Software and Programming 1

Transcription

1 Software and Programming 1 Lab 4: Exercises on Arrays 7 th February 2019 SP1-Lab pdf Tobi Brodie (tobi@dcs.bbk.ac.uk) 1

2 Array Exercises Lab objectives: Understand how to pass and return an array as a parameter variable for a method. Use an array to solve problems in a program. One class (ArrayExercises) will be created with several methods that will be tested with calls from a main method. Note: Marked Exercise 4 mirrorarray method is a checked exercise and you are required to complete it, then show and explain the finished program to one of the lab assistants, by the 21 st February. Make sure you have backed up your work on a memory stick or similar. This method should be written within the ArrayExercises class created in the lab today. 2

3 Array Exercises (2) Arrays a reminder An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed - The Java Tutorials, Oracle Sample code: /* declare an array of integers and allocate memory for 5 elements */ int[] anarray = new int[5]; 3

4 Array Exercises (3) import java.util.arrays; The Java Arrays utility class contains various methods for manipulating arrays, such as sorting and searching. It also includes the tostring()method that returns a string representation of the contents of a specified array. Sample code: /* print a string representation of anarray*/ System.out.print(Arrays.toString(anArray)); 4

5 Exercise 1: main method Create a main method in a class ArrayExercises. Declare and initialise a CONSTANT that determines the length of the array: final int SIZE = 5; Declare an array to store 5 integers, using SIZE as the array length: int[] anarray = new int[size]; 5

6 Exercise 1: main method (2) Create code that asks a user to enter 5 integers and store them in the array. This code should only be written once. Hint: Use a for loop utilising the anarray.length construct to reduce code. for(int i = 0; i < anarray.length ; i++) 6

7 Exercise 1: main method (3) - code 7

8 Exercise 1: printarrayelements method Once the user has assigned all 5 values to the array, the program should print the array contents to the terminal window using the Java Arrays utility class as described in slide 4. Write a helper method that takes 2 parameters, a String which describes the array, and an array, both of which should be printed to the terminal window. Example method call: printarrayelements("you input the following", anarray); 8

9 Exercise 1: main method (4) Returning to the main method, you will need to write calls for the methods in the three following exercises Use the array as an argument for the following methods, all of which should return results: arraysum altsum mirrorarray You should print the returned result from each method before calling the next, using the printarrayelements method if appropriate. 9

10 Exercise 2: arraysum method Implement the method: public static int arraysum(int[] incomingarray) that returns the sum of all elements in an array: For example, if you call it with [1, 4, 9, 16, 9] then it returns 39 ( ) Before writing the code, consider the following: Will the method require new variable/s? Without knowing the length of the array, how can this be accomplished? 10

11 Exercise 3: altsum method Implement a method that returns the alternating sum of all elements in an array (that is, the sum of all elements with odd indexes minus the sum of all elements with even indexes). For example, if you call it with [1, 4, 9, 16, 9] then it returns -1 ( ) Once again, ensure your code would work with a different number of elements in the incoming array. 11

12 Marked Exercise 4: mirrorarray method Implement the method: public static int[] mirrorarray(int[] data) that returns an array containing the original array element followed by another copy in reverse order: Hints: For example, if you call it with [1, 4, 9, 16, 9] The method should return a new array containing [1, 4, 9, 16, 9, 9, 16, 9, 4, 1]. 1. Create a new array with size determined by data.length 2. Use a for loop(s) to copy values from one array to the other in the required order. 12