CS2336 Ch. 7 Checkpoints
True or false? When an array is passed to a method, a new array is created and passed to the method.
False. When an array is passed to a method, the reference value of the array is passed. No new array is created. Both argument and parameter point to the same array.
Suppose the following code is written to reverse the contents in an array, explain why it is wrong. How do you fix it? int[] list = {1, 2, 3, 5, 4}; for (int i = 0, j = list.length - 1; i < list.length; i++, j--) { // Swap list[i] with list[j] int temp = list[i]; list[i] = list[j]; list[j] = temp; }
It swaps the elements twice. For example, the first element is swapped with the last element once and then the last element is swapped with the first element another time. In effect, the elements are not swapped. Here is the fix: int[] list = {1, 2, 3, 5, 4}; for (int i = 0, j = list.length - 1; i < list.length / 2; i++, j--) { // Swap list[i] with list[j] int temp = list[i]; list[i] = list[j]; list[j] = temp; }
If the binary search method returns -4, is the key in the list? Where should the key be inserted if you wish to insert the key into the list?
The key is not in the list. The key should be inserted at â€"(-4 + 1) = 3.
Can you invoke the printMax method in Listing 7.5 using the following statements? printMax(1, 2, 2, 1, 4); printMax(new double[]{1, 2, 3}); printMax(new int[]{1, 2, 3});
The last one printMax(new int[]{1, 2, 3}); is incorrect, because the array must of the double[] type.
When is the memory allocated for an array?
The memory is allocated when an array is created.
Once an array is created, its size cannot be changed. Does the following code resize the array? int[] myList; myList = new int[10]; // Sometime later you want to assign a new array to myList myList = new int[20];
The second assignment statement myList = new int[20] creates a new array and assigns its reference to myList.
Show the output of the following code: int[] list1 = {2, 4, 7, 10}; java.util.Arrays.fill(list1, 7); System.out.println(java.util.Arrays.toString(list1)); int[] list2 = {2, 4, 7, 10}; System.out.println(java.util.Arrays.toString(list2)); System.out.print(java.util.Arrays.equals(list1, list2));
[7, 7, 7, 7] [2, 4, 7, 10] False
Show the output of the following program when invoked using 1. java Test I have a dream 2. java Test “1 2 3†3. java Test public class Test { public static void main(String[] args) { System.out.println("Number of strings is " + args.length); for (int i = 0; i < args.length; i++) System.out.println(args[i]); } }
(1) Number of strings is 4 I have a dream (2) Number of strings is 1 1 2 3 (3) Number of strings is 0
Indicate true or false for the following statements: (a) Every element in an array has the same type. (b) The array size is fixed after an array reference variable is declared. (c) The array size is fixed after it is created. (d) The elements in an array must be a primitive data type.
(a) Answer: True (b) Answer: False (c) Answer: True (d) Answer: False
What happens when your program attempts to access an array element with an invalid index?
A runtime exception ArrayIndexOutOfBounds occurs.
To apply java.util.Arrays.binarySearch(array, key), should the array be sorted in increasing order, in decreasing order, or neither?
To apply java.util.Arrays.binarySearch(array, key), the array must be sorted in increasing order.
What is the output of the following code? int x = 30; int[] numbers = new int[x]; x = 60; System.out.println("x is " + x); System.out.println("The size of numbers is " + numbers.length);
x is 60 The size of numbers is 30
What is the array index type? What is the lowest index? What is the representation of the third element in an array named a?
The array index type is int and its lowest index is 0. a[2]
How do you modify the selectionSort method in Listing 7.8 to sort numbers in decreasing order?
Omitted
This book text declares the main method as public static void main(String[] args) Can it be replaced by one of the following lines? public static void main(String args[]) public static void main(String[] x) public static void main(String x[]) static void main(String x[])
public static void main(String[] args) can be replaced by public static void main(String args[]) public static void main(String[] x) public static void main(String x[]) but not static void main(String x[]) because it is not public.
Write statements to do the following: a. Create an array to hold 10 double values. b. Assign the value 5.5 to the last element in the array. c. Display the sum of the first two elements. d. Write a loop that computes the sum of all elements in the array. e. Write a loop that finds the minimum element in the array. f. Randomly generate an index and display the element of this index in the array. g. Use an array initializer to create another array with the initial values 3.5, 5.5, 4.52, and 5.6.
(a) double[] list = new double[10]; (b) list[list.length â€" 1] = 5.5; (c) System.out.println(list[0] + list[1]); (d) double sum = 0; for (int i = 0; i < list.length; i++) sum += list[i]; (e) double min = list[0]; for (int i = 1; i < list.length; i++) if (min > list[i]) min = list[i]; (f) System.out.println(list[(int)(Math.random() * list.length)]); (g) double[] list = {3.5, 5.5, 4.52, 5.6};
Show the output of the following two programs: (a) public class Test { public static void main(String[] args) { int number = 0; int[] numbers = new int[1]; m(number, numbers); System.out.println("number is " + number + " and numbers[0] is " + numbers[0]); } public static void m(int x, int[] y) { x = 3; y[0] = 3; } } (b) public class Test { public static void main(String[] args) { int[] list = {1, 2, 3, 4, 5}; reverse(list); for (int i = 0; i < list.length; i++) System.out.print(list[i] + " "); } public static void reverse(int[] list) { int[] newList = new int[list.length]; for (int i = 0; i < list.length; i++) newList[i] = list[list.length - 1 - i]; list = newList; } }
(a) number is 0 and numbers[0] is 3 (b) 1 2 3 4 5
What is the output of the following code? 1 public class Test { 2 public static void main(String[] args) { 3 int list[] = {1, 2, 3, 4, 5, 6}; 4 for (int i = 1; i < list.length; i++) 5 list[i] = list[i - 1]; 6 7 for (int i = 0; i < list.length; i++) 8 System.out.print(list[i] + " "); 9 } 10 }
1 1 1 1 1 1
Identify and fix the errors in the following code: 1 public class Test { 2 public static void main(String[] args) { 3 double[100] r; 4 5 for (int i = 0; i < r.length(); i++); 6 r(i) = Math.random * 100; 7 } 8 }
Line 3: the array declaration is wrong. It should be double[]. The array needs to be created before its been used. e.g. new double[10] Line 5: The semicolon (;) at the end of the for loop heading should be removed. Line 5: r.length() should be r.length. Line 6: random should be random() Line 6: r(i) should be r[i].
Will the program pick four random cards if you replace lines 22â€"27 in Listing 7.2 DeckOfCards.java with the following code? for (int i = 0; i < 4; i++) { int cardNumber = (int)(Math.random() * deck.length); String suit = suits[cardNumber / 13]; String rank = ranks[cardNumber % 13]; System.out.println("Card number " + cardNumber + ": " + rank + " of " + suit); }
No. You may the same card more than once.
Use Figure 7.11 as an example to show how to apply the selection-sort approach to sort {3.4, 5, 3, 3.5, 2.2, 1.9, 2}.
Omitted
Use Figure 7.9 as an example to show how to apply the binary search approach to a search for key 10 and key 12 in list {2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79}.
Omitted
What is wrong with each of the following method headers? public static void print(String... strings, double... numbers) public static void print(double... numbers, String name) public static double... print(double d1, double d2
Only one variable-length parameter may be specified in a method and this parameter must be the last parameter. The method return type cannot be a variable-length parameter.
If high is a very large integer such as the maximum int value 2147483647, (low + high) / 2 may cause overflow. How do you fix it to avoid overflow?
Replace (low + high) / 2 with (-low + high) / 2 + low
How do you declare an array reference variable and how do you create an array?
See the section "Array Basics."
Use the arraycopy method to copy the following array to a target array t: int[] source = {3, 4, 5};
System.arraycopy(source, 0, t, 0, source.length);
How do you access elements in an array?
You access an array using its index. arrayRefVar[index] is known as an array indexed variable.
What types of array can be sorted using the java.util.Arrays.sort method? Does this sort method create a new array?
You can sort an array of any primitive types except boolean. The sort method is void, so it does not return a new array.
Which of the following statements are valid? int i = new int(30); double d[] = new double[30]; char[] r = new char(1..30); int i[] = (3, 4, 3, 2); float f[] = {2.3, 4.5, 6.6}; char[] c = new char();
int i = new int(30); Answer: Invalid double d[] = new double[30]; Answer: Valid char[] r = new char(1..30); Answer: Invalid int i[] = (3, 4, 3, 2); Answer: Invalid float f[] = {2.3, 4.5, 5.6}; Answer: Valid char[] c = new char(); Answer: Invalid