Expressions

From Context Free Art

Expressions

Context Free allows expressions to be used anywhere that the compiler accepts a number, except for rule/path weights and loop counts.

Because the cfdg syntax does not separate size or skew parameters with commas, expressions like 2 - 3 are ambiguous. Does size 2 - 3 mean size -1 or does it mean size 2 -3? To resolve this issue, expressions must be put in parentheses. The only exception to this rule is that single invocations of functions do not require parentheses. So, r (360/5) and x (sin(45) + cos(60)) require parentheses, but size sqrt(1/2) does not.

The following components are allowed in an expression:

• + - : addition and subtraction
• * / : multiplication and division
• - : negation
• ^ : exponentiation
• (expression) : parentheses
• functions

Standard rules of operator ordering and precedence are supported; i.e., 2 + 3 * 4 is equivalent to 2 + (3 * 4). The list of supported functions are:

• cos(x) - cosine of x in degrees
• sin(x) - sine of x in degrees
• tan(x) - tangent of x in degrees
• acos(x) - arc-cosine of x, returned in degrees
• asin(x) - arc-sine of x, returned in degrees
• atan(x) - arc-tangent of x, returned in degrees
• atan2(y, x) - arc-tangent of y/x, returned in degrees
• log(x) - natural logarithm of x
• log10(x) - decimal logarithm of x
• exp(x) - e^x
• sqrt(x) - square root of x
• abs(x) - absolute value of x
• mod(x, y) - x modulo y
• rand_static() - returns a static random number in the interval [0,1)
• rand_static(x) - returns a static random number in the interval [0,x) (if x > 0) or [x,0) (if x < 0)
• rand_static(x, y) - returns a static random number in the interval [x,y) (if y > x) or [y,x) (if x > y)

The rand_static() functions are converted into a random number when the cfdg file is compiled. So the value is constant for the entire run, but it is different for each variation. A rand_static() function inside of a loop has the same value for each iteration of the loop. This is not as useful as the full dynamic random function that everyone fervently desires. But it is the best that can be done with the Context Free 2.x execution model and it is moderately useful.

Let us know if there is a function that you would like to see added to Context Free.

startshape foo
 
path trill {
    MOVETO {x cos(234) y sin(234)}		// no parentheses required
    5* {r -144} 
        CURVETO {y 1 x1 (cos(234) + cos(324)) y1 (sin(234) + sin(324)) x2 1 y2 1}
        // parentheses required because of addition
    CLOSEPOLY {p align}
    FILL {p evenodd}
    STROKE {p buttjoin a -0.5}
}
 
rule foo {
    trill{}
}