COSC 236 Section 101 Computer Science 1 -- Prof. Michael A. Soderstrand

Transcription

1 COSC 236 Section 101 Computer Science 1 -- Prof. Michael A. Soderstrand

2 COSC 236 Web Site You will always find the course material at: or or From this site you can click on the COSC-236 tab to download the PowerPoint lectures, the Quiz solutions and the Laboratory assignments. 2

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4 Review of Quiz 20 Use Quiz 19 as your model Only the second part of Quiz 19 needs to be modified You will print the array in the order it was entered using Arrays.toString You will the use Arrays.sort to sort the array into ascending order and print it using Arrays.toString Finally you will use your method to reverse the array and print the array in descending order using Arrays.toString 4

5 Review of Quiz 20 The Program form Quiz 19: import java.util.*; public class Quiz19 { public static void main(string[] args) { Scanner console = new Scanner(System.in); System.out.print("How many test scores do you wish to enter? "); int number = console.nextint(); double[] scores = new double[number]; // array to store scores double sum = 0; for (int i = 0; i < number; i++) { // read/store each score System.out.print("Please enter test " + (i + 1) + "'s score: "); scores[i] = console.nextdouble(); sum += scores[i]; double mean = sum / number; System.out.printf("Original Arrays.sort(scores); array = %s\n", Arrays.toString(scores)); Arrays.sort(scores); double high = scores[number-1]; System.out.printf(" double low = scores[0]; Sorted array = %s\n", Arrays.toString(scores)); System.out.printf("Reversed double median = 0; array = %s\n", Arrays.toString(reverse(scores))); if(number%2 == 0) { median = (scores[number/2] + scores[number/2-1])/2; //Add else your { method to reverse the elements of an array public median static = scores[number/2]; double[] reverse(double[] array) { for // report (int i results = 0; i < array.length/2; i++) { System.out.printf("Test double temp = array[i]; Statistics:\n"); System.out.printf(" array[i] = array[array.length High Score: - i %6.2f\n", -1]; high); System.out.printf(" array[array.length - Mean i -1] Score: = temp; %6.2f\n", mean); return System.out.printf(" array; Median Score: %6.2f\n", median); System.out.printf(" Low Score: %6.2f\n", low); Replace this 5

6 Review of Quiz 20 Replaced code: System.out.printf("Original array = %s\n", Arrays.toString(scores)); Arrays.sort(scores); System.out.printf(" Sorted array = %s\n", Arrays.toString(scores)); System.out.printf("Reversed array = %s\n", Arrays.toString(reverse(scores))); public static double[] reverse(double[] array) { Print original array Sort the array for (int i = 0; i < array.length/2; i++) { double temp = array[i]; array[i] = array[array.length - i -1]; array[array.length - i -1] = temp; return array; Print sorted array Print the reversed array The print statement contains the call to your method reverse that reverses the array 6

7 Review of Quiz 20 Replaced code: System.out.printf("Original array = %s\n", Arrays.toString(scores)); Arrays.sort(scores); System.out.printf(" Sorted array = %s\n", Arrays.toString(scores)); System.out.printf("Reversed array = %s\n", Arrays.toString(reverse(scores))); public static double[] reverse(double[] array) { for (int i = 0; i < array.length/2; i++) { double temp = array[i]; array[i] = array[array.length - i -1]; array[array.length - i -1] = temp; return array; Accepts a double precision array as a parameter Explicitly returns the reversed array to the main program NOTE: The array passed by reference as a parameter has already been reversed. The explicit return allows us to use the method in a print statement. 7

8 Review of Quiz 20 OUTPUT: Printout verifies that the sort and reverse do work properly 8

9 7.5 Multidimensional Arrays (Lecture 21) Rectangular Two-Dimensional Arrays (pp ) Jagged Arrays (pp ) 7.6 Case Study: Benford s Law (Lecture 21) Introduction (pp ) Tallying Values (pp ) Completing the Program (pp ) 9

10 7.5 Multidimensional Arrays (Lecture 21) Rectangular Two-Dimensional Arrays (pp ) The arrays that we have used so far are onedimensional They can be considered a single row Or they can be considered a single column Often we need two dimensional arrays Contains both rows and columns double[][]: two dimensional array of rows and columns 10

11 7.5 Multidimensional Arrays (Lecture 21) Rectangular Two-Dimensional Arrays (pp ) Java allows for multidimensional arrays: double[][][]: Three dimensions double[][][][]: Four dimensions etc. We will only use one and two dimensional arrays in COSC-236 These are the most useful in practice The concept is easily extended once you know one and two dimensional arrays 11

12 7.5 Multidimensional Arrays (Lecture 21) Rectangular Two-Dimensional Arrays (pp ) The most common multidimensional array is a twodimensional array of a fixed number of rows and columns: double[][] temps = new double[3][5] The above could be used to store five temperature readings for three days 12

13 7.5 Multidimensional Arrays (Lecture 21) Rectangular Two-Dimensional Arrays (pp ) We can set the values of any element in the array: temps[0][3] = 98.3; temps[2][0] = 99.4 The above results in the following 13

14 7.5 Multidimensional Arrays (Lecture 21) Rectangular Two-Dimensional Arrays (pp ) We can pass an array as a parameter for a method: public static void print(double[][] grid) { for (int i = 0; i < grid.length; i++) { for (int j = 0; j < grid[i].length; j++) { System.out.print(grid[i][j] + " "); System.out.println(); Prints out row i, column j Start new line after each row Use [][] for 2-D array grid.length refers to number of rows grid[i].length refers to the number of columns in row i 14

15 7.5 Multidimensional Arrays (Lecture 21) Rectangular Two-Dimensional Arrays (pp ) We can print multi-dimensional arrays as strings: import java.util.*; public class Lecture21S15 { public static void main(string[] args) { Scanner console = new Scanner(System.in); double[][] temps = new double[3][4]; for (int i = 0; i < 3; i++) { for(int j = 0; j < 4; j++) { System.out.print("Please enter row " + (i+1) + ", col " + (j+1) + ": "); temps[i][j] = console.nextdouble(); System.out.println(Arrays.deepToString(temps)); NOTE: Arrays.toString() does not work for multi-dimensional arrays 15

16 Rectangular 2-D Arrays (pp ) Rectangular Two-Dimensional Arrays (pp ) We can print multi-dimensional arrays as strings: 7.5 Multidimensional Arrays (Lecture 21) import java.util.*; public class Lecture21S15 { public static void main(string[] args) { Scanner console = new Scanner(System.in); double[][] temps = new double[3][4]; for (int i = 0; i < 3; i++) { for(int j = 0; j < 4; j++) { System.out.print("Please enter row " + (i+1) + ", col " + (j+1) + ": "); temps[i][j] = console.nextdouble(); System.out.println(Arrays.deepToString(temps)); 16

17 import java.util.*; public class Lecture21S16 { public static void main(string[] args) { Scanner console = new Scanner(System.in); System.out.print("How many rows? "); int row = console.nextint(); System.out.print("How many columns? "); int col = console.nextint(); double[][] temps = new double[row][col]; for (int i = 0; i < row; i++) { for(int j = 0; j < col; j++) { System.out.print("Please enter row " + (i+1) + ", col " + (j+1) + ": "); temps[i][j] = console.nextdouble(); System.out.println(Arrays.deepToString(temps)); Rectangular 2-D Arrays (pp ) This program allows us to specify the number of rows and columns 17

18 import java.util.*; Rectangular 2-D Arrays (pp ) public class Lecture21S16 { public static void main(string[] args) { Scanner console = new Scanner(System.in); System.out.print("How many rows? "); int row = console.nextint(); System.out.print("How many columns? "); int col = console.nextint(); double[][] temps = new double[row][col]; for (int i = 0; i < row; i++) { for(int j = 0; j < col; j++) { System.out.print("Please enter row " + (i+1) + ", col " + (j+1) + ": "); temps[i][j] = console.nextdouble(); System.out.println(Arrays.deepToString(temps)); 18

19 7.5 Multidimensional Arrays (Lecture 21) Rectangular Two-Dimensional Arrays (pp ) Jagged Arrays (pp ) 7.6 Case Study: Benford s Law (Lecture 21) Introduction (pp ) Tallying Values (pp ) Completing the Program (pp ) 19

20 7.5 Multidimensional Arrays (Lecture 21) Rectangular Two-Dimensional Arrays (pp ) Jagged Arrays (pp ) 20

21 7.5 Multidimensional Arrays (Lecture 21) Jagged Arrays (pp ) Jagged Arrays (pp ) 21

22 public class PascalsTriangle { public static void main(string[] args) { int[][] triangle = new int[11][]; fillin(triangle); Jagged Arrays print(triangle); public static void fillin(int[][] triangle) { for (int i = 0; i < triangle.length; i++) { triangle[i] = new int[i + 1]; triangle[i][0] = 1; triangle[i][i] = 1; for (int j = 1; j < i; j++) { triangle[i][j] = triangle[i-1][j-1] + triangle[i-1][j]; public static void print(int[][] triangle) { for (int i = 0; i < triangle.length; i++) { for (int j = 0; j < triangle[i].length; j++) { System.out.print(triangle[i][j] + " "); System.out.println(); (pp ) 22

23 public class PascalsTriangle { public static void main(string[] args) { int[][] triangle = new int[11][]; fillin(triangle); Jagged Arrays print(triangle); public static void fillin(int[][] triangle) { for (int i = 0; i < triangle.length; i++) { triangle[i] = new int[i + 1]; triangle[i][0] = 1; triangle[i][i] = 1; for (int j = 1; j < i; j++) { triangle[i][j] = triangle[i-1][j-1] + triangle[i-1][j]; public static void print(int[][] triangle) { for (int i = 0; i < triangle.length; i++) { for (int j = 0; j < triangle[i].length; j++) { System.out.print(triangle[i][j] + " "); System.out.println(); (pp ) 23

24 7.5 Multidimensional Arrays (Lecture 21) Rectangular Two-Dimensional Arrays (pp ) Jagged Arrays (pp ) 7.6 Case Study: Benford s Law (Lecture 21) Introduction (pp ) Tallying Values (pp ) Completing the Program (pp ) 24

25 7.6 Case Study: Benford s Law (Lecture 21) Introduction (pp ) Tallying Values (pp ) Completing the Program (pp ) 25

26 7.6 Case Study: Benford s Law (Lecture 21) Introduction (pp ) When real-world data is linear: We expect a uniform distribution of data Looking at the most significant digit of data we would expect each number 1 through 9 to occur 11% of the time (or 0 through 9 10% of the time) But real-world data is more often exponential than it is linear! 26

27 7.6 Case Study: Benford s Law (Lecture 21) Introduction (pp ) When real-world data is exponential: We expect data to conform to Benford s Law Benford s Law predicts far more low numbers than high number for the most significant digit Benford provided a table that quantifies this result 27

28 7.6 Case Study: Benford s Law (Lecture 21) Introduction (pp ) When real-world data is exponential: Introduction We expect data to conform to Benford s (pp. Law ) Benford s Law predicts far more low numbers than high number for the most significant digit Benford provided a table that quantifies this result 28

29 7.6 Case Study: Benford s Law (Lecture 21) Introduction (pp ) Tallying Values (pp ) Completing the Program (pp ) 29

30 7.6 Case Study: Benford s Law (Lecture 21) Introduction (pp ) Tallying Values (pp ) 30

31 A multi-counter problem (pp ) Problem: Write a method mostfrequentdigit that returns the digit value that occurs most frequently in a number. Example: The number contains: one 0, two 2s, four 6es, one 7, and one 9. mostfrequentdigit( ) returns 6. If there is a tie, return the digit with the lower value. mostfrequentdigit( ) returns 3. 31

32 A multi-counter problem (pp ) We could declare 10 counter variables... int counter0, counter1, counter2, counter3, counter4, counter5, counter6, counter7, counter8, counter9; But a better solution is to use an array of size 10. The element at index i will store the counter for digit value i. Example for : index value How do we build such an array? And how does it help? 32

33 Creating an array of tallies (pp ) // assume n = int[] counts = new int[10]; while (n > 0) { // pluck off a digit and add to proper counter int digit = n % 10; counts[digit]++; n = n / 10; index value

34 Tally solution (pp ) // Returns the digit value that occurs most frequently in n. // Breaks ties by choosing the smaller value. public static int mostfrequentdigit(int n) { int[] counts = new int[10]; while (n > 0) { int digit = n % 10; // pluck off a digit and tally it counts[digit]++; n = n / 10; // find the most frequently occurring digit int bestindex = 0; for (int i = 1; i < counts.length; i++) { if (counts[i] > counts[bestindex]) { bestindex = i; return bestindex; Set up the tally array While number is non zero Tally Ready the next digit The array counts now has a tally of how many times each digit occurs 34

35 Tally solution (pp ) // Returns the digit value that occurs most frequently in n. // Breaks ties by choosing the smaller value. public static int mostfrequentdigit(int n) { int[] counts = new int[10]; while (n > 0) { int digit = n % 10; // pluck off a digit and tally it counts[digit]++; n = n / 10; // find the most frequently occurring digit int bestindex = 0; for (int i = 1; i < counts.length; i++) { if (counts[i] > counts[bestindex]) { bestindex = i; return bestindex; We can now check through the array count to look for the most frequently occurring digit Zero the index finder Check through the array for index with maximum count Return that maximum index 35

36 Array histogram question (pp ) Given a file of integer exam scores, such as: Write a program that will print a histogram of stars indicating the number of students who earned each unique exam score. 85: ***** 86: ************ 87: *** 88: * 91: **** NOTE: This histogram does not refer to the previous data 36

37 Array histogram answer (pp ) 37

38 Array // Reads a file histogram of test scores and shows answer a histogram of score (pp. distribution ) import java.io.*; import java.util.*; public class Histogram { public static void main(string[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("midterm.txt")); int[] counts = new int[101]; // counters of test scores while (input.hasnextint()) { int score = input.nextint(); counts[score]++; // read file into counts array // if score is 87, then counts[87]++ for (int i = 0; i < counts.length; i++) { if (counts[i] > 0) { System.out.print(i + ": "); for (int j = 0; j < counts[i]; j++) { System.out.print("*"); System.out.println(); // print star histogram 38

39 Section attendance question Read a file of section attendance (see next slide): yynyyynayayynyyyayanyyyaynayyayyanayyyanyayna ayyanyyyyayanaayyanayyyananayayaynyayayynynya yyayaynyyayyanynnyyyayyanayaynannnyyayyayayny And produce the following output: Section 1 Student points: [20, 17, 19, 16, 13] Student grades: [100.0, 85.0, 95.0, 80.0, 65.0] Section 2 Student points: [17, 20, 16, 16, 10] Student grades: [85.0, 100.0, 80.0, 80.0, 50.0] Section 3 Student points: [17, 18, 17, 20, 16] Student grades: [85.0, 90.0, 85.0, 100.0, 80.0] Students earn 3 points for each section attended up to

40 Section input file student week section 1 section 2 section 3 yynyyynayayynyyyayanyyyaynayyayyanayyyanyayna ayyanyyyyayanaayyanayyyananayayaynyayayynynya yyayaynyyayyanynnyyyayyanayaynannnyyayyayayny Each line represents a section. A line consists of 9 weeks' worth of data. Each week has 5 characters because there are 5 students. Within each week, each character represents one student. a means the student was absent n means they attended but didn't do the problems y means they attended and did the problems (+0 points) (+2 points) (+3 points) 40

41 Section attendance answer import java.io.*; import java.util.*; public class Sections { public static void main(string[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("sections.txt")); int section = 1; while (input.hasnextline()) { String line = input.nextline(); // process one section int[] points = new int[5]; for (int i = 0; i < line.length(); i++) { int student = i % 5; int earned = 0; if (line.charat(i) == 'y') { // c == 'y' or 'n' or 'a' earned = 3; else if (line.charat(i) == 'n') { earned = 2; points[student] = Math.min(20, points[student] + earned); double[] grades = new double[5]; for (int i = 0; i < points.length; i++) { grades[i] = * points[i] / 20.0; System.out.println("Section " + section); System.out.println("Student points: Arrays.toString(points)); System.out.println("Student grades: " + Arrays.toString(grades)); System.out.println(); section++; 41

42 Data transformations In many problems we transform data between forms. Example: digits count of each digit most frequent digit Often each transformation is computed/stored as an array. For structure, a transformation is often put in its own method. Sometimes we map between data and array indexes. by position (store the i th value we read at index i ) tally (if input value is i, store it at array index i ) explicit mapping (count 'J' at index 0, count 'X' at index 1) Exercise: Modify our Sections program to use static methods that use arrays as parameters and returns. 42

43 Array param/return answer // This program reads a file representing which students attended // which discussion sections and produces output of the students' // section attendance and scores. import java.io.*; import java.util.*; public class Sections2 { public static void main(string[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("sections.txt")); int section = 1; while (input.hasnextline()) { // process one section String line = input.nextline(); int[] points = countpoints(line); double[] grades = computegrades(points); results(section, points, grades); section++; // Produces all output about a particular section. public static void results(int section, int[] points, double[] grades) { System.out.println("Section " + section); System.out.println("Student scores: " + Arrays.toString(points)); System.out.println("Student grades: " + Arrays.toString(grades)); System.out.println();... 43

44 Array param/return answer... // Computes the points earned for each student for a particular section. public static int[] countpoints(string line) { int[] points = new int[5]; for (int i = 0; i < line.length(); i++) { int student = i % 5; int earned = 0; if (line.charat(i) == 'y') { // c == 'y' or c == 'n' earned = 3; else if (line.charat(i) == 'n') { earned = 2; points[student] = Math.min(20, points[student] + earned); return points; // Computes the percentage for each student for a particular section. public static double[] computegrades(int[] points) { double[] grades = new double[5]; for (int i = 0; i < points.length; i++) { grades[i] = * points[i] / 20.0; return grades; 44

45 Assignments for this week 1. Laboratory for Chapter 7 due next Monday 11/17 IMPORTANT: When you me your laboratory Word Document, be sure it is all in one file 2. Read pp (Chapter 13) for Monday 11/12 3. Be sure to complete Quiz 21 before leaving class tonight This is another program to write You must demonstrate the program to me before you leave lab 45