Commodore VIC-20 - Arrays

Chapter 3: Programming the VIC 20 Computer 77
Arrays are used in many types of computer programs. If you are not
already familiar with arrays, you will need
to learn about them. The infor-
mation
that follows will be very important to your programming efforts.
Conceptually, arrays are very simple. When you have two or more
related data items, instead
of giving each data item a separate variable name,
you give the collection
of related data items a single variable name. Then you
select individual items using a position number, which in computer jargon
is
referred to as a subscript, or index.
A grocery list, for example, may have six items from the meat and
poultry department, four fruit
and vegetable items, and three dairy prod-
ucts. These three groups of items could each be represented by a single
variable name as follows:
MPS(O) = "CHOPPED SIRLOIN"
MPS(I)
= "CHUCK STEAK"
MPS(2)
= "NEW YORK STEAK"
MPS(3)
= "CHICKEN"
MPS(4)
= "SALAMI"
MPS(S)
= "SAUSAGES"
FVS(O) = "ORANGES"
FVS(I)
= "APPLES"
FVS(2) = "BEANS"
FVS(3)
= "CARROTS"
DPS(O) = "MILK"
DPS(I) = "CREAM"
DPS(2) = "COTTAGE CHEESE"
MPS
is a single variable name that identifies all meat and poultry products.
FVS identifies fruits and vegetables. while DPS identifies dairy products.
A subscript follows each variable name. Thus a specific data item is
identified by a variable name and
an index.
Notice
that the first index value in the examples above is 0, not 1.
Subscripts in BASIC start from 0 because this simplifies the programming
of many scientific and mathematical problems. Many people are un-
comfortable with this practice, however.
If you don't feel at home with using
element 0 of
an array, simply ignore it and start with a subscript of 1. You
will waste a little memory space,
but you are less likely to make program-
ming mistakes if you are not trying
to "adapt" yourself to the machine.
We could take the array concept one step further, specifying a single
variable name for the entire grocery list, using two indexes. The first index
(or sUbscript) specifies the product type, and the second index specifies the
item within the product type. This
is one way in which a single grocery list
variable array with two subscripts could replace the three arrays with single
subscripts illustrated above.