In the previous chapters, we have shown how literal values within resources can be assembled to form static content for a 3-D world. In this chapter, we break the straight-jacket of static content, by showing how to use dynamically calculated expressions in the place of literal values.

Essentially, an expression is an organized sequence of operations which result in a value. Expressions make use of a broad range of capabilities: variables, assignment, methods, members, and operators. Each of these are explained in this chapter.

## Variables and Assignment

Let's start by talking about how to store and retrieve values using variables.

### Variable Names

A variable is a named identifier used to store and retrieve a single value of any type. In effect, a variable name is like a claim ticket you get when checking in your baggage; hand in the ticket (variable) and you get back your baggage (value). It is a way to have Acorn remember a value and then get that value back later when you need it.

A variable name starts with a letter, underscore or dollar sign. The rest of the variable name can include numbers as well as letters, underscores, and dollar signs. a21, first, Animal, _private_var, camelCase and $ are all valid variable names.

**Warning:** Acorn reserves several identifiers
(e.g., if or true) for specific language features.
These cannot be used as variable names.

### Assignment

The assignment operator '=' is used to store a value into a variable. The right side is an expression, which is evaluated to a value, then stored into the variable on the left:

pi = 3.15159 # stores the value of pi into a variable named pi x = pi # the value of a pi is retrieved and stored into the variable named x x = pi = 3.14159 # Equivalent to above, since assignment expressions return a value x = 1.618 # The value in a variable can be changed as often as desired # Create a indexed look-up for scientific constants constants = Index[ pi: pi # equivalent to the property 'pi': 3.14159 e: 2.71828

The last example shows that a variables can just as easily store a collection, such as an Index. So, you could create a 3-D snowman, and assign that part value to a variable named frosty. Later on, you might use the variable frosty to make changes to the 3-D snowman (maybe by telling him to walk across the street). All changes will be visible to anyone else looking at frosty's value.

### Multi-value assignments

As a convenient shortcut, the assignment operator supports the assignment of multiple, comma-separated values at one time. All values on the right are evaluated before storing them into the variables on the left.

a,b = b,a # swaps the values of a and b a,b = 3 # a is 3, b is null c = a,b # changes c. b is evaluated but not assigned to anything

### Global vs. Local variables

Acorn supports two very different types of variables:

- A
**global**variable begins with either a capital letter or a '$'. A global variable's value may be accessed or changed by any program. Type names are typically global, so they are capitalized (e.g., Integer). Environment values (such as the current world $) are global variables that begin with the '$'. Global variables persist forever. - A
**local**variable begins with a lower-case letter or a '_'. Local variables are private, only usable within the code they are part of (explained more fully in chapter describing Methods and Functions. Space for local variables are allocated and destroyed each time the method or function is called.

In general, it is better to use lower-case local variables which perform faster and make less messy code (unless, of course, global access to the value is needed). Using local variables reduces the risk of variable collision with other parts of the Acorn code (which might use the same variable name for a very different purpose).

## Methods

Methods are the workhorses of Acorn, the engine for getting stuff done. Pretty much everything that is done with a value is done using methods.

Every type has its own methods for manipulating values of that type. The Types appendix lists and describes all methods for each type.

A method is invoked by placing a period between the value and a method name.. Comma-separated parameter values, if any, follow the method name.

2.0.sqrt # returns 1.4142135, the square root of 2 "Expression".size # returns 10 List[12, 2].size # returns 2

In those examples, the .size method is applied to both a Text and List value. Although the concept is the same (to return how many members are in a collection), each type implements its method in its own way. Duplicate use of method names is common across types.

Methods can be chained together in arbitrarily long sequences, and are performed in left-to-right order. For example,

-4.0.abs.sqrt # returns 2.0

**Note** A method name may look like a variable,
but actually it is an implied symbol (due to the preceding period).
To use a non-alphabetic method, put single quotes around it.
To use a method name calculated using an expression,
enclose it in parentheses after the period:

# Another example of duplicate use of a method name 3.'+' 4 # The '+' method applied to integers adds them, resulting in 7 add = '+' # Put the method name (a symbol) into a variable "He".(add) "llo" # Applying the '+' method concatenates texts into "Hello"

Hopefully, you are about to complain about this cumbersome way of doing addition. Surely, Acorn allows you to say "3+4" without the dot and quotes? Surely?

Why yes ... yes it does. Phew!

## Arithmetic Operators

What a lovely way to transition into the rest of this chapter, where we discuss several common methods and a tidier notation for using them. We begin with arithmetic.

Acorn offers several arithmetic operators: add (+), subtract (-), multiply (*), divide (/), modulo (%) and power (**). The negative operator (-) is also supported.

In general, calculations are performed from left-to-right. However, power operations are calculated first, then multiply or divide, and then finally add and subtract. Parentheses can be used to override the order for performing the operations.

-2 + 3*4 # Returns 10 (spaces may be sprinkled in to improve readability) -(2+3)*4 # Returns -20 (shows use of parentheses) a = 2.6 + 3.*4.**2. # Stores 50.6 into a (assignment is done after calculations)

### Add and Subtract Assignment

As another nice shortcut, Acorn also offers two operators that are a nice hybrid arithmetic and assignment: += and -=. For numbers, these are a useful way to increase or decrease an existing value by a certain amount:

val = 7 val += 4 # val is now 11 val -= 1 # val is now 10

### Non-numeric Arithmetic

So, what happens if we try to use the arithmetic operators on values that are not numbers? Conceptually, they are meaningful when applied to quasi-numeric values, such as numeric matrices, Xyz coordinates, or Colors. Some of the operators are meaningful when applied to collections.

In practice, the result depends entirely on whether the value's type implements the operator's underlying method:

a = List[1; 2] b = List[1; 4] c = a+b # List[1;2;1;4] (+ is concatenation for collections) d = c-List[1] # List[2;4] (- removes matching members)

The add and subtract assignment operators also work on collections, making them a great way to add or subtract members from a collection. If desired, the right hand can be just a [] collection of members (the type does not need to be specified).

a += [1; 4] # Adds two members to a. a now looks like c (above). a -= [1] # Looks like d (above)

For an Index or Part, if the member being deleted is a symbol, it deletes the attribute whose key is that symbol.

frosty = Part[ age: 2 frosty -= 'age' # frosty's age is now unknown

If a type does not support the arithmetic operator, null is returned.

e = c**2.0 # returns null, since collections do not implement '**'

The last example shows that Acorn uses the type of the value to the left of the operator to determine which method to use. Order matters:

"-" * 5 # returns "-----", using the Text '*' method 5 * "-" # returns null, since the Integer '*' method cannot handle a Text value

### Type Conversion

If two values are of different types, will Acorn automatically convert the second value to match the first value's type? As a general rule, no, as many methods want parameter values of a different type. However, some methods will perform a limited form of type conversion. For example, the arithmetic operators will do this between integers and floats with the type of operator calculation determined by the value on the left:

4.5 * 2 # 9.0 (2 is converted to 2.0) # Use decimal points in floats to ensure the expected result... 2 * 4.5 # 8 (4.5 is converted to 4) 5 / 2 # Returns the Integer 2 2 + 4.1 # Returns 6. Since 2 is an integer, 4.1 is converted to the Integer 4 2.0 + 4**2.5 # Returns 18.0

If type conversion is explicitly desired, the standard way to request it (where supported by the originating type) is to use a method whose name is the same as the desired type, for example:

2.0.Integer # converts the Float number 2.0 to the Integer 2 2..sqrt.Text # "1.4142135"

### Uh-Oh! To Infinity and Beyond!

As already mentioned, integers and floats have built-in precision limits. Multiply two gigantic numbers and you will blow past those limits. Acorn won't warn you. Instead, it will calculate a really strange number that you would never expect.

So, be careful with your calculations and perhaps do appropriate tests ahead of time to check whether you are getting too close to those limits. If you are not sufficiently careful with your math, your 3-D scenes may exhibit some truly bizarre behaviors!

Floats do offer in interesting safeguard, should you decide to divide a number by zero or take the square-root of a negative number. The result is still a Float, but no longer looks like a number:

-1.0/0.0 # Division by zero, result is -Float::Infinity 0.0.sqrt # Imaginary number i, result is Float::NaN

Infinity and not-a-number can be used in further calculations, but the results are rarely useful for 3-D situations.

## Comparison and Boolean Operators

Acorn also provides operators for comparing values and for boolean logic. Let's delay talking about these until a later chapter on control flow decisions, where those operators are mostly commonly used.

## Collection Selector

Let's talk now about a very different type of operator, used to access members of a collection. When you put a selector value within parentheses right after a collection value, you can work with the selected member(s) of that collection. How this work depends on the type of the selector.

### Integer Selector

To retrieve a value from an ordered collection, such as a Text, List or Part, use an integer to represent its position in the collection (where 0 is the first member). Use negative numbers to count backwards from the end (with -1 being the last member:

mix = List[23; "Hello", true] mbr = mix(1) # "Hello", the second member of the list mbr(-1) # Returns the Text value of "o", the last character in "Hello" mbr(6) # Returns null, as there is no 7th character

Used on the left side of an assignment, integer indexing will alter that member of the collection:

item(-1) = "e" # item now has the value "Helle"

### Range Selector

Use of a Range value, instead of an integer, will access all the indexed members in that range:

item(0..-2) # Returns "Hell" item(1,-2)="op" # Stores "Hope" in item

### Symbol Selector

Using a symbol index accesses the value of a property within an Index or Part value:

tree = Part[ age: 4 species: 'oak' tree('age') # Returns the value 4

Acorn provides the member operator (double colon) as a shortcut whenever the symbol selector is an alphabetic literal:

tree::age # An alternative to above, also returning 4 tree::age = 5 # Resets the tree's age to 5 tree::x # returns null, as there is no 'x' property tree::name = "Big Brother" # Adds the new 'name' property to tree tree::species = 'oak' # changes the species tree::color # null, because the tree does not have a color property tree::age = null # A null value deletes a property. The age is now unknown.

The member operator is similarly useful for accessing type-specific constants (generally capitalized) or functions:

Float::Pi # returns the float value 3.14159 Float::rand # returns a random number between 0.0 and 1.0

### Pattern Selector

Using a pattern (such as a Regex or Selector) as an index retrieves the members that match the pattern. These are discussed later.

## Assembling Expressions

Early on, we said an expression is an organized sequence of operations which result in a value. Then we dug into the details of several techniques for building expressions: variables, assignment, methods, arithmetic operators and collection selectors. These are flexible building blocks, which can be assembled in so many myriad ways, both simple and complex, whatever is needed to get the solve the problem at hand. So many combinations are possible.

For example, an assignment is itself an expression that returns a value. Thus, it can be embedded into another expression.

area = 3.141592 * (r=4.1)*r # Sets the radius, then calculates the area of a circle

Even though we have not yet talked about collection iterators, The tools presented so far are very useful for managing collections:

prompts = List[] # Creates an empty list prompts.empty? # this list method returns true prompts += [ "Who are you?" "How old are you?" "Why are you here?" prompts(0) = "Who r u?" # Changes the first prompt prompts.empty? # returns false now prompts -= prompts(1) # Deletes the 2nd element: "How old are you?"

## Conclusion: Creating Dynamic Content

At the beginning of this chapter, we talked about moving beyond static content. Expressions allows us to create dynamic 3-D content whose properties and parts can be calculated based on other values and parts.

The original scene in the second chapter was built entirely with literal values. Any time we want, we can use expressions in place of literal values. Here are a few examples:

# A range from,sz = 4,3 from..(from+sz) # A range built from two variables, same as 4..7 # Properties prop,val = 'password','twink28' (prop): val # Same as: password: twink28 force: obj::mass*a # force property is a calculated value # Escape allows an expression inside a text or symbol # prompt becomes "I have a question: Why are you here?" prompt = "I have a question: \(prompts(1))" @(myhouse) # A resource whose Url is stored in a variable tree::pos += Xyz(10, 0, 0) # Moves the tree # Parts scene += [ Frosty # Adds the frosty part to the scene

The use of expressions starts the journey towards dynamic content. Now let's look at how to make parts smart.