Arrays, Strings and References

Here is a simple example of an array of three characters.

    {
         char ca[3] ;    // three characters variables
         ca[0] = 't' ;
         ca[1] = 'o' ;
         ca[2] = 'p' ;  
         cout << ca[0] << ca[1] << ca[2] ;
         cout << endl ;  // newline, so it looks nice.
    }

Here is an example of calculating the index to the array as the program runs:

    {
     // using indices which are the result of evaluating
     // and expression.

         char ca[3] ;
         ca[0] = 't' ;
         ca[1] = 'o' ;
         ca[2] = 'p' ;
 
         int i ;
         for ( i=0; i<3; i++ ) 
         {
            cout << ca[i] ;
         }
         cout << endl ;  // newline, so it looks nice.

         for ( i=0; i<3; i++ ) 
         {
            cout << ca[2-i] ;
         }
         cout << endl ;  // newline, so it looks nice.
      }

The name of an array not followed by an integer expression enclosed in square brackets is something we have not yet encountered. It is not directly a place to store data. It is a reference to a place to store data. Here is an analogy with luggage lockers in a train station.

In Victoria Train Station, London, there are luggage lockers where you can lock away your luggage while you visit the city. You place some money in the vending portion of the locker, this unlocks the locker and gives you a paper ticket to use later as the key. There is a timer, so that you might have to pay more to get your stuff out of the locker if you overstay the original time limit.

A variable is like one of these lockers. In the case of a simple variable, the ticket is unimportant. You refer to the lock by some pet name, like i or j, and you never wander too far from the locker.

An array is a sequence of lockers. You contract for N lockers, int a[N], and to make finding the lockers easy, and to cut down on the number of individual tickets, the lockers are all given in a single row. You only keep track of the first locker in the sequence, and the number of lockers thereafter.

The ticket which references the locker or sequence of lockers is a very interesting item. This ticket is portable. It is much more convienient to walk around town with the ticket rather than the locker. When the locker's contents are needed, the ticket provides the necessary information for the access. This is called dereferencing the ticket. The dereference could be for retrieval of the locker's contents, or the placing of new items in the locker. Moreover, we allow the ticket to be copied. Both the holders of the ticket and the ticket's copy can access the lockers. Tickets can themselves be placed in lockers.

Arguments are passed by value, hence the ticket giving access to the array is to be copied into a variable local to the functions. The type of the variable must match the type of the ticket. In our example, this variable will be of type array of char, or pointer to char, or reference to char, or char-star. Here is an example:

    void printIt(char arrayToPrint[])
    {
        int i ;
        i = 0 ;
        while ( arrayToPrint[i]!='\0' )
        {
            cout << arrayToPrint[i] ;
            i++ ;
         }
     }

     void main()
     {
         char hi[3] ;
         hi[0] = 'h' ;
         hi[1] = 'i' ;
         hi[2] = '\0' ;
         printIt( hi ) ;
     }

1. Access arrayToPrint to get the ticket.
2. Change the number on the ticket by incrementing by i
3. Access the resulting location.

It is possible to write the function header as:

    void printIt( char arrayToPrint[3] )
    {
       // and so on
    }

As added clarification:

   void copyIt( char arrayToCopy[] )
   {
       char aRealArray[100] ;
       int i ;
       i = 0 ;
       while ( arrayToCopy[i]!='\0' ) 
       {
          aRealArray[i] = arrayToCopy[i] ;
          i++ ;
       }
       aRealArray[i] = '\0' ;
   }