Introduction to Computer Science - Java

Using Arrays

        If we wanted to keep track of the fees we charged cars for parking in our garage we would need a variable for the fee for a given car. And perhaps another varaible for the total daily charges. To do this we might write something like this.

boolean quit = false;
double fee, total_fees;
while( ! quit )
   fee = get_charges();
   total_fees += fee;

This seems fine except what if after we were done computing all the fees and the total fee we wanted to know what the 4th car had paid, How could we recall this information? We could store each fee in a different variable. We could make
double fee1,fee2,fee3,fee4,fee5;
Then we would have to copy the fee into a different variable each time. We would have to use a lot of if and else's to check which iteration number we were on so we would know which fee number to assign. This would produce very messy code, but it could work. In stead we can create an array of ints.

        If we want to create a set of variable of the same type we can do this by creating an array. An array is a set of variables of one type all having the same name but with different index numbers. We can declare an array like this:
int a[]= new int[12] ;
This will create 12 integers located in contiguous spaces in memory, i.e. one after the other. By using the number 12 we have instructed the computer to find us 12 new integer locations in memory. The new command is an instruction to the operating system to find whatever number of contiguous spaces we specify afterwards. In this case we specified 12 int's. If an int is 4 bytes then the computer will look for 12 * 4 or 48 bytes of memory in adjacent places. If we ask for a very large space, the computer may fail to find the space and the new command will fail at run time. The name of this array will be "a".

        We can reference different parts of the array by using indexes, also called offsets. Here is how it is done.
a[0] = 18 ;
This assigns the first element of array a the value 18. Since we asked for 12 spaces, we got 12 spaces, but the indexes will start with zero ( 0 ) and go until eleven ( 11 ). So to assign the last element in the array a value we would do the following.
a[11] = 625 ;

        We can assign different values to the different elements in the array inside of a loop. We can increment through all the indexes of the array by using our counter as the index of the array. Let's see the above example rewritten using an array.

double fee[] = new double [ 12 ];
double total_fees; for(int counter =0; counter < 12; counter++ )
   fee[ counter ] = get_charges();
   total_fees += fee [ counter ] ;

In this example counter will start with the value of 0 and increment upwards till it reaches 12. The first car will have its fee stored in the variable fee [ 0 ] . The second car will be in fee [ 1 ] , and so on until the last or 12th car will have its fee assigned to fee [11] . If at some later time I wish to recall the fee of the 4th car I can access it by writing fee [ 3 ] . This provides a simple way to store a large set of data in an easilly accessable format.

        The name of the array in this case is "fee". What is the type of this name? It is not simply a double. It is a reference to a set of doubles. If I want to dereference a particular element in the array I must specify which element. As we have seen I do this by using the braces [ ] and including a number inside them. That number is called the index of the array, or the array offset. If I were to passing an array in a method, what type would I write in the method definition? I would specify that it is an array. I would do this like this:

int myMethod ( double someName [ ] )


This means that the method myMethod has a parameter which is an array of doubles. This array can be of any size.
I would call the function like this:

myMethod ( fee );

The call will pass the whole array named fee to the method myMethod. Now teh method can access all the element in the array. In the method myMethod the elements will be refered to by the new name I gave the array, someName. I.e. to print the 4th element in the array inside the method I would write

System.out.println( someName [ 3 ] );

        We have learned that in methods any changes to the values that were passed as variables to the method (through the parameters) if changed inside the method, do not affect the original variable in the calling mathod. This is try because the variable itself is not passed to the method, but only the value of the variable is passed. That value is then copied into the variable which appears in the parameter list in the function defininition in the order it was written. Thus changes in the method variable do not affect the original variable. This is not true with arrays. Since an array name is a reference to the variables inside it, when a copy of this reference is made, the copy still refers to the same variables. So when I dereference the elements of the method's array, they are the same variables as the calling method's array. Changes in the array elements in a passed array will affect the array in the calling function. If I give a hit-man an address of a person to be killed, even if the hit-man makes a copy of the address on a separate paper, when he goes to the house at the address on the copied paper and kills the man inside that house, the man who lives in the house at the address on the original paper is also dead - it is the same man! . The paper is a copy, but the thing it refers to is only one thing.
Try an example and see for yourself, but don't get killed.

© Nachum Danzig November 2003