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Java For Kids Tutorial 5

Java Arrays


Arrays are lists of similar objects or datatypes. they are ordered in a numbered sequence starting from 0 until the end or last item in the array. Here is an array of integers (int's). Note the format.
{12, 7, 32, 15, 113, 0, 7}
Here is how they will be stored with their number in the list :
index0 1 2 3 4 5 6
value127 32 15 113 0 7
The array items' number in the list is called its index. To get the value of an array item or element you give its index inside [ ] square brackets after the array's name. We could call the above array scoreArray.

Declaration of Arrays


We can declare arrays in Java in 2 ways:
	int scoreArray[] ;
	int[] scoreArray ;
	
This declaration tells the compilers what type of variables (datatype) the array contains. No actual array is created until the keyword NEW is invoked or used. e.g.

	int [] scoreArray ;
	scoreArray = new int[7] ;
	

The 2 lines above put an array of 7 ints into the array called scoreArray and they are all given a default value of 0 .
An array can be declared, constructed, and initialized in a single statement . e.g.

int[] scoreArray = {12, 7, 32, 15, 113, 0, 7 } ;

Note the use of the { } curly brackets and commas between the elements of the array.

Calling an Array Element


To retrieve an element in an array we simply put its index number in square brakets [] after the array name. e.g. scoreArray[0] ;

Exercise:

  1. what does scoreArray[0] equal ?
  2. What is the value of scoreArray[5] ?

Remember - arrays start at index 0 . So, the 5th element in an array will be arrayName[4] .
Here is a formula:

	nth Element     =>     arrayName[n-1]

Changing Java Array values


If you want to change an array element value, just go something like this :

	scoreArray[3] = 56 ;
	scoreArray[1] = 27 ;

To put an array element into another variable , just do something like this :

	int myScore ;
	myScore = scoreArray[4] ;

If you printed out myScore(printLine(myScore) ; ) , it would print 113.
Lets write a program after all that theorizing !


	void main() {
		// declare , initialize array
	int[] scoreArray =  {12, 7, 32, 15, 113, 0, 7 } ;
		// print scores
	for(int i = 0; i<7 ; i++ )
	{
		printLine("Score "+ i + " = " + scoreArray[i]);
	}
	}

Java Problem :


You have been given the job by the ACB to write a program to enter the scores for the Australian cricket teams one day match scores.
Output the scores and the Runs Per Over. A One Day Game has 50 overs.
It could have an output like this :

Player 1 : 23
Player 2 : 66
...
Player 11 : 7

Runs Per Over = 4.03

Lets first write out the steps that our program will take, in plain English .

  1. Declare array of ints containing 11 elements
  2. Loop through these items to read each score- input
  3. Add each score to a total
  4. Calculate the Runs Per Over
  5. Print out the scores.
  6. Print out the Runs Per Over

What we have just done is write the program's

ALGORITHM

.
An Algorithm is the series of steps written in plain English that would make our program run successfully or accomplish its task.

Lets write it ! You have a go on your own first.
...
What did you get ?
..
...

When you have done it, try to read in and output each cricketer's name instead of Player 3 e.g.
Calculate the Average score for the team.
And print out those above it and those below the average.

Exercise:

  1. Are these statements legal ?
    a. int[ ] numberArray[ ] ;
    b. String[ ] name ;
    c. double price[ ] ;
    price = new double[12] ;
  2. What are the default initialization values for int, double, String , boolean ?
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