Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!uwvax!umn-d-ub!umn-cs!nis!ntmtka!mike
From: mike@ntmtka.mn.org (Mike Tietel)
Newsgroups: comp.lang.postscript
Subject: Re: PostScript question (help needed)
Message-ID: <1967@ntmtka.mn.org>
Date: 21 Dec 89 17:03:41 GMT
References: <314@vidiot.UUCP>
Organization: Northern Telecom Inc., Minnetonka, MN
Lines: 101

In article <314@vidiot.UUCP>, brown@vidiot.UUCP (Vidiot) writes:
> 
> /$DITroff 140 dict def $DITroff begin

The above line is the key to the misunderstanding.  The
"/$DITroff 140 dict def" creates a dictionary named "$DITroff"
with enough space for 140 key-value pairs.  The "$DITroff begin"
just makes the dictionary "$DITroff" current (i.e., puts the
dictionary "$DITroff" on top of the dictstack).  Placing the
dictionary on top of the dictstack allows you to place key-value
pairs in the dictionary using the "def" operator instead of the
"put" operator (i.e., "/hi (hello) def" instead of
"$DITroff /hi (hello) put").

> .
> many definitions created
> .
> end

The "end" pops the topmost dictionary off of the dictstack, note
however that this dictionary is still known by the name "$DITroff".
All that has been done up to this point is define a few key-value
pairs in the dictionary "$DITroff". NOTHING HAS BEEN THROWN AWAY!
The end does not cause this dictionary to be lost forever, because
the dictionary is known by the name "$DITroff"!!!

> Then a definition is created for ditstart that does another dictionary for
> $DITroff.  This time there isn't an end, so far so good.
> 
> /ditstart{$DITroff begin

The above line does not "do another dictionary" it makes the
"$DITroff" dictionary current so that some new key-value pairs
may be added to it.

>  /nfonts 60 def			% NFONTS makedev/ditroff dependent!
>  ...
> }def
> .
> more dictionaries and macro definitions
> .
> ditstart
> (psc)xT
> 576 1 1 xr
> ...
> 1 f
> xi
> %%EndProlog
> %%Page: 1 1
> 10 s 0 xH 0 xS 1 f
> %body of PostScript pages goes here
> %%Trailer
> xt
> %This procedure does the end for the dictionary.
> xs
> 

Note from the red book that:
dict - *creates* an *empty* dictionary and places it on the operand stack
begin - pushes a dictionary on the dictstack
end - pops a dictionary off the dictstack

If I write a program like the following:

    2 dict begin
	/hi (hello) def
	/bye (goodbye) def
    end

the dictionary created *is* lost forever.
However, if I write a program:

    /mydict 2 dict def	% create a dict called "mydict"
    mydict begin
	% define key-value pairs
	/hi (hello) def
	/bye (goodbye) def
    end

the dictionary is not lost because we have named it "mydict".
We could also have written this program as follows:

    /mydict 2 dict def	% create a dict called "mydict"
    mydict /hi (hello) put	% define one key-value pair
    mydict /bye (goodbye) put	% define one key-value pair

or:

    /mydict 2 dict def	% create a dict called "mydict"
    mydict begin /hi (hello) def	% define one key-value pair
    mydict begin /bye (goodbye) def	% define one key-value pair

In all three cases "mydict" will contain the same key-value pairs.

If there are more questions, don't hesitate...
mjt
-- 
Mike Tietel
Northern Telecom, Inc.       (612) 932-8017
9701 Data Park, H-200        mike@ntmtka.mn.org
Minnetonka, MN 55343         {rosevax,bungia}!ntmtka!mike