [2 lab points] This Lab will give you the opportunity to explore using images in your Java programs. Note that you don t have to write that much code this Lab is more about you reading the existing program and learning how you can do things with images in Java. We are doing this now because an image is really just a fancy 2D array. From the Sketchpad folder in the Labs folder download the file ExploreImages.java and the new version of Sketchpad.java into a folder that also contains Basic.java. Study the code in the constructor for ExploreImages, in lines Note how a BufferedImage object is created from an image file using the static method read in the ImageIO class. Note that six 2D arrays are set up, each with the same number of rows and columns as the image. Finally, note that the BufferedImage class has instance methods getrgb (line 53) and setrgb (line 84) that allow us to get an individual pixel from the image and to change the pixel stored at any location in the image. Actually, each pixel in a BufferedImage is stored as a single int value. An int in Java takes 4 bytes of memory, and each byte is used to store part of the color: Bits: alpha red green blue If you are really into this stuff, look at the helper methods at the end of the file to see how Java lets us conveniently work with bits using the shift operator >>, the bitwise and operator &, and writing int literals in hexadecimal! For convenience, we are extracting the red, green, and blue levels for each pixel and storing them in three separate 2D arrays. CS 1050 Spring 2018 Page 43

3 Run ExploreImages. You will need to click back in the output window in order to type in the name of the image file you want to use (you can use any jpeg file you want, or you can download pups.jpg from the Sketchpad folder if you don t want to use some other image). Once you have typed the file name, the image file will be read into a BufferedImage object and the step method will display the image in the sketchpad. You will see that the image looks a little weird. Study the code in lines that goes through every location in the parallel 2D arrays red, green, and blue, and copies transformed color numbers (just switching red and green and leaving blue unchanged) into the same locations in red2, green2, and blue2. Note that the code in lines copies all these array values into the corresponding spots in the image. Then, just to show you how it can be done, the current image is saved to an image file on line 91, with "odd-" prepended to the original file name. Now for the only code you have to write for this Lab: replace the code in lines (the body of the nested for loop) by your own code that will average each pixel with its 8 neighbors. This should be done separately for red, green, and blue. Use the 2 arrays to hold the results, and use the plain arrays to get the original image data. For example, here are the 9 cells near red[row][col] that should be averaged and stored in red2[row][col]: col 1 col col+1 row 1 row row+1 When you run your code, it will crash because the pixels in the outer rectangle don t have all 8 neighbors. You could fix this with a bunch of if statements, taking care of all the different cases (some locations have only three neighbors, some have five, most have eight), but to keep it simple just change the limits of the row/column nested for loops to start with row 1 and column 1 and only go up to the next to the last row and column. Your final, correct image will probably only be subtly different from the original. This averaging with neighbors tends to smooth out the image a little, and you might have to study the original image and the averaged one side-by-side to see the differences. CS 1050 Spring 2018 Page 44

4 Core Competency EBL (Execute Branching and Looping) Given code that does reasonably complex nested branching and looping statements, trace execution. Trace the execution of the program listed below, showing the successive contents of each memory cell and exactly what is displayed on screen. 1 public class EBL { 2 3 public static void main(string[] args) { 4 5 int x = 6; 6 7 while( x!= 1 ) { 8 9 System.out.println("top: " + x ); if( x % 2 == 0 ) { 12 x = x / 2; else { 15 x = 3 * x + 1; System.out.println("end: " + x ); Local Variables x top: 6 top: 3 top: 10 top: 5 top: 16 top: 8 top: 4 top: 2 end: Display CS 1050 Spring 2018 Page 45

5 Practice Problem: trace the execution of the program listed below. public class EBL2 { public static void main(string[] args) { int a = 12; int s = 0; while ( s <= 10 ) { if (a > 6) { s = s + 2; a = a - 3; System.out.println("1, a: " + a + " s: " + s ); else if (a<=4) { s = s + 3; a = a - 2; System.out.println("2, a: " + a + " s: " + s ); else { s = s + 4; a = a - 3; System.out.println("3, a: " + a + " s: " + s ); System.out.println("4, a: " + a + " s: " + s ); CS 1050 Spring 2018 Page 46

6 Core Competency WML (Write Menu-Loop) Given a chart showing the desired actions for various commands, write a menu-loop structure to implement, including display of relevant information at top, prompt for command, getting the command, and branching to perform command. Write a fragment of code, starting with the given code, that will allow the user to interactively change the value of a variable x, using the commands/actions specified in the chart below, Be sure to show the user the value of x at the top of the loop. Assume that the commands are full line strings. Command Action(s) r increase x by 1 l Decrease x by 1 i q any other string Allow the user to input any desired value for x Exit the loop and print the final value of x Print an error message // given code: double x = 1; String command; boolean done = false; Scanner keys = new Scanner(System.in); // this code is what you would write for your answer: while (! done ) { System.out.println("current value of x is " + x ); System.out.print("command? "); command = keys.nextline(); if ( command.equals("r") ) { x = x + 1; else if ( command.equals("l") ) { x = x - 1; else if ( command.equals("i") ) { System.out.print("enter desired value for x: " ); CS 1050 Spring 2018 Page 47

7 x = keys.nextdouble(); keys.nextline(); // crucial to finish line else if( command.equals("q") ) { done = true; else { System.out.println("Illegal command!"); System.out.println("final value of x is " + x ); Practice Problem: Write a menu-loop structure to implement a simple banking application following the chart below, where the savings and checking account balances are stored in the int variables savings and checking which are both displayed at the top of the loop. Command wc ws dc ds quit Action(s) Withdraw $20 from checking Withdrdaw $20 from savings Deposit $20 into checking Deposit $20 into savings Exit the loop CS 1050 Spring 2018 Page 48

8 Note: the following three Core Competencies are closely related. They all require traversing an array and performing some task. Because this is such an important thing to do in programming, we focus on three useful tasks that follow this same pattern. Core Competency TAPT (Traverse Array of Primitives and Total) Write a fragment of code that will traverse the specified items in a primitive array and add them up. Given an array a, where all its locations have a double value stored in them, total and display the items in the entire array that are in the even-numbered indices. Here is the solution: double total = 0; for (int k=0; k<a.length; k=k+2) { total = total + a[k]; System.out.println("Total of even index items is " + total ); Practice Problem: Assume that an array named scores contains some scores, where the number of scores stored in the array is stored in the variable number. In other words, assume that scores[0], scores[1],..., scores[number-1] hold a bunch of scores. Write a fragment of code that will compute and display the total of all these scores that are less than 100. Core Competency TAPM (Traverse Array of Primitives and find Minimum) Write a fragment of code that will traverse the specified items in a primitive array and find the location of either the minimum or the maximum item. Given an array a, where all its locations have a double value stored in them, find and display the location of the maximum item in the range from index first to index last. Here is the solution: int loc = first; for (int k=first+1; k<last; k++) { if( a[k] > a[loc] ) { loc = k; System.out.println("Index of max item is " + loc ); CS 1050 Spring 2018 Page 49

9 Practice Problem: Assume that an array named scores contains some scores, where the number of scores stored in the array is stored in the variable num. In other words, assume that scores[0], scores[1],..., scores[number-1] hold a bunch of scores. Write a fragment of code that will find and display the location of the lowest score in this range. Core Competency TAPS (Traverse Array of Primitives and Search) Write a fragment of code that will traverse the specified items in a primitive array and find the location of a target item. Given an array a, where all its locations have a double value stored in them, and a double variable target that holds the target value, find and display the location of the first (lowest index) place the target value is found, or display a message saying that the item is not in the list. Here is the solution: int loc = -1; for (int 0; k<number && loc<0; k++) { if( a[k] == target ) { loc = k; if( loc >= 0 ) { System.out.println("Index of first target item is " + loc ); else { System.out.println("Target not found in the list"); Practice Problem: Assume that an array named scores contains some scores, where the number of scores stored in the array is stored in the variable num. In other words, assume that scores[0], scores[1],..., scores[number-1] hold a bunch of scores. Write a fragment of code that will find and report the location of the first occurrence of the target item 37 in the list, or display a message that it is not in the list. CS 1050 Spring 2018 Page 50