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====== Getting Stuff Done I ======

//Operations and expressions//

In this chapter, we will take a closer look at what makes up the commands you can use in Processing.

Processing is what computer science people call an //imperative// language. The essence of an imperative language is that programs written in it are a sequence of commands that the computer performs. Furthermore, those commands may (and typically do) change the //state// of the program. Here is an example of Processing code that commands three state changes (all on the variable ''%%foo%%''):

<code java>
int foo = 55;
foo = 99;
foo = 42;
</code>
In Processing (as in many other imperative languages) you can think of commands as consisting of a hierarchy of concepts (from lowest-level to highest level): operations, expressions, and statements.

===== Statements =====

In a previous chapter, we said that statements are the smallest complete executable element in an imperative language. In Processing, a semicolon is used to mark the end of a statement, as in the examples below:

<code java>
int theAnswer = 42;
println(theAnswer);
</code>
Statements are often built up from expressions, which are themselves built up from operations. We will talk about operations first and then discuss expressions.

===== Operations =====

You are probably familiar with operations in mathematics. **Operations** are made up of **operators** and **operands**. Operators indicate what you want to do and operands are the things you want that operation done to. In the following operation:

$12 + 30$

the operator, represented by the plus symbol, indicates what you want to do with the operands ''%%12%%'' and ''%%30%%''. The concept of an operation in imperative languages is taken directly from the concept in mathematics--although the notation and details can and do differ.

==== Arithmetic operators ====

Processing defines a number of arithmetic operators that work essentially the same as in pure math. These are summarized in the table below.

^Operator   ^Description   ^Example      ^
|''%%+%%''  |addition      |''%%a + b%%''|
|''%%-%%''  |subtraction   |''%%b - d%%''|
|''%%*%%''  |multiplication|''%%c * d%%''|
|''%%/%%''  |division      |''%%d / c%%''|
|''%%%%%''  |modulus       |''%%d % c%%''|

Processing uses //infix// notation for arithmetic operations, meaning that the operator is placed between the two operands.((Alternatives to //infix// used by some languages are //postfix// (operator after operands) or //prefix// (operator before operands) notation.
))

=== Addition ===

The ''%%+%%'' operator is used for addition. The two operands must be (or evaluate to) numeric values.

<code java>
foo + 5
foo + bar
</code>
=== Subtraction ===

TODO

=== Multiplication ===

TODO

=== Integer versus floating-point division. ===

TODO

=== modulo ===

TODO

==== Assignment ====

Assignment is an operation on two operands that sets the value of one of the operands (typically a variable) to the value of the other operand. In Processing, the equals symbol is used to indicate assignment. In the example below, the value 8 is assigned to ''%%foo%%'', then twice the value of ''%%baz%%'' is assigned to ''%%bar%%''.

<code java>
foo = 8;
bar = 2 * baz;
</code>
The assignment operation in Processing looks like the "equals" relationship in math, and this is can be a source of confusion to new programmers. //They are not the same thing.// "Equals" is a statement of fact: two things are the same. Assignment is an operation: it does something--specifically, it copies the value of whatever is on the right into what is on the left.

Assignment is one way that the state of a program can be changed.

==== Mixing numeric types ====

==== Logical operators ====

TODO

==== The number of operands ====

Operators can be categorized based on the number of operands they require.

**Unary** operators take //one// operand. An example of a unary operator is the logical NOT: ''%%!foo%%''.

**Binary** operators take //two// operands. The arithmetic operators above are all examples of binary operators as are the logical operators, with the exception of the logical NOT.

**Ternary** operators take //three// operands. Processing's conditional operator (not discussed here) is an example of a ternary operator.

Theoretically, it's possible for a language to define an operator with an arbitrary number of operands, but I am not aware of any operators that take more than three operands in Processing.

===== Expressions =====

Expressions are made of up one or more operations and return a value.

TODO

==== Order of precedence ====

TODO

==== Whitespace ====

TODO

(Spaces, tabs, and the invisible characters that mark the end of a line. In short, whitespace in Processing (and Java, C++, and C) is interchangeable. In other words, from Processing’s point of view, a space is the same thing as a tab is the same thing as the end of a line.)