10. Expressions
In general, any mathematical expression accepted by C, FORTRAN, Pascal, or
BASIC is valid. The precedence of these operators is determined by the
specifications of the C programming language. White space (spaces and tabs)
is ignored inside expressions.
Complex constants are expressed as {<real>,<imag>}, where <real> and <imag>
must be numerical constants. For example, {3,2} represents 3 + 2i; {0,1}
represents 'i' itself. The curly braces are explicitly required here.
Note that gnuplot uses both "real" and "integer" arithmetic, like FORTRAN and
C. Integers are entered as "1", "-10", etc; reals as "1.0", "-10.0", "1e1",
3.5e-1, etc. The most important difference between the two forms is in
division: division of integers truncates: 5/2 = 2; division of reals does
not: 5.0/2.0 = 2.5. In mixed expressions, integers are "promoted" to reals
before evaluation: 5/2e0 = 2.5. The result of division of a negative integer
by a positive one may vary among compilers. Try a test like "print -5/2" to
determine if your system chooses -2 or -3 as the answer.
The integer expression "1/0" may be used to generate an "undefined" flag,
which causes a point to ignored; the ternary operator gives an example.
The real and imaginary parts of complex expressions are always real, whatever
the form in which they are entered: in {3,2} the "3" and "2" are reals, not
integers.
10.1 Functions
The functions in gnuplot are the same as the corresponding functions in
the Unix math library, except that all functions accept integer, real, and
complex arguments, unless otherwise noted.
For those functions that accept or return angles that may be given in either
degrees or radians (sin(x), cos(x), tan(x), asin(x), acos(x), atan(x),
atan2(x) and arg(z)), the unit may be selected by set angles, which
defaults to radians.
10.1.1 abs
The abs function returns the absolute value of its argument. The returned
value is of the same type as the argument.
For complex arguments, abs(x) is defined as the length of x in the complex
plane [i.e., sqrt(real(x)**2 + imag(x)**2) ].
10.1.2 acos
The acos function returns the arc cosine (inverse cosine) of its argument.
acos returns its argument in radians or degrees, as selected by `set
angles`.
10.1.3 acosh
The acosh function returns the inverse hyperbolic cosine of its argument in
radians.
10.1.4 arg
The arg function returns the phase of a complex number in radians or
degrees, as selected by set angles.
10.1.5 asin
The asin function returns the arc sin (inverse sin) of its argument.
asin returns its argument in radians or degrees, as selected by `set
angles`.
10.1.6 asinh
The asinh function returns the inverse hyperbolic sin of its argument in
radians.
10.1.7 atan
The atan function returns the arc tangent (inverse tangent) of its
argument. atan returns its argument in radians or degrees, as selected by
set angles.
10.1.8 atan2
The atan2 function returns the arc tangent (inverse tangent) of the ratio
of the real parts of its arguments. atan2 returns its argument in radians
or degrees, as selected by set angles, in the correct quadrant.
10.1.9 atanh
The atanh function returns the inverse hyperbolic tangent of its argument
in radians.
10.1.10 besj0
The besj0 function returns the j0th Bessel function of its argument.
besj0 expects its argument to be in radians.
10.1.11 besj1
The besj1 function returns the j1st Bessel function of its argument.
besj1 expects its argument to be in radians.
10.1.12 besy0
The besy0 function returns the y0th Bessel function of its argument.
besy0 expects its argument to be in radians.
10.1.13 besy1
The besy1 function returns the y1st Bessel function of its argument.
besy1 expects its argument to be in radians.
10.1.14 ceil
The ceil function returns the smallest integer that is not less than its
argument. For complex numbers, ceil returns the smallest integer not less
than the real part of its argument.
10.1.15 cos
The cos function returns the cosine of its argument. cos accepts its
argument in radians or degrees, as selected by set angles.
10.1.16 cosh
The cosh function returns the hyperbolic cosine of its argument. cosh
expects its argument to be in radians.
10.1.17 erf
The erf function returns the error function of the real part of its
argument. If the argument is a complex value, the imaginary component is
ignored.
10.1.18 erfc
The erfc function returns 1.0 - the error function of the real part of its
argument. If the argument is a complex value, the imaginary component is
ignored.
10.1.19 exp
The exp function returns the exponential function of its argument (e
raised to the power of its argument). On some implementations (notably
suns), exp(-x) returns undefined for very large x. A user-defined function
like safe(x) = x<-100 ? 0 : exp(x) might prove useful in these cases.
10.1.20 floor
The floor function returns the largest integer not greater than its
argument. For complex numbers, floor returns the largest integer not
greater than the real part of its argument.
10.1.21 gamma
The gamma function returns the gamma function of the real part of its
argument. For integer n, gamma(n+1) = n!. If the argument is a complex
value, the imaginary component is ignored.
10.1.22 ibeta
The ibeta function returns the incomplete beta function of the real parts
of its arguments. p, q > 0 and x in [0:1]. If the arguments are complex,
the imaginary components are ignored.
10.1.23 inverf
The inverf function returns the inverse error function of the real part
of its argument.
10.1.24 igamma
The igamma function returns the incomplete gamma function of the real
parts of its arguments. a > 0 and x >= 0. If the arguments are complex,
the imaginary components are ignored.
10.1.25 imag
The imag function returns the imaginary part of its argument as a real
number.
10.1.26 invnorm
The invnorm function returns the inverse normal distribution function of
the real part of its argument.
10.1.27 int
The int function returns the integer part of its argument, truncated
toward zero.
10.1.28 lgamma
The lgamma function returns the natural logarithm of the gamma function
of the real part of its argument. If the argument is a complex value, the
imaginary component is ignored.
10.1.29 log
The log function returns the natural logarithm (base e) of its argument.
10.1.30 log10
The log10 function returns the logarithm (base 10) of its argument.
10.1.31 norm
The norm function returns the normal distribution function (or Gaussian)
of the real part of its argument.
10.1.32 rand
The rand function returns a pseudo random number in the interval [0:1]
using the real part of its argument as a seed. If seed < 0, the sequence
is (re)initialized. If the argument is a complex value, the imaginary
component is ignored.
10.1.33 real
The real function returns the real part of its argument.
10.1.34 sgn
The sgn function returns 1 if its argument is positive, -1 if its argument
is negative, and 0 if its argument is 0. If the argument is a complex value,
the imaginary component is ignored.
10.1.35 sin
The sin function returns the sine of its argument. sin expects its
argument to be in radians or degrees, as selected by set angles.
10.1.36 sinh
The sinh function returns the hyperbolic sine of its argument. sinh
expects its argument to be in radians.
10.1.37 sqrt
The sqrt function returns the square root of its argument.
10.1.38 tan
The tan function returns the tangent of its argument. tan expects
its argument to be in radians or degrees, as selected by set angles.
10.1.39 tanh
The tanh function returns the hyperbolic tangent of its argument. tanh
expects its argument to be in radians.
A few additional functions are also available.
10.1.40 column
column(x) may be used only in expressions as part of using manipulations
to fits or datafile plots. See plot datafile using.
10.1.41 tm_hour
The tm_hour function interprets its argument as a time, in seconds from
1 Jan 2000. It returns the hour (an integer in the range 0--23) as a real.
10.1.42 tm_mday
The tm_mday function interprets its argument as a time, in seconds from
1 Jan 2000. It returns the day of the month (an integer in the range 1--31)
as a real.
10.1.43 tm_min
The tm_min function interprets its argument as a time, in seconds from
1 Jan 2000. It returns the minute (an integer in the range 0--59) as a real.
10.1.44 tm_mon
The tm_mon function interprets its argument as a time, in seconds from
1 Jan 2000. It returns the month (an integer in the range 1--12) as a real.
10.1.45 tm_sec
The tm_sec function interprets its argument as a time, in seconds from
1 Jan 2000. It returns the second (an integer in the range 0--59) as a real.
10.1.46 tm_wday
The tm_wday function interprets its argument as a time, in seconds from
1 Jan 2000. It returns the day of the week (an integer in the range 1--7) as
a real.
10.1.47 tm_yday
The tm_yday function interprets its argument as a time, in seconds from
1 Jan 2000. It returns the day of the year (an integer in the range 1--366)
as a real.
10.1.48 tm_year
The tm_year function interprets its argument as a time, in seconds from
1 Jan 2000. It returns the year (an integer) as a real.
10.1.49 valid
valid(x) may be used only in expressions as part of using manipulations
to fits or datafile plots. See plot datafile using.
10.2 Operators
The operators in gnuplot are the same as the corresponding operators in the
C programming language, except that all operators accept integer, real, and
complex arguments, unless otherwise noted. The ** operator (exponentiation)
is supported, as in FORTRAN.
Parentheses may be used to change order of evaluation.
10.2.1 Unary
The following is a list of all the unary operators and their usages:
| | Symbol Example Explanation
- -a unary minus
+ +a unary plus (no-operation)
~ ~a * one's complement
! !a * logical negation
! a! * factorial
$ $3 * call arg/column during using manipulation
|
(*) Starred explanations indicate that the operator requires an integer
argument.
Operator precedence is the same as in Fortran and C. As in those languages,
parentheses may be used to change the order of operation. Thus -2**2 = -4,
but (-2)**2 = 4.
The factorial operator returns a real number to allow a greater range.
10.2.2 Binary
The following is a list of all the binary operators and their usages:
| | Symbol Example Explanation
** a**b exponentiation
* a*b multiplication
/ a/b division
% a%b * modulo
+ a+b addition
- a-b subtraction
== a==b equality
!= a!=b inequality
< a<b less than
<= a<=b less than or equal to
> a>b greater than
>= a>=b greater than or equal to
& a&b * bitwise AND
^ a^b * bitwise exclusive OR
| a|b * bitwise inclusive OR
&& a&&b * logical AND
|| a||b * logical OR
|
(*) Starred explanations indicate that the operator requires integer
arguments.
Logical AND (&&) and OR (||) short-circuit the way they do in C. That is,
the second && operand is not evaluated if the first is false; the second
|| operand is not evaluated if the first is true.
10.2.3 Ternary
There is a single ternary operator:
| | Symbol Example Explanation
?: a?b:c ternary operation
|
The ternary operator behaves as it does in C. The first argument (a), which
must be an integer, is evaluated. If it is true (non-zero), the second
argument (b) is evaluated and returned; otherwise the third argument (c) is
evaluated and returned.
The ternary operator is very useful both in constructing piecewise functions
and in plotting points only when certain conditions are met.
Examples:
Plot a function that is to equal sin(x) for 0 <= x < 1, 1/x for 1 <= x < 2,
and undefined elsewhere:
| | f(x) = 0<=x && x<1 ? sin(x) : 1<=x && x<2 ? 1/x : 1/0
plot f(x)
|
Note that gnuplot quietly ignores undefined values, so the final branch of
the function (1/0) will produce no plottable points. Note also that f(x)
will be plotted as a continuous function across the discontinuity if a line
style is used. To plot it discontinuously, create separate functions for the
two pieces. (Parametric functions are also useful for this purpose.)
For data in a file, plot the average of the data in columns 2 and 3 against
the datum in column 1, but only if the datum in column 4 is non-negative:
| | plot 'file' using 1:( $4<0 ? 1/0 : ($2+$3)/2 )
|
Please see plot data-file using for an explanation of the using syntax.
10.3 User-defined
New user-defined variables and functions of one through five variables may
be declared and used anywhere, including on the plot command itself.
User-defined function syntax:
| | <func-name>( <dummy1> {,<dummy2>} ... {,<dummy5>} ) = <expression>
|
where <expression> is defined in terms of <dummy1> through <dummy5>.
User-defined variable syntax:
| | <variable-name> = <constant-expression>
|
Examples:
| | w = 2
q = floor(tan(pi/2 - 0.1))
f(x) = sin(w*x)
sinc(x) = sin(pi*x)/(pi*x)
delta(t) = (t == 0)
ramp(t) = (t > 0) ? t : 0
min(a,b) = (a < b) ? a : b
comb(n,k) = n!/(k!*(n-k)!)
len3d(x,y,z) = sqrt(x*x+y*y+z*z)
plot f(x) = sin(x*a), a = 0.2, f(x), a = 0.4, f(x)
|
Note that the variable pi is already defined. But it is in no way magic;
you may redefine it to be whatever you like.
Valid names are the same as in most programming languages: they must begin
with a letter, but subsequent characters may be letters, digits, "$", or "_".
Note, however, that the fit mechanism uses several variables with names
that begin "FIT_". It is safest to avoid using such names. "FIT_LIMIT",
however, is one that you may wish to redefine. See the documentation
on fit for details.
See show functions, show variables, and fit.
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