CS151 lecture notes, Fall 1999
Week 10, Wednesday

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

  public static void main (String[] args) {
    int[] array = new int[10];

    // put values into the array

    System.out.println ("sum = " + sum(array));   
  }

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


Traversals
----------

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;
  }

Think of max as "the biggest thing we've seen so far"
	

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

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


for loops
---------

Exactly the same as a while loop, different syntax

  INITIALIZER
  while (CONDITION) {
    BODY
    INCREMENTOR
  }

Problem: the loop-controlling statements are scattered
all over.  How many times do you forget the INCREMENTOR?

Try this...

    for (INITIALIZER; CONDITION; INCREMENTOR) {
      BODY
    }