NAME LOOKUP

The standard method for retrieving something that has been stored into a

dictionary is to use the name lookup mechanism. An executable name

encountered by the interpreter is looked up in the context of the current

dictionary stack, and if there is an entry corresponding to that name, the

value will be placed on the operand stack—with one notable exception. If

you happen to retrieve a procedure body (an executable array object)

under the name you look up, that procedure will be placed on the

execution stack and immediately executed, rather than being placed on

the operand stack and treated as data.

TIP

If you need to retrieve a procedure body that you have stored into a dictio-

nary, but don’t want to execute it just yet, you can get a copy of it onto the

operand stack using the load operator, using the name of your procedure

(with the leading slash, to make it a literal name) as the argument to load.

Chapter 5: UNDERSTANDING THE STACK

53

Example 5.1: Simple Use of the Operand Stack

/Times-Roman 12 selectfont

/Xlocation 100 def

/Ylocation 200 def

Xlocation Ylocation moveto

(text sample) show

This code segment looks like a definition of two variables, Xlocation and

Ylocation, which are then used as the coordinates for the moveto

operator. From the point of view of stack manipulation, the def operator

simply takes two objects from the stack and puts them in the current

dictionary, and the subsequent name lookup of, say, Xlocation, retrieves

the object associated with that key from the current dictionary. As shown

in Figure 5.2, by the time you get to moveto, there are no variables or

anything else left; there are just two numbers on the operand stack, which

is what is required by the moveto operator. So in effect, the numbers 100

and 200 start out on the operand stack, are stored into a dictionary,

retrieved from that dictionary back onto the operand stack, and used by

the moveto instruction. A variable is a name used to reference data.

TIP

When you define a variable in your PostScript program, you are actually

taking something off the operand stack and putting it into a dictionary. In

order to use the variable, you must always recall it from the dictionary

back onto the operand stack. Be aware that using “variables” is always

somewhat slower than using the operand stack directly.

To understand how the operand stack works, it is worthwhile to step

through a simple program, executing it in your head. It is important

enough that you might spend some real time at this, until you are

extremely comfortable talking through a program, understanding exactly

what happens to it as it is being executed.

54

Chapter 5: UNDERSTANDING THE STACK

represents the operand stack

lines from the

100

program

/Xlocation

/Xlocation

/Xlocation

100

def

200

/Ylocation

/Ylocation

/Ylocation

200

def

200

100

100

Xlocation

Ylocation

moveto

(text sample)

(text sample)

show

Chapter 5: UNDERSTANDING THE STACK

55