New user-defined variables and functions of one through twelve variables may be declared and used anywhere, including on the `plot` command itself.
User-defined function syntax:
<func-name>( <dummy1> {,<dummy2>} ... {,<dummy12>} ) = <expression>
where <expression> is defined in terms of <dummy1> through <dummy12>.
User-defined variable syntax:
<variable-name> = <constant-expression>
Examples:
w = 2
q = floor(tan(pi/2 - 0.1))
f(x) = sin(w*x)
sinc(x) = sin(pi*x)/(pi*x)
delta(t) = (t == 0)
ramp(t) = (t > 0) ? t : 0
min(a,b) = (a < b) ? a : b
comb(n,k) = n!/(k!*(n-k)!)
len3d(x,y,z) = sqrt(x*x+y*y+z*z)
plot f(x) = sin(x*a), a = 0.2, f(x), a = 0.4, f(x)
file = "mydata.inp"
file(n) = sprintf("run_%d.dat",n)
The final two examples illustrate a user-defined string variable and a user-defined string function.
Note that the variables `pi` (3.14159...) and `NaN` (IEEE "Not a Number") are already defined. You can redefine these to something else if you really need to. The original values can be recovered by setting:
NaN = GPVAL_NaN
pi = GPVAL_pi
Other variables may be defined under various gnuplot operations like mousing in interactive terminals or fitting; see variables for details.
You can check for existence of a given variable V by the exists("V") expression. For example
a = 10
if (exists("a")) print "a is defined"
if (!exists("b")) print "b is not defined"
Valid names are the same as in most programming languages: they must begin with a letter, but subsequent characters may be letters, digits, or "_".
Each function definition is made available as a special string-valued variable with the prefix 'GPFUN_'.
Example:
set label GPFUN_sinc at graph .05,.95