Operators

Pre-defined

First, note that pretty much any valid Julia function which takes one or two scalars as input, and returns on scalar as output, is likely to be a valid operator[^1]. A selection of these and other valid operators are stated below.

Binary

+
-
*
/
^
max
min
mod
cond
- Equal to (x, y) -> x > 0 ? y : 0
greater
- Equal to (x, y) -> x > y ? 1 : 0
logical_or
- Equal to (x, y) -> (x > 0 || y > 0) ? 1 : 0
logical_and
- Equal to (x, y) -> (x > 0 && y > 0) ? 1 : 0

Unary

neg
square
cube
exp
abs
log
log10
log2
log1p
sqrt
sin
cos
tan
sinh
cosh
tanh
atan
asinh
acosh
atanh_clip
- Equal to atanh(mod(x + 1, 2) - 1)
erf
erfc
gamma
relu
round
floor
ceil
round
sign

Custom

Instead of passing a predefined operator as a string, you can just define a custom function as Julia code. For example:

    PySRRegressor(
        ...,
        unary_operators=["myfunction(x) = x^2"],
        binary_operators=["myotherfunction(x, y) = x^2*y"],
        extra_sympy_mappings={
            "myfunction": lambda x: x**2,
            "myotherfunction": lambda x, y: x**2 * y,
        },
    )

Make sure that it works with Float32 as a datatype (for default precision, or Float64 if you set precision=64). That means you need to write 1.5f3 instead of 1.5e3, if you write any constant numbers, or simply convert a result to Float64(...).

PySR expects that operators not throw an error for any input value over the entire real line from -3.4e38 to +3.4e38. Thus, for invalid inputs, such as negative numbers to a sqrt function, you may simply return a NaN of the same type as the input. For example,

my_sqrt(x) = x >= 0 ? sqrt(x) : convert(typeof(x), NaN)

would be a valid operator. The genetic algorithm will preferentially selection expressions which avoid any invalid values over the training dataset.

[^1]: However, you will need to define a sympy equivalent in extra_sympy_mapping if you want to use a function not in the above list.