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Code Editor : pic-21.html

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<h2>21. PIC Reference
<a name="21. PIC Reference"></a>
</h2>

This is an
annotated grammar of pic.

<h3>21.1. Lexical Items
<a name="21.1. Lexical Items"></a>
</h3>

In
general, pic is a free-format, token-oriented
language that ignores whitespace outside strings. But
certain lines and contructs are specially interpreted at the
lexical level:

A comment
begins with # and continues to \n (comments
may also follow text in a line). A line beginning with a
period or backslash may be interpreted as text to be passed
through to the post-processor, depending on command-line
options. An end-of-line backslash is interpreted as a
request to continue the line; the backslash and following
newline are ignored.

Here
are the grammar terminals:

INT </td>
<td width="4%"></td>
<td width="38%">

A positive
integer. </td>
<td width="42%">
</td></tr>
</table>

NUMBER

A floating point numeric constant.
May contain a decimal point or be expressed in scientific
notation in the style of printf(3)&rsquo;s %e escape.
A trailing &lsquo;i&rsquo; or &lsquo;I&rsquo; (indicating
the unit &lsquo;inch&rsquo;) is ignored.</td></tr>
<tr valign="top" align="left">
<td width="10%"></td>
<td width="8%">

TEXT </td>
<td width="2%"></td>
<td width="80%">

A string
enclosed in double quotes. A double quote within
TEXT must be preceded by a backslash. Instead
of TEXT you can use</td></tr>
</table>

sprintf
( TEXT [, &lt;expr&gt; ...] )

except
after the &lsquo;until&rsquo; and &lsquo;last&rsquo;
keywords, and after all ordinal keywords (&lsquo;th&rsquo;
and friends).

VARIABLE </td>
<td width="74%">
</td></tr>
</table>

A string
starting with a character from the set [a-z], optionally
followed by one or more characters of the set [a-zA-Z0-9_].
(Values of variables are preserved across
pictures.)

LABEL </td>
<td width="80%">

A string
starting with a character from the set [A-Z], optionally
followed by one or more characters of the set
[a-zA-Z0-9_]. </td></tr>
</table>

COMMAND-LINE

A line starting with a command
character (&lsquo;.&rsquo; in groff mode, &lsquo;\&rsquo; in
TeX mode).</td></tr>
</table>

BALANCED-TEXT

A string either enclosed by
&lsquo;{&rsquo; and &lsquo;}&rsquo; or with X and
X, where X doesn&rsquo;t occur in the
string. </td></tr>
</table>

BALANCED-BODY

Delimiters as in
BALANCED-TEXT ; the body is interpreted as
&lsquo;&lang; command&rang; ...&rsquo;.</td></tr>
</table>

FILENAME

The name of a file. This has the
same semantics as TEXT .</td></tr>
</table>

MACRONAME

Either VARIABLE or
LABEL .</td>
<td width="30%">
</td></tr>
</table>

<h3>21.2. Semi-Formal Grammar
<a name="21.2. Semi-Formal Grammar"></a>
</h3>

Tokens not
enclosed in &lang; &rang; are literals, except:

1. </td>
<td width="6%"></td>
<td width="90%">

\n
is a newline.</td></tr>
<tr valign="top" align="left">
<td width="4%">

2. </td>
<td width="6%"></td>
<td width="90%">

Three dots
is a suffix meaning &lsquo;replace with 0 or more
repetitions of the preceding element(s).</td></tr>
<tr valign="top" align="left">
<td width="4%">

3. </td>
<td width="6%"></td>
<td width="90%">

An
enclosure in square brackets has its usual meaning of
&lsquo;this clause is optional&rsquo;.</td></tr>
<tr valign="top" align="left">
<td width="4%">

4. </td>
<td width="6%"></td>
<td width="90%">

Square-bracket-enclosed
portions within tokens are optional. Thus,
&lsquo;h[eigh]t&rsquo; matches either &lsquo;height&rsquo;
or &lsquo;ht&rsquo;.</td></tr>
</table>

If one of
these special tokens has to be referred to literally, it is
surrounded with single quotes.

The
top-level pic object is a picture.

&lt;picture&gt;
::= 
.PS [NUMBER [NUMBER]]\n 
&lt;statement&gt; ... 
.PE \n

The
arguments, if present, represent the width and height of the
picture, causing pic to attempt to scale it to the
given dimensions in inches. In no case, however, the X and
Y&nbsp;dimensions of the picture exceed the values of the
style variables maxpswid and maxpsheight
(which default to the normal 8.5i by 11i page
size).

If the
ending &lsquo;.PE&rsquo; is replaced by &lsquo;.PF&rsquo;,
the page vertical position is restored to its value at the
time &lsquo;.PS&rsquo; was encountered. Another alternate
form of invocation is &lsquo;.PS&nbsp;&lt;
FILENAME &rsquo;, which replaces the
&lsquo;.PS&rsquo; line with a file to be interpreted by
pic (but this feature is deprecated).

The
&lsquo;.PS&rsquo;, &lsquo;.PE&rsquo;, and &lsquo;.PF&rsquo;
macros to perform centering and scaling are normally
supplied by the post-processor.

In the
following, either &lsquo;|&rsquo; or a new line starts an
alternative.

&lt;statement&gt;
::= 
&lt;command&gt; ; 
&lt;command&gt; \n

&lt;command&gt;
::= 
&lt;primitive&gt; [&lt;attribute&gt;] 
LABEL : [;] &lt;command&gt; 
LABEL : [;] &lt;command&gt; [&lt;position&gt;] 
{ &lt;command&gt; ... } 
VARIABLE [:] = &lt;any-expr&gt; 
figname = MACRONAME 
up | down | left | right 
COMMAND-LINE 
command &lt;print-arg&gt; ... 
print &lt;print-arg&gt; ... 
sh BALANCED-TEXT 
copy FILENAME 
copy [FILENAME] thru MACRONAME [until TEXT] 
copy [FILENAME] thru BALANCED-BODY [until TEXT] 
for VARIABLE = &lt;expr&gt; to &lt;expr&gt; [by [*]
&lt;expr&gt;] do BALANCED-BODY 
if &lt;any-expr&gt; then BALANCED-BODY [else BALANCED-BODY]
 
reset [VARIABLE [[,] VARIABLE ...]]

&lt;print-arg&gt;
::= 
TEXT 
&lt;expr&gt; 
&lt;position&gt;

The
current position and direction are saved on entry to a
&lsquo;{&nbsp;...&nbsp;}&rsquo; construction and restored on
exit from it.

Note that
in &lsquo;if&rsquo; constructions, newlines can only occur
in BALANCED-BODY . This means that

if
 
{ ... } 
else 
{ ... }

fails. You
have to use the braces on the same line as the
keywords:

if
{ 
... 
} else { 
... 
}

This
restriction doesn&rsquo;t hold for the body after the
&lsquo;do&rsquo; in a &lsquo;for&rsquo;
construction.

At the
beginning of each picture, &lsquo;figname&rsquo; is reset to
the vbox name &lsquo;graph&rsquo;; this command has only a
meaning in TeX mode. While the grammar rules allow digits
and the underscore in the value of &lsquo;figname&rsquo;,
TeX normally accepts uppercase and lowercase letters only as
box names (you have to use &lsquo;\csname&rsquo; if you
really need to circumvent this limitation).

&lt;any-expr&gt;
::= 
&lt;expr&gt; 
&lt;text-expr&gt; 
&lt;any-expr&gt; &lt;logical-op&gt; &lt;any-expr&gt; 
! &lt;any-expr&gt;

&lt;logical-op&gt;
::= 
== | != | &amp;&amp; | &rsquo;||&rsquo;

&lt;text-expr&gt;
::= 
TEXT == TEXT 
TEXT != TEXT

Logical
operators are handled specially by pic since they can
deal with text strings also. pic uses
strcmp(3) to test for equality of strings; an empty
string is considered as &lsquo;false&rsquo; for
&lsquo;&amp;&amp;&rsquo; and &lsquo;||&rsquo;.

&lt;primitive&gt;
::= 
box # closed object &mdash; rectangle 
circle # closed object &mdash; circle 
ellipse # closed object &mdash; ellipse 
arc # open object &mdash; quarter-circle 
line # open object &mdash; line 
arrow # open object &mdash; line with arrowhead 
spline # open object &mdash; spline curve 
move 
TEXT TEXT ... # text within invisible box 
plot &lt;expr&gt; TEXT # formatted text 
&rsquo;[&rsquo; &lt;command&gt; ...
&rsquo;]&rsquo;

Drawn
objects within &lsquo;[&nbsp;...&nbsp;]&rsquo; are treated
as a single composite object with a rectangular shape (that
of the bounding box of all the elements). Variable and label
assignments within a block are local to the block. Current
direction of motion is restored to the value at start of
block upon exit. Position is not restored (unlike
&lsquo;{&nbsp;}&rsquo;); instead, the current position
becomes the exit position for the current direction on the
block&rsquo;s bounding box.

&lt;attribute&gt;
::= 
h[eigh]t &lt;expr&gt; # set height of closed figure 
wid[th] &lt;expr&gt; # set width of closed figure 
rad[ius] &lt;expr&gt; # set radius of circle/arc 
diam[eter] &lt;expr&gt; # set diameter of circle/arc 
up [&lt;expr&gt;] # move up 
down [&lt;expr&gt;] # move down 
left [&lt;expr&gt;] # move left 
right [&lt;expr&gt;] # move right 
from &lt;position&gt; # set from position of open figure
 
to &lt;position&gt; # set to position of open figure 
at &lt;position&gt; # set center of open figure 
with &lt;path&gt; # fix corner/named point at specified
location 
with &lt;position&gt; # fix position of object at specified
location 
by &lt;expr-pair&gt; # set object&rsquo;s attachment point
 
then # sequential segment composition 
dotted [&lt;expr&gt;] # set dotted line style 
dashed [&lt;expr&gt;] # set dashed line style 
thick[ness] &lt;expr&gt; # set thickness of lines 
chop [&lt;expr&gt;] # chop end(s) of segment 
&rsquo;-&gt;&rsquo; | &rsquo;&lt;-&rsquo; |
&rsquo;&lt;-&gt;&rsquo; # decorate with arrows 
invis[ible] # make primitive invisible 
solid # make closed figure solid 
fill[ed] [&lt;expr&gt;] # set fill density for figure 
xscaled &lt;expr&gt; # slant box into x direction 
yscaled &lt;expr&gt; # slant box into y direction 
colo[u]r[ed] TEXT # set fill and outline color for figure
 
outline[d] TEXT # set outline color for figure 
shaded TEXT # set fill color for figure 
same # copy size of previous object 
cw | ccw # set orientation of curves 
ljust | rjust # adjust text horizontally 
above | below # adjust text vertically 
aligned # align parallel to object 
TEXT TEXT ... # text within object 
&lt;expr&gt; # motion in the current direction

Missing
attributes are supplied from defaults; inappropriate ones
are silently ignored. For lines, splines, and arcs, height
and width refer to arrowhead size.

The
&lsquo;at&rsquo; primitive sets the center of the current
object. The &lsquo;with&rsquo; attribute fixes the specified
feature of the given object to a specified location. (Note
that &lsquo;with&rsquo; is incorrectly described in the
Kernighan paper.)

The
&lsquo;by&rsquo; primitive is not documented in the tutorial
portion of the Kernighan paper, and should probably be
considered unreliable.

The
primitive &lsquo;arrow&rsquo; is a synonym for
&lsquo;line&nbsp;-&gt;&rsquo;.

Text is
normally an attribute of some object, in which case
successive strings are vertically stacked and centered on
the object&rsquo;s center by default. Standalone text is
treated as though placed in an invisible box.

A text
item consists of a string or sprintf-expression, optionally
followed by positioning information. Text (or strings
specified with &lsquo;sprintf&rsquo;) may contain font
changes, size changes, and local motions, provided those
changes are undone before the end of the current item. Text
may also contain \-escapes denoting special characters. The
base font and specific set of escapes supported is
implementation dependent, but supported escapes always
include the following:

\fR,
\f1 </td>
<td width="84%">
</td></tr>
</table>

Set Roman
style (the default)

\fI,
\f2 </td>
<td width="84%">
</td></tr>
</table>

Set
Italic style

\fB,
\f3 </td>
<td width="84%">
</td></tr>
</table>

Set Bold
style

\fP&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </td>
<td width="82%">
</td></tr>
</table>

Revert to
previous style; only works one level deep, does not
stack.

Color
names are dependent on the pic implementation, but in all
modern versions color names recognized by the X&nbsp;window
system are supported.

A position
is an (x,y) coordinate pair. There are lots of different
ways to specify positions:

&lt;position&gt;
::= 
&lt;position-not-place&gt; 
&lt;place&gt; 
( &lt;position&gt; )

&lt;position-not-place&gt;
::= 
&lt;expr-pair&gt; 
&lt;position&gt; + &lt;expr-pair&gt; 
&lt;position&gt; - &lt;expr-pair&gt; 
( &lt;position&gt; , &lt;position&gt; ) 
&lt;expr&gt; [of the way] between &lt;position&gt; and
&lt;position&gt; 
&lt;expr&gt; &rsquo;&lt;&rsquo; &lt;position&gt; ,
&lt;position&gt; &rsquo;&gt;&rsquo;

&lt;expr-pair&gt;
::= 
&lt;expr&gt; , &lt;expr&gt; 
( expr-pair )

&lt;place&gt;
::= 
&lt;label&gt; 
&lt;label&gt; &lt;corner&gt; 
&lt;corner&gt; [of] &lt;label&gt; 
Here

&lt;label&gt;
::= 
LABEL [. LABEL ...] 
&lt;nth-primitive&gt;

&lt;xxx-of&gt;
::= 
xxx # followed by &lsquo;of&rsquo;

&lt;nth-primitive&gt;
::= 
&lt;ordinal&gt; &lt;object-type&gt; 
[&lt;ordinal&gt;] last &lt;object-type&gt;

&lt;ordinal&gt;
::= 
INT th 
INT st | INT nd | INT rd 
&lsquo; &lt;any-expr&gt; &rsquo;th

&lt;object-type&gt;
::= 
box 
circle 
ellipse 
arc 
line 
arrow 
spline 
&rsquo;[]&rsquo; 
TEXT

As
Kernighan notes, &ldquo;since barbarisms like 1th and
3th are barbaric, synonyms like 1st and
3rd are accepted as well.&rdquo; Objects of a given
type are numbered from 1 upwards in order of declaration;
the last modifier counts backwards.

The
&ldquo;&rsquo;th&rdquo; form (which allows you to select a
previous object with an expression, as opposed to a numeric
literal) is not documented in DWB&rsquo;s
pic(1).

The &lang;
xxx-of&rang; rule is special: The lexical parser
checks whether xxx is followed by the token
&lsquo;of&rsquo; without eliminating it so that the grammar
parser can still see &lsquo;of&rsquo;. Valid examples of
specifying a place with corner and label are thus

A
.n 
.n of A 
.n A 
north of A

while

north
A 
A north

both cause
a syntax error. (DWB pic also allows the weird form
&lsquo;A&nbsp;north&nbsp;of&rsquo;.)

Here the
special rules for the &lsquo;with&rsquo; keyword using a
path:

&lt;path&gt;
::= 
&lt;relative-path&gt; 
( &lt;relative-path&gt; , &lt;relative-path&gt; )

&lt;relative-path&gt;
::= 
&lt;corner&gt; 
. LABEL [. LABEL ...] [&lt;corner&gt;]

The
following style variables control output:

Any of these can be set by
assignment, or reset using the reset statement. Style
variables assigned within &lsquo;[&nbsp;]&rsquo; blocks are
restored to their beginning-of-block value on exit;
top-level assignments persist across pictures. Dimensions
are divided by scale on output.

All
pic expressions are evaluated in floating point;
units are always inches (a trailing &lsquo;i&rsquo; or
&lsquo;I&rsquo; is ignored). Expressions have the following
simple grammar, with semantics very similar to
C&nbsp;expressions:

&lt;expr&gt;
::= 
VARIABLE 
NUMBER 
&lt;place&gt; &lt;place-attribute&gt; 
&lt;expr&gt; &lt;op&gt; &lt;expr&gt; 
- &lt;expr&gt; 
( &lt;any-expr&gt; ) 
! &lt;expr&gt; 
&lt;func1&gt; ( &lt;any-expr&gt; ) 
&lt;func2&gt; ( &lt;any-expr&gt; , &lt;any-expr&gt; ) 
rand ( )

&lt;place-attribute&gt;
 
.x | .y | .h[eigh]t | .wid[th] | .rad

&lt;op&gt;
::= 
+ | - | * | / | % | ^ | &rsquo;&lt;&rsquo; |
&rsquo;&gt;&rsquo; | &rsquo;&lt;=&rsquo; |
&rsquo;&gt;=&rsquo;

&lt;func1&gt;
::= 
sin | cos | log | exp | sqrt | int | rand | srand

&lt;func2&gt;
::= 
atan2 | max | min

Both
exp and log are base 10; int does
integer truncation; and rand() returns a random
number in [0-1).

There are
define and undef statements which are not part
of the grammar (they behave as pre-processor macros to the
language). These may be used to define
pseudo-functions.

define
name { replacement-text
}

This
defines name as a macro to be replaced by the
replacement text (not including the braces). The macro may
be called as

name(arg1,
arg2, ..., argn)

The
arguments (if any) are substituted for tokens $1, $2 ... $n
appearing in the replacement text. To undefine a macro, say
undef name, specifying the name to be
undefined.
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