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<h2>13. Expressions, Variables, and Assignment
<a name="13. Expressions, Variables, and Assignment"></a>
</h2>

A number
is a valid expression, of course (all numbers are stored
internally as floating-point). Decimal-point notation is
acceptable; in GNU gpic, scientific notation in
C&rsquo;s &lsquo;e&rsquo; format (like 5e-2) is
accepted.

Anywhere a
number is expected, the language also accepts a variable.
Variables may be the built-in style variable described in
the last section, or new variables created by
assignment.

DWB
pic supports only the ordinary assignment via
=, which defines the variable (on the left side of
the equal sign) in the current block if it is not already
defined there, and then changes the value (on the right
side) in the current block. The variable is not visible
outside of the block. This is similar to the
C&nbsp;programming language where a variable within a block
shadows a variable with the same name outside of the
block.

GNU
gpic supports an alternate form of assignment using
:=. The variable must already be defined, and the
value is assigned to that variable without creating a
variable local to the current block. For example,
this

x=5
 
y=5 
[ 
x:=3 
y=3 
] 
print x &quot; &quot; y

prints
3 5.

You can
use the height, width, radius, and x and y coordinates of
any object or corner in expressions. If A is an
object label or name, all the following are
valid:

A.x
# x coordinate of the center of A 
A.ne.y # y coordinate of the northeast corner of A 
A.wid # the width of A 
A.ht # and its height 
2nd last circle.rad # the radius of the 2nd last
circle

Note the
second expression, showing how to extract a corner
coordinate.

Basic
arithmetic resembling those of C operators are available;
+, *, -, /, and %. So is
^ for exponentiation. Grouping is permitted in the
usual way using parentheses. GNU gpic allows logical
operators to appear in expressions; ! (logical
negation, not factorial), &amp;&amp;, ||,
==, !=, &gt;=, &lt;=,
&lt;, &gt;.

Various
built-in functions are supported:
sin(x), cos(x),
log(x), exp(x),
sqrt(x),
max(x,y),
atan2(x,y),
min(x,y),
int(x), rand(), and
srand(). Both exp and log are
base&nbsp;10; int does integer truncation;
rand() returns a random number in [0-1), and
srand() sets the seed for a new sequence of
pseudo-random numbers to be returned by rand()
(srand() is a GNU extension).

GNU
gpic also documents a one-argument form or rand,
rand(x), which returns a random number
between 1 and x, but this is deprecated and may be
removed in a future version.

The
function sprintf() behaves like a C sprintf(3)
function that only takes %, %e, %f, and %g format
strings.
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