CS 1713/1711 Introduction to Computer Science Lecture 27

CS 1713 Lecture 27: Arrays
November 1, 1999

The array data structure:
- An ordered collection of data items of the same type.
- Each item is identified by its position (index).
- In Java, the index values start at 0.
- In Java, use the new to allocate space for an array.
- Alternatively, you can allocate space by giving an explicit list of values in braces.
Syntax display for the array in Java:
Form: elementType [] arrayName = new elementType[ size ];
elementType [] arrayName = { list-of-values };
Example: double [] rands = new double[50];
int [] coins = {1, 5, 10, 25, 50};
Interpretation: The arrayName represents a collection of items of type elementType. The number of items is size in the first form or the number of items in the list in the second form.
Example: Create an array of n boolean values:
```
     boolean [] tests = new boolean[n];
```
The value of n must be known at the point this statement is executed.
Syntax display for accessing array elements in Java:
Form: array[ index ]
Example: x[4] = 10;
Interpretation: The array specifies which array. The index specifies the position of the item in the array, starting at 0. If index is less than 0 or bigger than the size of the array, you will get an Out of bounds error at run time.

Example: Declare an array of doubles containing 50 random numbers.

      double [] rands = new double[50];  // set aside space
      for (int i = 0; i < 50; i++)
          rands[i] = Math.random();

Example: Find the maximum and minimum values of rands in the previous example.

      double maximum = rands[0];
      double minimum = rands[0];
      for (int j = 1; j < 50; j++) {
          if (maximum < rands[j]) 
             maximum = rands[j];
          else if (minimum > rands[j])
             minimum = rands[j];
      }

Example: Create a boolean array called upper which contains true in entries corresponding to entries of rands that are greater than 0.5.

     boolean [] upper = new boolean[50];
     for (int k = 0; k < 50; k++)
         if (rands[k] > 0.5)
            upper[k] = true;
         else
            upper[k] = false;

Example: Sieve for designating prime numbers. (Just briefly discuss in preparation for Lab 7.)

Example: Create an array containing grades of class members.

      SimpleGUI wind = new SimpleGUI();   // create an input window
      int numberGrades = wind.getInt("How many grades are there?");
      double [] grades = new double[numberGrades];
      for (int i = 0; i < numberGrades; i++) 
          grades[i] = wind.getDouble("Enter grade " + (i+1));

Note: You will need to include import simpleIO.*; at the top of the file where this code appears and set the classpath to include the simpleIO root directory.

Example: Find the class average in the previous example and create a new array with the deviations of the grades from the average.

                     //compute the average of the elements in the array
       double average = 0;
       for (int j = 0; j < numberGrades; j++) 
           average += grades[j]; 
       if (numberGrades > 0)
          average = average/numberGrades;

                    //create the deviation array
       double deviation = new double[numberGrades];
       for (int k = 0; k < numberGrades; k++)
           deviation[k] = grades[k] - average;

The length data field: gives the number of elements in an array.
Example: Find the sum of the entries in the array a.
```
      int sum = 0;
      for (int i = 0; i < a.length; i++)
          sum += a[i];
```
The length of the array a is a.length. Notice that there are no parentheses because length is a field, not a method.

Objective: Introduction to arrays in Java.

Last revision: October 30, 1999 at 9:30 am