Latest revision as of 04:41, 1 January 2009

Global Constants and Values

Assigning recognizable names to numbers is a convenient shortcut, as you've seen with named input parameters and local variables. But as you saw, both of those kinds of names are local, they apply only to a very limited section of the program, inside a definition. But Mops also has a provision called VALUE for assigning readily-identifiable names to numbers such that they can be used throughout a program.

Your program can contain many different values because you define each value by giving it a unique name and a number that it is to hold. You define a value like this:

25 value Jane

In other words, the value named JANE is holding the number 25. To recall a value's number, all you do is type the value name, and a copy of the number is placed on the parameter stack. Type:

Jane

and the number 25 is placed on the stack.

Note: If you already have some familiarity with other (much older) Forths, a VALUE is what is sometimes called a self-fetching variable.

A value is essentially a global version of a local variable (it accessible to all definitions in a program), and responds to similar operations. To store a different number in a value, you use the gazinta (the store arrow), like this:

37 -> Jane

which writes a 37 over the original number (25). Or you can increment or decrement the number stored in a value name with the ++> or --> operations, like this:

17 ++> Jane
4 --> Jane

which adds 17 to number already stored in JANE (37), then subtracts 4. Of course you can also do a subtraction by incrementing the value by a negative number:

-10 ++> Jane

If you want to define your values at the beginning of a program without placing specific numbers in them, you can simply place zeros in them all:

0 value Joe
0 value Nancy

and then store (->) numbers to them when necessary.

Note: The initial numbers you specify for your VALUEs are set up when your program is loaded by Mops. If you restart your application without reloading it, your VALUEs will still contain whatever you last put in them, not their initial numbers.

So much for theory. Now it's time to pull together all the discussions and examples from the preceding lessons and dive into some real Mops applications.