REARRANGING THE STACK

If you want to use the operand stack directly in your program, the chances are you will need to perform some stack manipulation. It’s almost unavoidable. There are a few operators that you will find indispensable for stack manipulation. There are also some techniques worth learning to help you keep bugs out of your stack exercises. Table 6.1 contains a summary of the stack operators in the PostScript language for quick reference. Don’t worry if you don’t understand how to use all the operators in the table; it will become more clear as you read on.

Table 6.1: PostScript Stack Manipulation Operators

The most important operators for stack manipulation are dup, exch, index, and roll. Of these four operators, dup and exch are fairly easy to understand, since they apply only to the topmost item or items on the stack, but index and roll can be a bit more confusing.

Using the dup and index Operators

Although the dup operator is very simple in its operation, knowing when and how to use it effectively requires some skill in manipulating the operand stack. The index operator has almost exactly the same function as dup, except it will let you duplicate an object further down on the operand stack (dup duplicates only the topmost object). There are two or three places where these operators are used most commonly.

1.If you need to use a piece of data twice, use dup to make a temporary copy of the data. The copy will be used by the first operation, leaving the original data for the second operation.

2.When you are debugging, you can make a temporary copy of an object (using dup) to print or write to a debugging file without disturbing the execution of the program.

3.In a loop where you may need the same piece of data each time around the loop, that data can be left on the operand stack and it can be copied (with dup or index) each time before it is used inside the loop. Just make sure to remove it from the stack when you exit the loop. This technique can be seen in Example 6.2.

Example 6.2: Using index in a Loop Body

/Encoding 256 array def

Encoding % leave on operand stack throughout loop 0 1 255 { %for

1 index exch /.notdef put

} bind for pop

Using the roll Operator

The roll operator is one of the most useful stack operators, and one of the most confusing. Its purpose is to rearrange several of the topmost operands on the stack by rolling them. It takes two operands: one to supply the total number of elements to participate in the roll operation (this total is called the roll group in the following discussion) and the other to indicate how many elements within that total should actually be rolled (called the roll amount).

Let’s look at the anatomy of the roll operator (Figure 6.1).