# Expression select

This modifier allows you to select particles based on a user-defined criterion. You enter an Boolean expression, which is evaluated by the modifier for every particle in the system. Those particles for which the expression yields a non-zero value ("true") will be selected; all other particles, for which the expression evaluates to zero ("false"), are deselected.

The Boolean expression can contain references to particle properties and global quantities like the simulation cell size or animation timestep. Hence, the modifier can be used to select particles based on properties such as position, type, energy etc., and any combination thereof. The list of available input variables is displayed in the lower panel as shown in the screenshot.

Boolean expressions can contain comparison operators like `==`, `!=`, `>=`, etc., and several conditions can be combined using logical AND and OR operators (`&&` and `||`).

Note that variable names and function names are case-sensitive.

## Expression syntax

The expression syntax supported by the modifier is very similar to the one used by the C programming language. Arithmetic expressions can be created from float literals, variables or functions using the following operators in this order of precedence:

Operator | Description
--- | ---
(...) | expressions in parentheses are evaluated first
A^B | exponentiation (A raised to the power B)
A*B, A/B | multiplication and division
A+B, A-B | addition and subtraction
A==B, A!=B, A<B, A<=B, A>B, A>=B | comparison between A and B (result is either 0 or 1)
A && B | logical AND operator: result is 1 if A and B differ from 0, else 0
A || B | logical OR operator: result is 1 if A or B differ from 0, else 0
A ? B : C | If A differs from 0 (i.e. is true), the resulting value of this expression is B, else C.

The expression parser supports the following functions:

Function name | Description
--- | ---
abs(A) | Absolute value of A. If A is negative, returns -A otherwise returns A.
acos(A) | Arc-cosine of A. Returns the angle, measured in radians, whose cosine is A.
acosh(A) | Same as acos() but for hyperbolic cosine.
asin(A) | Arc-sine of A. Returns the angle, measured in radians, whose sine is A.
asinh(A) | Same as asin() but for hyperbolic sine.
atan(A) | Arc-tangent of A. Returns the angle, measured in radians, whose tangent is A.
atan2(Y,X) | Two argument variant of the arctangent function. Returns the angle, measured in radians. This function is documented here.
atanh(A) | Same as atan() but for hyperbolic tangent.
avg(A,B,...) | Returns the average of all arguments.
cos(A) | Cosine of A. Returns the cosine of the angle A, where A is measured in radians.
cosh(A) | Same as cos() but for hyperbolic cosine.
exp(A) | Exponential of A. Returns the value of e raised to the power A where e is the base of the natural logarithm, i.e. the non-repeating value approximately equal to 2.71828182846.
fmod(A,B) | Returns the floating-point remainder of A/B (rounded towards zero).
rint(A) | Rounds A to the closest integer. 0.5 is rounded to 1.
ln(A) | Natural (base e) logarithm of A.
log10(A) | Base 10 logarithm of A.
log2(A) | Base 2 logarithm of A.
max(A,B,...) | Returns the maximum of all values.
min(A,B,...) | Returns the minimum of all values.
sign(A) | Returns: 1 if A is positive; -1 if A is negative; 0 if A is zero.
sin(A) | Sine of A. Returns the sine of the angle A, where A is measured in radians.
sinh(A) | Same as sin() but for hyperbolic sine.
sqrt(A) | Square root of a value.
sum(A,B,...) | Returns the sum of all parameter values.
tan(A) | Tangent of A. Returns the tangent of the angle A, where A is measured in radians.

## Examples

Here are two examples for Boolean expressions:

    ParticleType==1 || ParticleType==2

    sqrt(Position.X*Position.X + Position.Y*Position.Y) < 10