Example 11.3: Creating and Filling an Empty Array

10 array dup

/one /one /one (four) /five

6 /seven { 8 (the whole procedure) } 9 10 astore

CONSTRUCTING A STRING

Strings can be constructed in much the same way as arrays. You can create one at run-time in the original input program by using the notation found in Example 11.4 for simple strings or hexadecimal strings (also called hex strings).

Example 11.4: Creating Strings with the Scanner

(This is a string created at run-time; following is a hex string:) <a32989e4ff>

Hexadecimal strings are scanned two bytes at a time to create the single byte represented by the 8-bit number. (One byte of hexadecimal, since it is base 16, provides only 4 bits of data; it requires two bytes of hexadecimal to represent a full 8 bits.) Hex strings are generally used only for image data, strings for Kanji character sets, or other places where full 8-bit string data must be used. The string operator can also be used to create an empty string body, and you can use put or putinterval to put bytes into the string.

MANIPULATING DATA WITH PUT AND GET

Strings and arrays are composite data structures. They are both represented by single objects, but really they are composed of collections of objects. There are a few different ways to read and write the contents of these composite data structures, included in Table 11.1. The fundamental mechanism is the family of put and get operators. We’ll start with the simplest ones.

Table 11.1: Operators to put and get Data

The put operator requires three arguments: the thing you want data put into, the key or position in that composite data structure where you want the data put, and finally the piece of data itself that is to be added (see Example 11.5).

Example 11.5: Anatomy of the put Operator

% data_structure position_key new_data put currentdict /key (value) put

10 array 0 (first element) put (XXXXX) 4 (a) put

The putinterval operator is similar to put, but the piece of data you supply needs to be of the same type as the data structure into which you are putting it. The data gets copied from the supplied data into the destination, rather than putting just one piece of data (see Example 11.6).