Defining Expression and Evaluation

Learning Goals

• Define "Expression"
• Identify the Parts of an Expression
• Identify Core Operators in Ruby
• Define "Evaluation"

Introduction

We've seen that we can have a conversation with Ruby by using the IRB program. Conversations, we've seen, are the things that result when two individuals — be they human or machine — communicate expressions to one another.

We've been imprecise in defining "expression" while we were getting the hang of it. Let's propose formal definitions for expression and evaluation.

Define Expression

An expression in a programming language is like a sentence in a spoken language.

Some sentences are simple: "He wept." Some sentences are complex: "I sing of weapons and a man, an outcast of Troy who was driven to the shores of Italy..."

Some expressions are simple: 2. Some expressions are complex 1 + 2. Some expressions are really complex: 10 + (3 * ( -1 ** 3) + 2) / 18.

Definition: Expression: A combination of information called data and symbols indicating how to combine data called operators.

Define "Evaluation"

Evaluation is the process of interpreting an expression, according to rules, to produce a return value.

Expression and Evaluation with Ruby

These definitions should align with your experience of having a conversation with IRB from the previous lesson. Think about 255 / 5. Which parts of the expression are data? Which parts are operators?

PRO-TIP: Think it through yourself. Which is a given thing (data) and which parts tell you how to combine things (operators)? When reading technical documents you can't simply read the answers, you have to think along not merely read. You're not reading a gossip site or a sports story, co-participation is needed to help your brain learn that this stuff is important!

Identify the Parts of an "Expression"

The data are: 255 and 5 The operator: is /

In this example, there is only one operator. It's certainly possible for expressions to have multiple operators like 100 + 10 - 3. In this example, the operators are + and -.

Identify Core Operators in Ruby

Here's a table of other operators and their operations. Fortunately, all these operators are either the same or nearly the same as their mathematical partner, so you probably know all the operators you're going to need to get started.

Operator	Operation	Note
+	Addition
-	Subtraction
*	Multiplication	We use * instead of × because it looks like x-the-letter
/	Division	We use / instead of ÷ because that's not on a keyboard
**	Exponentiation	We use ** instead of ^ because ^ means something else in programming languages
()	Association	Expressions inside of () get evaluated earlier

Conclusion

In the next few lessons, we're going to introduce the Essential Three Expressions:

1. The constant expression
2. The assignment expression (variable assignment)
3. The variable lookup expression

All expressions, which are the core of every programming language are built on these Essential Three Expressions.

Let's visit the mysterious, yet oddly dull first of the essential expressions: The Constant Expression.