Sample Quiz Answers
For Chapter 8
THIS PAGE CONTAINS SAMPLE ANSWERS to the Quiz on Chapter 8 of this on-line Java textbook. Note that in many cases, there are lots of correct answers to a given question.
Question 1: What does the computer do when it executes the following statement? Try to give as complete an answer as possible.
Color[] pallette = new Color[12];Answer: This is a declaration statement, that declares and initializes a variable named pallette of type Color[]. The initial value of this variable is a newly created array that has space for 12 items. To be specific about what the computer does: It creates a new 12-element array object on the heap, and it fills each space in that array with null. It allocates a memory space for the variable, pallette. And it stores a pointer to the new array object in that memory space.
Question 2: What is meant by the basetype of an array?
Answer: The base type of an array refers to the type of the items that can be stored in that array. For example, the base type of the array in the previous problem is Color.
Question 3: What does it mean to sort an array?
Answer: To sort an array means to rearrange the items in the array so that they are in increasing or decreasing order (according to some criterion).
Question 4: What is meant by a dynamic array? What is the advantage of a dynamic array over a regular array?
Answer: A dynamic array is like an array in that it is a data structure that stores a sequence of items, all of the same type, in numbered locations. It is different from an array in that there is no preset upper limit on the number of items that it can contain. This is an advantage in situations where a reasonable value for the size of the array is not known at the time it is created.
Question 5: What is the purpose of the following subroutine? What is the meaning of the value that it returns, in terms of the value of its parameter?
static String concat( String[] str ) { if (str == null) return ""; String ans = ""; for (int i = 0; i < str.length; i++) { ans = ans + str[i]; return ans; }Answer: The purpose of the subroutine is to chain all the strings in an array of strings into one long string. If the array parameter is null, then there are no strings, and the empty string is returned. Otherwise, the value returned is the string made up of all the strings from the array. For example, if stringList is an array declared as
String[] stringList = { "Put 'em ", "all", " together" };then the value of concat(stringList) is "Put 'em all together".
Question 6: Show the exact output produced by the following code segment.
char[][] pic = new char[6][6]; for (int i = 0; i < 6; i++) for (int j = 0; j < 6; j++) { if ( i == j || i == 0 || i == 5 ) pic[i][j] = '*'; else pic[i][j] = '.'; } for (int i = 0; i < 6; i++) { for (int j = 0; j < 6; j++) System.out.print(pic[i][j]); System.out.println(); }Answer: The output consists of six lines, with each line containing six characters. In the first line, i is 0, so all the characters are *'s. In the last line, i is 5, so all the characters are *'s. In each of the four lines in the middle, one of the characters is a * and the rest are periods. The output is
****** .*.... ..*... ...*.. ....*. ******It might help to look at the array items that are printed on each line. Note that pic[row][col] is '*' if row is 0 or if row is 5 or if row and col are equal.
pic[0][0] pic[0][1] pic[0][2] pic[0][3] pic[0][4] pic[0][5] pic[1][0] pic[1][1] pic[1][2] pic[1][3] pic[1][4] pic[1][5] pic[2][0] pic[2][1] pic[2][2] pic[2][3] pic[2][4] pic[2][5] pic[3][0] pic[3][1] pic[3][2] pic[3][3] pic[3][4] pic[3][5] pic[4][0] pic[4][1] pic[4][2] pic[4][3] pic[4][4] pic[4][5] pic[5][0] pic[5][1] pic[5][2] pic[5][3] pic[5][4] pic[5][5]
Question 7: Write a complete subroutine that finds the largest value in an array of ints. The subroutine should have one parameter, which is an array of type int[]. The largest number in the array should be returned as the value of the subroutine.
Answer:
public static int getMax(int[] list) { // Find and return the largest item in the array, list. int max = list[0]; // This is the largest item seen so far. for (int i = 1; i < list.length; i++) { // Look at each item in the array. If the item is // bigger than max, then set max equal to the item. if (list[i] > max) max = list[i]; } // At this point, max is the largest item in the whole array. return max; } // end subroutine getMax
Question 8: Suppose that temperature measurements were made on each day of 1999 in each of 100 cities. The measurements have been stored in an array
int[][] temps = new int[100][365];where temps[c][d] holds the measurement for city number c on the dth day of the year. Write a code segment that will print out the average temperature, over the course of the whole year, for each city. The average temperature for a city can be obtained by adding up all 365 measurements for that city and dividing the answer by 365.0.
Answer: A pseudocode outline of the answer is
For each city { Add up all the temperatures for that city Divide the total by 365 and print the answer }Adding up all the temperatures for a given city itself requires a for loop, so the code segment looks like this:
for (int city = 0; city < 100; city++) { int total = 0; // total of temperatures for this city for (int day = 0; day < 365; day++) total = total + temps[city][day]; double avg = total / 365.0; // average temp for this city System.out.println("Average temp for city number " + city + " is " + avg); }
Question 9: Suppose that a class, Employee, is defined as follows:
class Employee { String lastName; String firstName; double hourlyWage; int yearsWithCompany; }Suppose that data about 100 employees is already stored in an array:
Employee[] employeeData = new Employee[100];Write a code segment that will output the first name, last name, and hourly wage of each employee who has been with the company for 20 years or more.
Answer: (The data for the i-th employee is stored in an object that can be referred to as employeeData[i]. The four pieces of data about that employee are members of this object and can be referred to as:
- employeeData[i].firstName
- employeeData[i].lastName
- employeeData[i].hourlyWage
- employeeData[i].yearsWithCompany
The code segment uses a for loop to consider each employee in the array.)
for (int i=0; i < 100; i++) { if ( employeeData[i].yearsWithCompany >= 20 ) System.out.println(employeeData[i].firstName + " " + employeeData[i].lastName + ": " + employeeData[i].hourlyWage); }
Question 10: Suppose that A has been declared and initialized with the statement
double[] A = new double[20];
And suppose that A has already been filled with 20 values. Write a program segment that will find the average of all the non-zero numbers in the array. (The average is the sum of the numbers, divided by the number of numbers. Note that you will have to count the number of non-zero entries in the array.) Declare any variables that you use.
Answer: (There is one problem with this problem. What happens if all the entries in the array A are zero? In that case, the number of non-zero entries is zero, and the average of non-zero entries is undefined. In my answer, I assign a value of zero to the average in this case, but this is somewhat arbitrary.)
int nonzeroCt = 0; // The number of non-zero entries in the array. double total = 0; // The total of all the grades in the array. double average; // The average of the non-zero entries in the array. for (int i = 0; i < 20; i++) { if (A[i] != 0) { // Process this non-zero entry. total += A[i]; // Add it to the total. nonzeroCt++; // Count it. } } if (nonzeroCt > 0) average = total / nonzeroCt; else // (Actually, the average is undefined in this case) average = 0;
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