Parallel arrays often come up in the examination.

An altyernative data structure would be an array of records.

using arrays as method parameters - an SL mastery aspect

Parallel Arrays

Consider two arrays, scores and names:

Record structures are discussed further elswhere .

Worked example

Given an array scores and another array pass

Write the function that examines scores and assigns the value true or false to the corresponding element of pass when the score is over 50:

The value in scores[0] is examined. It is <= 50 so the value false is assigned to pass[0]. A simple method to do this is shown below:

private void allocate( int scores[], bool pass[], int size )
{
   for(int i = 0; i < size; i++)
   {
     pass[i] = (scores[i] >= 50);
   }
}

This illustrates some interesting points:

1.  Objects and Primitives as Parameters
Arrays are objects (instances) as we have seen. This means that, unlike primitives they are passed-by-reference . Any change in the contents of an array (eg pass[]) in the method are made to the original array.

Primitives, by comparison are passed-by-value . Therefore a primitive (eg size) is not altered in the calling routine. For example if the line:

  allocate( scores[], pass[], size );

is used to call allocate, scores and pass could change but size cannot, even if a line like:

  size = size - 1;

is placed in the allocate method.

Incidentally, if you do need to pass primitives by reference ( not a recommended practice but sometimes neccessary) you can use the Java wrapper classes (Integer, Double, Boolean, etc).

2.  Assigning boolean expressions to boolean primitives
Boolean primitives can be directly assigned the result of a boolean expression - like the <conditions> we have seen with if and while statements.

  boolean check = ( (x < y) && (z > m) );

is a perfectly valid statement.

3.  The "off-by-one" error
It is quite easy to make a mistake when setting up a loop to process an array and this leads to runtime errors in Java (ie the program stops unexpectedly). Consider the following, almost correct, loop:

  void allocate( int scores[], bool pass[], int size )
   {
     for(int i = 0; i <= size; i++)
     {
       pass[i] = (scores[i] >= 50);
     }
   }

The pointer ( i ) used to access the array continues until it is equal to size . Unfortunately, since Java arrays are 0-based (their first element is 0), the array has elements only 0 to ( size - 1 ). Such an error can have serious consequences in other languages (such as C++) but are caught by Java when the application runs.

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