Shelves and Constants

You can create shelves in your OmniMark program. Shelves may be of any supported data type, including record data types defined using declare record.

Shelves can be either global or local. The difference between these is that global shelves exist (and can be used) everywhere within a program, whereas local shelves only exist (and can be used) within the rule or function where they are declared.

Since shelves cannot be used until they are declared, global shelf declarations usually appear at the top of an OmniMark program, and local shelves appear at the beginning of the rule or function in which they are to be used. The scope of a shelf (global or local) must be indicated in the shelf declaration.

A shelf declaration that creates a global integer shelf named count1 looks like this:

  global integer count1

Once declared, count1 can be used to store any positive or negative integer value.

To create a local string shelf named quotation, you would use the shelf declaration:

  local string quotation

To store a value in a shelf, you can use the set keyword. For example:

  set quotation to "Is this a dagger which I see before me?"

integer shelf values can be assigned and changed the same way as string shelves using the set action, but integer shelves can also be manipulated using the increment and decrement actions. For example, to increase the value of the count1 shelf by one, you need only say:

  increment count1

It is possible to increment or decrement the value of an integer shelf by the value of another integer shelf. For example, you could decrement the value of count1 by the value of count2 with the following code:

  decrement count1 by count2

The following is a program that makes use of a global switch shelf to decide which output action should be executed:

  global switch question
  
  process
     set question to true
  
  process
     do when question  ;checks if question is true
        output "to be"
     else
        output "not to be"
     done

Note that the output of this program will always be to be.

Initial values

It is possible to declare a shelf with an initial value:

  global integer count2     initial { 3 }
  global string  quotation2 initial { "A horse!" }
  global switch  status2    initial { true }

You can set a shelf to the value of another shelf. For example, the process rule in the following program will set the value of the global integer shelf var1 to the value of the local integer shelf var2 and give you the output 8:

  global integer var1
  
  process
     local integer var2
  
     set var2 to 8
     set var1 to var2
  
  process
     output "%d(var1)"

Constants

A constant is like a global shelf, except that it must be given an initial value, and its initial value cannot be changed. A constant is declared by using the keyword constant instead of global. A constant is a good way to give a name to a value that the program will never change.

  constant integer days-in-week   initial { 7 }
  constant integer months-in-year initial { 12 }

Related Topics