Expressions and Statements

We have used the terms expression, statement, and statement block in previous chapters without explicitly defining them. However, in order to understand what follows in this chapter, you will need to know exactly what these terms mean.

From a stylistic perspective, an expression is like a phrase; a statement is like a complete sentence; and a statement block is like a paragraph. For example, the following is an expression:

2+3

This code does not do anything other than calculates a result. The following is a statement:

x=2+3;

This code not only calculates a result, it also assigns the result to a variable. Just like a sentence represents a complete thought, a statement represents a complete action. Finally, the following code is a statement block:

{
x=2+3;
y=x;
}

From a syntax perspective, the difference between these three constructs is more basic. Quite simply, a statement is an expression that is followed by a semicolon, and a statement block is a list of statements enclosed in braces. Usually you create expressions, statements, and blocks that follow the phrase/sentence/paragraph parallel; this makes your code easy to read. However, you need to understand the definitions of expression and statement from a syntax perspective.

Expressions

A simple expression is nothing more than a function call. This can be two data items joined by an operator; it can be a call to a primitive; or it can be a call to a user-defined function. For example, each of the following is a simple expression:

2*3
x+y
5!
sqrt(4)
MyFunction(3,4)

A compound expression is just like a simple expression except that one or more of the data items is another expression. For example, 2*3 and 4*5 are both simple expressions, so 2*3+4*5 is a compound expression. There is no practical limit to how long a compound expression can be. Here are some examples:

2*3/(x+y)
sqrt(5!)
MyFunction(sqrt(5!),4)
makelist(makelist(f(x)+g(y),x,1,10),y,1,10)

Anywhere you can enter a simple expression, you can also enter a compound expression. Since there is no real need to draw a distinction between these types, we will refer to both simple and compound expressions as just expressions.

Statements

A statement is nothing more than an expression followed by semicolon (;). For example, x=2+3 is an expression and x=2+3; is a statement.

You probably remember a few paragraphs back we said that the code x=2+3 represents a complete action so it is a statement. To be more specific, we should say that because this code represents a complete action, you could choose to enter it as a statement. However, it is not a statement without the terminating semicolon. As an analogy, consider the following phrase:

John went to the store

If you add a terminating period (.), then this phrase becomes a sentence. Without the period, however, this is just a phrase and can be used as part of a longer sentence such as:

Mike and John went to the store.

To follow this analogy, x=2+3 is an expression, x=2+3; is a statement, and y=x=2+3 is a longer statement using the initial expression.

Statement Blocks

A statement block is two or more statements enclosed in braces { }. For example, the following is a statement block:

{
x=2+3;
y=x;
}

Note that a statement block does not end in a semicolon.

When a statement block is executed, DScript first creates a new block scope (to keep variables local), and then executes each statement in the order listed. Finally, the value of the last statement is returned.

Wherever you can enter a single statement you can instead enter a statement block.

Compound Statements

A compound statement is a sequence of reserved words that combine expressions and statements into a logical group that performs a specific task.

For example, the if compound statement has the format

if( expression )
  statement

Although there are numerous ways you can combine compound statements to create a complete application, there are only a small number of distinct statement types. The remainder of this chapter outlines the basic syntax for each type.