Arrays. Introduction. Figure 3: Examples for Arrays.

Transcription

1 Arrays Introduction Arrays are special data structures that help to handle many pieces of similar data. To be specific, an array can store a fixed number of objects or primitive data values of the same type. Therefore, each array is said to be of one type: there are arrays of int, arrays of boolean, arrays of String, or arrays of Movie. Arrays are not objects and not primitive data types. They don't have methods, and they don't only hold one piece of data. Rather, they are simple lists holding a fixed number of elements. Each element has an index, starting from 0. Fig. 3 gives a few examples. Note that not all array elements are used in some of the arrays. Those elements are simply empty. Figure 3: Examples for Arrays.

2 Using Arrays One can define array-variables and use them just like any other variables. The advantages are that (1) an array can store many objects of the same type and (2) each element can be accessed by the name of the array and the index number of the element. Because of that, arrays dramatically simplify many common computer algorithms, like searching, sorting, storing databases, etc. An array variable is declared as follows: <data type> [] <variable name> An array needs to be instantiated just like an object, in order to reserve the memory for that array. However, different from object instantiation, the instantiation usually does not fill the array with data (although that is possible, see textbook). Filling the array must be done explicitly with code. An array is instantiated as follows: <array variable> = new <data type> [<number of elements>]; This looks more complicated than it is. The three arrays in Fig. 3 would be declared and instantiated as follows: Rational[] rationalarray = new Rational[6]; int[] intarray = new int[9]; String[] stringarray = new String[6]; Important points: not all elements of an array have to be used. That is a useful feature, since often one does not know initially how large one wants the array to be. The unused elements of an array will have certain initial values, depending on the data type of the array. If the data type is an object, the initial value is the null-reference. For the primitive data types, the initial value is 0 (false for boolean, and '\0' for char). The indices of an array of elements run from 0 to n - 1(similarly to the indices of the characters of a string, as obtained by the charat-method of Chapter 2). It is not possible to resize an array once it is instantiated. The only solution to deal with full arrays is to declare a larger array of the same type and copy every element from the old array into the new array with a for-loop. The length of an array can be obtained by the special syntax <variable name>.length Note that this returns the maximum length of the array, as it was declared, but not the number of elements used. The main program needs to maintain a record of how many and which elements are used.

3 Remark: An array could be declared, constructed and initialized simultaneously if you give the list of all its elements in the declaration part, ie. for instance, you could write or float [] Temperature = 82.3, 71.4, 33.1, 44.6, 102.1; This would be equivalent to the following two ways that we already saw: 1) float [ ] Temperature; Temperature = new float[5]; Temperature[0] = 82.3; Temperature[1] = 71.4; Temperature[2] = 33.1 Temperature[3] = 44.6; Temperature[4] = 102.1; 2) float [ ] Temperature = new float[5]; Temperature[0] = 82.3; Temperature[1] = 71.4; Temperature[2] = 33.1 Temperature[3] = 44.6; Temperature[4] = 102.1; But be aware that you can t write (because it is not done simultaneously) // float [] Temperature; // Temperature = 82.3, 71.4, 33.1, 44.6, 102.1; // float [] Temperature; // Temperature = new float [5]; // Temperature = 82.3, 71.4, 33.1, 44.6, 102.1;

4 Run the examples TestArray.java, TestArray1.java and TestArray2.java. The code below summarizes all that is important about arrays: public class MyFirstArray public static void main (String[] args) // declare an array of strings, which holds at most 10 strings String[] friends = new String[10]; // fill the first four elements with values friends[0] = "John"; friends[1] = "Paul"; friends[2] = "George"; friends[3] = "Ringo"; // at this point, elements [4] through [9] are empty and unused // print all the elements so far: for (int i=0; i<=3; i++) System.out.println(friends[i]); System.out.println("This array was declared to have " +friends.length+" elements."); This code will print: John Paul George Ringo This array was declared to have 10 elements. Using Arrays for Databases Arrays provide a very elegant way to keep databases (i.e., collections of similar objects in which one wants to search). For the following example of a Movie-database one could rewrite the main program to use an array of Movie-objects, rather than a fixed number of variables of type Movie. import cs1.keyboard; public class MiniDB public static void main(string[] args) // insert movies movie1 = new Movie(1,"Dr. Strangelove",8.9,"Stanley Kubrick",1963); movie2 = new Movie(2,"Full Metal Jacket",8.1,"Stanley Kubrick",1987); movie3 = new Movie(3,"Citizen Kane",8.8,"Orson Welles",1941); movie4 = new Movie(4,"Killer Tomatoes",2.5,"John de Bello",1978); movie5 = new Movie(5,"A.I.",7.1,"Steven Spielberg",2001);

5 public class Movie private int MovieIndex, MovieYear; private String MovieTitle, MovieDirector; private double MovieScore; // Setup a new movie with the information provided at construction time public Movie(int index, String title, double score, String director, int year) MovieIndex = index; MovieTitle = title; MovieScore = score; MovieDirector = director; MovieYear = year; public double getscore() return MovieScore; public String getmovietitle() return MovieTitle; public String tostring() String result; result = MovieIndex + MovieTitle + MovieScore + \n + MovieDirector + MovieYear; return result; ************************************************** The rewritten main code, including a simple loop that finds the best movie, is given below. For practice, I have put the code to find the best movie into a method. import cs1.keyboard; public class MiniDBArray public static void main(string[] args)

6 // space for 10 movies max. Movie[] moviedb= new Movie[10]; // count movies actually in DB int moviesindb=0; ; // insert movies moviedb[0] = new Movie(1,"Dr. Strangelove",8.9,"Stanley Kubrick",1963); moviedb[1] = new Movie(2,"Full Metal Jacket",8.1,"Stanley Kubrick",1987) moviedb[2] = new Movie(3,"Citizen Kane",8.8,"Orson Welles",1941); moviedb[3] = new Movie(4,"Killer Tomatoes",2.5,"John de Bello",1978); moviedb[4] = new Movie(5,"A.I.",7.1,"Steven Spielberg",2001); Movie bestmovie = bestmovie(moviedb, moviesindb); System.out.println("The best movie is "+ bestmovie.getmovietitle()); System.out.println("Its ranking is "+ bestmovie.getscore()); //this method finds the best movie in the movie array, by going through // the first moviesindb movies in the array. It returns the movie // with the highest score. public static Movie bestmovie(movie[] moviedb, int moviesindb) // find best movie by going through all used elements in array double maxscore = moviedb[0].getscore(); Movie bmovie = moviedb[0]; for (int i=1; i<moviesindb; i++) if (moviedb[i].getscore()>maxscore) maxscore = moviedb[i].getscore(); bmovie = moviedb[i]; return bmovie; Note that (1) the code for finding the best movie is much simpler, and (2) extending the database to as many movies as needed does not require to write any additional code (besides the code that

7 is needed to instantiate new Movie objects and fill them with data). The loop finding the best movie could be used for 5, 50, or 5,000 movies without any changes. Just like any other variable, arrays can be overwritten (or single elements of an array can be overwritten), and arrays can be passed to methods as parameters. Note that due to the nature of array variables as references (not covered in this course), an array that has been passed to a method and is changed inside the method will change outside the method as well. Passing arrays to methods must be done very carefully when an array's elements are changed. Since arrays are references, if we define two arrays A and B, of the same type, and with the same number of elements, the assignment statement A = B; changes the reference of A to be the one of B. So A is an alias for B. If you want to copy the elements of B into A, you have to do element by element, i.e., A[0] = B[0]; A[1] = B[1], etc.. This way you have two different arrays that contain the same values. Multidimensional Arrays The arrays discussed so far correspond to the mathematical concept of vector. When one wants to store things like matrices, one needs multi-dimensional arrays. Two-dimensional arrays correspond to mathematical matrices. Arrays can have more than two dimensions (up to 255), although more than 3 are usually not useful (and very difficult to handle). I will only give a small piece of an example since this is beyond the scope of the course. The two-dimensional array created with the following code is equivalent to the matrix below. int[][] matrix = new int[3][3]; for (int i=0; i<=2; i++) for (int j=0; j<=2; j++) matrix[i][j]=i+j;