How to Declare and Initialize Arrays

Part I: How to Declare and Initalize an Array

You can declare an array of any type. Below we create an array of ints and we only initialized the first three values.

Below is an example of an array of Strings. This array can store 400 Strings! We only used the first three spots.

	String[] anArrayofStrings = new String[400];
	anArrayofStrings[0]  = "1" ;
	anArrayofStrings[1] = "aldjlajld;";
	anArrayofStrings[2] = "djka";

Uninitalized elements : In our example above there are 397 slots that we haven't yet inialized. The default value of an uninitalized element is null. See the example below

System.out.print( anARrayofStrings[3] ) will print null

The Length Property of Arrays

The length of an array is a data member, or a property, of the array. If you declare the array to hold 4 values its length will be 4. Remember thugh that its last index value will be 3.

Run the loop below to understand what it does.

int[] anArrayOfints = new int[100];
		for(int i = 0;i< anArrayOfints.length; i++){
			anArrayOfints[i] = i*3;
		}

		for(int i = 0;i< anArrayOfints.length; i++){
			System.out.println(anArrayOfints[i]) ;
		}

Projects
• Javabat: first last
• Javabat: same First Last
• Javabat: make Pi
• Javabat : common end
• javabat : sum
• javabat: rotate left
• javabat: reverse 3
• javabat: max end 3
• javabat sum 2
• Javabat: middle Way
• Javabat: Make Ends
• Javabat: has 23
• Javabat: no 23
• Javabat: makeLast
• Javabat: double 23
• Create an array of 8 doubles that stores random numbers

How to loop over an Array in Java

The general method for looping through an array is similar to how you loop through a String . In both cases, you use the length of the object and an index value based on a loop.

Below are two different loops. Each one iterates over the entire array. If you're familiar with the % operator, you should be able to figure out what the programmer below was trying to do: to print out all of the elements that are divisible by 10. However, only one of the loops actually does this. Can you determine which loop works properly.

Answer

Often times you want to compare elements within an array to try to figure somethign out. Look at the loop below. What is this loop 'figuring out' about the array of ints

Deadline: The last day that we will have class time dediated to solving thse will be Friday.

Javabat Practice Problems : looping through an array in Java
• Count Events
• Write a program in Eclipse that
  • Creates an array of doubles. Each double should have a random value between 1 and 10
  • Calculates the average of all of the elements in the double[]
  • prints out the average
• Centered average
  • for "Centered Average" use Eclipse and just print out the centered average. First, run this code to learn how to use a very, handy method of the Arrays class
• has 22
• sum 67
• Lucky 13
• Is Everwhere
• Either 24
• Match Up
• sum 28
• only 14
Extra Credit Exercises
• have three
• Big Diff (**Extra Credit)