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Expressions
This chapter describes:
- What Is an Expression?
- Precedence of Operations
- Expression Example
Notes and samples in this chapter are based Visual Basic 6.0.
What Is an Expression?
Giving a precise single definition of an expression is not an easy task. So I will try to define
it in a recursive way:
1. A simple expression is a presentation of a data value like, a literal, a variable, an element
of an array, or a function call.
2. A complex expression is an operation represented by an operator, and one or two expressions
as operands. The operation will result a data value.
If you apply rule #2 recursively, an expression may contain multiple operations in a sequence.
When this happens, operations must be carried out in an order defined by the following rules:
A. Operations enclosed in a pair of parentheses must be carried out before operations outside the parentheses.
B. Operations with higher precedence must be carried out before operations with lower precedence.
C. Operation on the left must be carried out before the operation on the right side.
D. Rule A must be applied before Rule B, which must be applied before Rule C.
Precedence of Operations
The following table shows you the relative precedence of some commonly used operations:
Precedence Operations Notes
1 ^ Exponentiation
2 * / \ Mod Multiplication, division, ...
3 + - Addition and subtraction
4 = <> < > <= >= Comparisons
5 And Or Xor Logical operations
There is nothing new in this table. We should have learned this in high school classrooms.
Expression Example
Now let me try to show you some expressions in an example, expression.html:
<html>
<body>
<!-- expression.html
Copyright (c) 2006 by Dr. Herong Yang. http://www.herongyang.com/
-->
<pre>
<script language="vbscript">
' Expressions with a single data literal
document.writeln("")
document.writeln(777)
document.writeln(0.00314159e30)
document.writeln(TRUE)
' Expressions with a single operation
document.writeln("")
document.writeln(3.333e200 + 0.111e200)
document.writeln(11.0 * 1.0e20)
document.writeln(1 = 1)
document.writeln(9.9999e-1 < 1)
document.writeln(True Or False)
' Expressions with multiple operations
document.writeln("")
document.writeln(+ 1 - 2 * 3 / 4)
document.writeln(+ 1 - 2 * 3 / 4 = ( + 1 - 2 ) * 3 / 4)
document.writeln(1 - 2 * 3 / 4 = (1 - 2) * 3 / 4 And 0.9999 < 1)
</script>
</pre>
</body>
</html>
Here is the output:
777
3.14159E+27
True
3.444E+200
1.1E+21
True
True
True
-0.5
False
False
All results look correct to me.
Conclusions
- Expression always results a single value.
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