Copyright © 1997-2013
Robert W. Hasker

Arrays

• Ch. 9
• Suppose we want to collect temperature data for a month.
• Need to hold these values in variables, so declare 30 of them:

  	double temp0, temp1, ..., temp29;
  

  • tedious, hard to change
• Solution: create a collection of temperatures.
  • [draw picture of array of 30 temperatures, label it temp]
  • How to declare this type of item?

    	double[] temp = new double[30];
    

  • 2 parts to this: type of each temperature, number of temperatures
  • This one declares 30 variables, named temp[0] to temp[29].
  • This is known as an array.
    • arrays allow us to create a variable with "slots" so that we can access parts of it by numbers
  • Should use a constant for the length:

    	static private final int NUM_DAYS = 30;
    	...
    	double[] temp = new double[NUM_DAYS];
    

• Accessing the array:
  • Use an index: a number to refer to a slot
  • The index of the first slot is 0; the index of the last slot is n - 1 where n is the number of slots in the array
  • Set temp of first day to 61:

    	temp[0] = 61.0;
    

  • Watch out: the index of the kth position is k - 1
    • Storing an item in the 5th position (a cold day):

      	temp[4] = 15.2;
      

  • Printing out the temperature for the last day [HOW?]:

    	System.out.println(temp[29]);
    

    Better:

    	System.out.println(temp[temp.length - 1]);
    

    In general, length is a (final) constant giving the size of the array
  • Set last day's temp to today's temp [HOW?]:

    	temp[temp.length - 1] = temp[0];
    

  • Read in temperatures from the keyboard; what type of loop?

    	for(int day = 0; day < temp.length; day++)
               temp[day] = in.nextInt();
    

• Printing the average temp:
  • Pseudocode:

    	sum = 0;
    	for each day
    	   add day's temperature to sum
    	print sum / number of days
    

  • Code:

    	double sum = 0.0;
    	for(int day = 0; day < temp.length; day++)
    	   sum += temp[day];
            System.out.println("Average temp: " + (sum / temp.length));
    

• What is the point of an array?
  • Create bunch of values that can be accessed by number.
  • This is a sequence of items.
  • Example: how would we compute the average with temp0, temp1, etc?

    	double sum;
    	sum = temp0 + temp1 + temp2 + ... + temp29;
    

• General form of an array declaration:

  	element-type[] array-name = new element-type[length];
  

Array of Objects

• class for temperature readings: DailyRecord: high, low, getHigh, getLow
• class MonthlyStats: DailyRecord[] dailies = new DailyRecord[30];
• averageHigh, averageTemp, averageLow
• hasFreezingTemp
• firstHardFreeze
• Exercise: reverse a list of names

          System.out.print("Enter number of names: ");
          int num_names = in.nextInt();
          String[] names = new String[num_names];
          for(int i = 0; i < num_names; i++)
             names[i] = in.next();
          System.out.print("Names in reverse order:");
          for(int i = num_names - 1; i >= 0; i--)
             System.out.println(names[i]);
  

Array Gotcha's

• The array indices always start at 0 and go to length - 1.
• Assignment: what would x = y mean?
• Many operations are not useful with arrays:
  • <, >, etc
  • What would x == y, x != y mean?

Example: Reading Numbers Until Reach Sentinel

• Pseudocode, version 1:

  	inputs: max number of items to read, sentinel
  	outputs: array (nums), actual count of items read
  
  	pos = 0		// always: position to put next item
  	read next
  	while pos < items to read and next not= sentinel
  	   nums[pos] <- next
  	   pos <- pos + 1
  	   read next
  	actual_length <- pos
  

• Problem: may read one too many items; improved version of while loop:

  	while pos < length and next not= sentinel
  	   nums[pos] <- next
  	   pos++
  	   if pos < length then
  	      read next
  

Review

• arrays: sequences of items
• access item by indexing it