MathAbs(f1 - 30 * f2) < 00001; // Use whatever level of accuracy in Visual C#.NET

Print Code-39 in Visual C#.NET MathAbs(f1 - 30 * f2) < 00001; // Use whatever level of accuracy

This calculation returns true for both cases You can use the constant DoubleEpsilon instead of 00001 to produce the maximum level of accuracy Epsilon is the smallest possible difference between two nonequal double variables

For a self-guided tour of the SystemMath class, where Abs and many other useful mathematical functions live, look for math in Help

The bool variables have another set of operators defined just for them, as shown in Table 4-3

Table 4-3

Operator Is True If a is false (also known as the not operator) a and b are true (also known as the and operator) Either a or b or else both are true (also known as a and/or b) a is true or b is true but not both (also known as a xor b) a is true and b is true with short-circuit evaluation a is true or b is true with short-circuit evaluation (I discuss short-circuit evaluation in this section)

The ! operator (NOT) is the logical equivalent of the minus sign For example, !a (read not a ) is true if a is false and false if a is true Can that be true The next two operators in the table are straightforward First, a & b is true only if both a and b are true And a | b is true if either a or b is true (or both are) The exclusive or (xor) operator, or ^, is sort of an odd beast An exclusive or is true if either a or b is true but not if both a and b are true All three operators produce a logical bool value as their result The &, |, and ^ operators also have a bitwise operator version When applied to int variables, these operators perform their magic on a bit-by-bit basis Thus 6 & 3 is 2 (01102 & 00112 is 00102), 6 | 3 is 7 (01102 | 00112 is 01112), and 6 ^ 3 is 5 (01102 ^ 00112 is 01012) Binary arithmetic is cool but beyond the scope of this book You can search for it at your favorite search engine The remaining two logical operators are similar to, but subtly different from, the first three Consider the following example:

bool b = (boolExpression1) & (boolExpression2);

In this case, C# evaluates boolExpression1 and boolExpression2 It then looks to see whether they both are true before deciding the value of b However, this may be a wasted effort If one expression is false, there s no reason to perform the other Regardless of the value of the second expression, the result will be false Nevertheless, & goes on to evaluate both expressions The && operator avoids evaluating both expressions unnecessarily, as shown in the following example:

bool b = (boolExpression1) && (boolExpression2);

In this case, C# evaluates boolExpression1 If it s false, then b is set to false and the program continues on its merry way On the other hand, if boolExpression1 is true, then C# evaluates boolExpression2 and stores the result in b The && operator uses this short-circuit evaluation because it short-circuits around the second Boolean expression, if necessary Most programmers use the doubled forms most of the time The || operator works the same way, as shown in the following expression:

bool b = (boolExpression1) || (boolExpression2);

If boolExpression1 is true, there s no point in evaluating boolExpression2 because the result is always true You can read these operators as short-circuit and and short-circuit or

In calculations, an expression s type is just as important as its value Consider the following expression:

int n; n = (5 * 5) + 7;

My calculator says the resulting value of n is 32 However, that expression also has an overall type based on the types of its parts Written in type language, the preceding expression becomes

int [=] (int * int) + int;

To evaluate the type of an expression, follow the same pattern you use to evaluate the expression s value Multiplication takes precedence over addi-