Computer Science 1 Top Lectures Assignments Noticeboard Arrays

Introduction

• An array is a language element that is a feature of most programming languages
• It supports collections of data, like a Vector:
• consists of one or more positions
• posistions indexed by an int
• first position is 0
• must be created with new
• is an object
• However it is different from a Vector:
• Not a class - it is built into the language
• There are no methods
• There is a field, called length, to give the number of elements (c.f. a method size for Vectors)
• Arrays can hold primitive data types (like ints)
• The type of elements in the array must be specified when the array is declared. Everything stored in that array must be of that type.
• Arrays can't grow.

Using arrays

• To declare an array, you write the type of the data element it will hold, followed by square brackets and then the name:

int [] lottoNumbers;

String [] winners;

Employee [] personnel;

• Compare this with the parameter to the main method, which we've seen from day one.
• To create an array, use new, followed by element type, followed by desired length in square brackets.

lottoNumbers = new int [6];

winners = new String [20];

personnel = new Employee [1000];

• Anywhere in the program, we can now find out their lengths:

int numEmployees = personnel.length;

Notice, no brackets after length - it is not a method call.
• To set the value of a particular position in the array, you use an assignment:

lottoNumbers[1] = 27;

• To use the value of a position, you index it with square brackets

String line = winners[0] + winners[1];

• Example, read in 100 integers and print them in reverse order

import java.io.*;
import cucs.*;
class ReverseIntegers {

  public static void main (String [] args) {
     BasicDataReader in = new BasicDataReader ();
       int[] z = new int[100];
       int i;

       for (i=0; i != 100; i++) {
         z[i] = in.readInt ();
       }

       for (i=100; i != -1; i--) {
         System.out.println (z[i]);
       }
  }
}

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Last modified: 4/19/99