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CSIS 110 – Lecture 33
Chapter 8 - Arrays What is an array? An array is a set of variables of the same type that have a single identifier associated with them. Each of the variables is called an element of the array. Declaration of an array variable: [] ; [] = new [ ]; Two forms:
- the first form declares the array variable, but does not allocate the array elements
- The second form declares the array variable, and allocates space for the array elements - uses the operator 'new' as if we were creating a new object. Examples int[] studentID; int[] quizScores = new int[ 13 ]; boolean[] passingCourse = new boolean[13]; double[] temperature = new double[ 365 ]; int daysPerMonth = new int[ 12 ]; String[] monthNames = new String[12]; Point[] polygon; Initialization Draw pictures of above. Show that an array is a reference type. The array identifier refers to the array reference. For those arrays that are initialized, show that numeric types are initialized to 0, booleans to false, and reference types to null. Emphasize that declaring the array only creates the reference. You have to allocate the array elements also! Accessing Array Elements We access the array elements using subscripting or indexing. Each array element has a subscript (or index ) that we can use to refer to that element. The subscripts are integer
values, starting at 0. Thus, the first element of the array has subscript 0, the second has subscript 1, and the last has a subscript of the array-size - 1. int[] daysPerMonth = new int[12]; Draw picture, showing the array subscripts, and the names of each element (daysPerMonth[0], daysPerMonth[1], ...) The elements of an array are referenced using the array name and a subscript. So, we can say daysPerMonth[6] or quizScores[4] or temperature[300]. The value of the index or subscript MUST BE an integer expression. That means we don't have to use constants. temperature[ i ] quizScore[ last / 2 ] Assigning values to array elements Use an assignment statement, but the variable on the lhs is indexed. monthName[0] = "January"; daysPerMonth[0] = 31; // January (1 - 1) daysPerMonth[10] = 30; // November (11 - 1) quizScores[0] = 15; polygon[0] = new Point( 3, 5 ); polygon[1] = new Point( 7, 10 ); polygon[2] = new Point( 12, 2 ); Accessing array elements In other situations, a subscripted array element is an expression that can be used anywhere that an expression of that type is allowed. System.out.println( "There are " + daysPerMonth[ 3 ] + " in " + monthName[3] ); Point firstPoint = polygon[0]; sum += quizScore[ i ]; if ( passingCourse[ 0 ] ) System.out.println( "Student is passing." ); *** Look at array1.java *** Allocating the array elements Let's look at some of our array declarations again
Indexing The value of an array subscript must be an integer type with a value between 0 and the array-size - 1. If you attempt to access an array element with a subscript outside of this range, Java will throw an IndexOutOfBoundsException, and your program will terminate. daysPerMonth[12] = 31; // Error quizScore[ -1 ] = 40; // Error *** See Array3.java *** Explicit Initialization Remember, after you allocate an array, its elements are initialized to 0, false, or null. These are probably not the values that you want. For some arrays, you know the desired initial values: int[] daysPerMonth = new int[ 12 ]; daysPerMonth[0] = 31; daysPerMonth[1] = 28; ... daysPerMonth[11] = 31; This is tedious. Java provides nice shorthand for this: int[] daysPerMonth = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }; *** See Array4.java *** for loops and arrays For loops are frequently used to access all of the elements of an array - Look at ArrayLoop1.java and ArrayLoop2.java. int[] anArray = { 15, 9, 12 }; int sum = 0; for ( int i = 0; i < anArray.length; i++ ) { sum += anArray[i]; } System.out.println("The sum of the array elements is " + sum );
Remember the difficulty we would have finding the smallest of a set of variables? It is much easier with arrays - how would we do it? Another example (see ArrayLoop3.java): double[] array2 = { 5.6, 7.2, 3.9, 8.4, 12.1 }; double min = array2[0]; for ( int i = 0; i < array2.length; i++ ) { if ( array2[i] < min ) min = array2[i]; } Common errors - incorrect indexing! // Skips first element of array - // not always wrong, but usually is for ( int i = 1; i < array2.length; i++ ) { ... } // Goes past last element of array - always wrong for ( int i=0; i <= array2.length; i++ ) { ... } *** See ArrayLoop4.java *** Random Numbers Let's say that I wanted to generate a sequence of random numbers in the range 1 to 6. How could I do it in the real world (without a computer)?
- coins
- dice
- drawing ping pong balls What are random numbers used for?
- Games/Lottery
- Controlled Experiments (Control/Treatment groups)
- Simulations
- Cryptography
- Randomly selecting advertisements to appear on web pages Java has a class named Random that can be used to generate sequences of psuedo-random numbers. import java.util.Random;
