CS115 lecture notes, Spring 1999
Week 11, Wednesday

How are arrays similar to objects?

How are arrays different from objects?

Arrays (continued)
------------------

If you use an index that is negative or >= size, you get
an ArrayOutOfBoundsException.

You can use loops to traverse arrays:

    int i = 0;
    while (i<4) {
      System.out.println (count[i]);
      i++;
    }

Example:

Given an array of doubles, a, how would you make a copy?

  double[] a = new double[3];
  double[] b = a;

This copies a reference to the array, which means that
a and b are now aliased.

To build a new array, we have to use new to allocate
space, and a loop to copy the elements of the array.

    double[] b = new double [3];

    int i = 0;
    while (i < 4) {
      b[i] = a[i];
      i++;
    }


Passing arrays as parameters
----------------------------

Good news.  Everything is the same as passing objects
as parameters.

1) A reference gets passed

2) Caller and callee have two references to the same object
   (aliasing)

3) Callee can modify the contents of the array.


Example:

  public static int banana (int[] a) {
    int i = 0;
    int grape = 0;
    while (i < a.length) {
      grape += a[i];
      i++;
    }
    return grape;
  }

  public static void main (String[] args) {
    int[] a = new int[10];
    // put values into the array

    System.out.println ("max = " + max2(a));   
  }

Hey, look!  The parameter to main makes sense, finally!

Quiz
----

Different kinds of traversals:

ACCUMULATION

  public static int sum (int[] a) {
    int i = 0;
    int total = 0;
    while (i < a.length) {
      total += a[i];
      i++;
    }
    return total;
  }
	
COUNTING

  public static int frequency (int[] a, int p) {
    int i = 0;
    int count = 0;
    while (i < a.length) {
      if (a[i] == p) count++;
      i++;
    }
    return count;
  }

EUREKA: finding the first thing that satisfies a criterion

  public static int find (int[] a, int p) {
    int i = 0;
    while (i < a.length) {
      if (a[i] == p) return i;
      i++;
    }
    return -1;
  }		

EXTREME VALUE FINDING

  public static int max (int[] a) {
    int max = a[0];
    int i = 1;
    while (i < a.length) {
      if (a[i] > max) max = a[i];
      i++;
    }
    return max;
  }
	

How to generalize these things?
------------------------------

Specify a range of indices over which to sum/count/search.


This week's homework
--------------------

Lots of problems where I say what to do, but not how.

I will be providing hints in lecture.

Work for a while, think for a while, ask questions,
think for a while, repeat.

This is homework 11; only one more after this one!!!