92 PROGRAMMING CONCEPTS: THE FUNDAMENTALS

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You must also exercise care when using numeric types of different sizes. Data conversion statements are typically provided in strong data-typing programming languages to convert 8-, 16-, 32-, and (sometimes) 64-bit numbers from one form to another. If the programming language supports floating-point numbers (numbers that have digits to the right of the decimal point), then there will likely be data conversion statements for those as well.

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CREATING EXPRESSIONS

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An expression tells the program what you want it to do with information given to it. An expression consists of two parts:

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I One or more values I An operator that specifies what you want to do with these values

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In most programming languages, expressions can be used when you are defining the contents of variables, as in the following:

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Test1 = 1 + 1 Test2 = (15 * 2) + 1 Test3 = "This is" & "a test"

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The program processes the expression and places the result in the variable. Expressions can also be part of a more elaborate scheme that uses other program commands. Used in this way, expressions provide a way for your programs to think on their own (although they may seem to be more independent than you d like them to be!). The following sections present the commonly used operators and how they are used to construct expressions. Some operators work with numbers only, and some can also be used with strings. The list is divided into two parts: math operators (which apply to number values) and relational operators (which work with both numbers and in some programming languages strings as well). Depending on the language, you can use literal values or variables for v1 and v2.

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Math operators

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OPERATOR

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FUNCTION

value v1 v2

Treats the value as a negative number. Adds values v1 and v2 together. Subtracts value v2 from v1. Multiplies values v1 and v2. Divides value v1 by v2. Sometimes also expressed as v1 DIV v2. Divides value v1 by v2. The result is the floating-point remainder of the division. Sometimes also given as v1 MOD v2.

v1 - v2 v1 * v2 v1 / v2 v1 % v2

VARIABLES, EXPRESSIONS, AND OPERATORS 93

Relational operators

OPERATOR

FUNCTION

Not value v1 And v2 v1 Or v2 v1 v2

Evaluates the logical Not of value. The logical Not is the inverse of an expression: True becomes False, and vice versa. Evaluates the logical And of v1 and v2. Evaluates the logical Or of v1 and v2. Tests that v1 and v2 are equal. Tests that v1 and v2 are not equal. Tests that v1 is greater than v2. Tests that v1 is greater than or equal to v2. Tests that v1 is less than v2. Tests that v1 is less than or equal to v2.

v1 <> v2 v1 > v2 v1 > v1 v1 v2 v2 v2

Relational operators are also known as Boolean or True/False operators. Whatever they test, the answer is either yes (True) or no (False). For example, the expression 2 2 would be True, but the expression 2 3 would be False.

Using And and Or Relational Operators

The And and Or operators work with numbers (depending on the language) and expressions that result in a True/False condition. The And operator is used to determine if two values in an expression are both True (equal to something other than zero). If both A and B are True, then the result is True. But if A or B is False (equal to 0), then the result of the And is False. The Or operator works in a similar way, except that it tests if either of the values in an expression is True. If at least one of them is True, then the result is True. Only when both values are False is the result of the Or expression False. It s often helpful to view the action of the And and Or operators by using a truth table like the two that follow. The tables show all the possible outcomes given to values in an expression: