# Expression

An **Expression** is a representation of a value that is resolved while the deck is playing. Some common uses for expressions include: Reading the value of variables, performing simple math, or creating randomness. Expressions can also be nested and combined to support more complex things, such as calculating health for characters, crafting procedural games and animations, and even making generative artwork.

Expressions always resolve to a numeric value.

Some expressions take arguments. For example, when reading the value of a Variable, you need to specify which variable to read. When adding two numbers, you need to specify which two numbers (or expressions) to add. Other expressions don't take any arguments, such as the **Time elapsed since the card started**.

## Using Expressions

Expressions are an important component of Rules in Castle. Typically, when editing a rule, any numeric value can be an expression. To edit an expression, look for the round beaker icon to the left of a number input. Press it to open a sheet to edit that expression.

## List of Expression Types

Some expressions are contextual to the enclosing Response. For example, the "Number of repeats finished" expression only appears if already inside a Repeat response.

### Values

- A constant number. This is the same as not using an expression.
- The value of a variable
- The value of a Behavior property

### Choices

- Weighted choose
- Choose
- Maximum
- Minimum

### Randomness

- A random number with a Gaussian distribution
- Perlin noise in 2 dimensions
- A random number in a range

### Spatial relationships

- The Y coordinate of the user's touch
- The X coordinate of the user's touch
- The speed of an actor
- The angle of motion of an actor
- The angle from one actor to another (degrees)
- The distance between two actors

### Arithmetic

- The number of Repeats finished
- Logarithm
- Power
- Modulo
- Divide
- Multiply
- Subtract
- Add

### Clock

- Time elapsed since the card started
- The clock tempo
- Time since the last beat
- The current bar
- The current beat in the current bar
- Beats elapsed since the card started

### Music

- The note played (only appears under the "track plays note" trigger)