Using Mathematical Expressions

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At times, you’ll want to perform some kind of mathematical operation in your scripts and assign the results to a variable. As with most programming languages, the command shell allows you to write mathematical expressions using a variety of operators, including

• Arithmetic operators to perform standard mathematical operations (such as addition, subtraction, multiplication, and division)

• Assignment operators that combine an assignment operation (symbolized by the equal sign) with an arithmetic operation

• Comparison operators that compare values and are usually used with if statements

• Bitwise operators that allow you to manipulate the sequences of binary values

Math operations are performed using set with the /A (arithmetic) parameter, such as

set /a theTotal=18+2
set /a theTotal=18*2
set /a theTotal=18/2

All mathematical expressions are evaluated using 32-bit signed integer arithmetic. This allows for values –2³² to 2^32–1. If you exceed this range, you’ll get an arithmetic error (code –2) instead of the intended value.

The most commonly used operators are those for arithmetic, assignment, and comparison. Arithmetic and assignment operators are discussed in the sections that follow. Comparison operators are discussed in the section titled “Making Comparisons in If Statements,” later in this chapter. Pay particular attention to the additional discussions on operator precedence and simulating exponents in scripts.

Working with Arithmetic and Assignment Operators

You use arithmetic operators to perform basic math operations on numerical values. These values can be expressed literally as a number, such as 5, or as a variable that contains the value you want to work with, such as %TOTAL%.

Table 3-2 summarizes the available arithmetic and assignment operators. Most of the arithmetic operators are fairly straightforward. You use * in multiplication, / in division, + in addition, and – in subtraction. You use the equal sign (=) to assign values to variables. You use % (modulus) to obtain the remainder from division. For example, if you divide 8 into 60, the answer is 7 Remainder 4; the value 4 is what the result would be if you use the modulus operator.

Table 3-2: Arithmetic and Assignment Operators

Arithmetic Operators	Assignment Operators
+ (Addition)	+= (Increment, that is, add and assign)
- (Subtraction)	-= (Decrement, that is, subtract and assign)
* (Multiplication)	*= (Scale up, that is, multiply and assign)
/ (Division)	/= (Scale down, that is, divide and assign)
% (Modulus)	%= (Modulus and assign)

Examples of working with arithmetic operators follow:

set /a theCount=5+3

set /a theCount=%nServers% + %nWstations%

set /a theCount=%nServers% - 1

Tip

Earlier, I stated that everything stored in a variable is a string, and that remains true. However, the command shell can detect when a string contains only numerals, and this is what allows you to use variables in expressions. The key detail to remember is to use the proper syntax for substitution, which is %variableName%.

You use assignment operators to increment, decrement, scale up, or scale down. These operators combine arithmetic and assignment operation functions. For example, the += operator is used to increment a value and combines the effects of the + operator and the = operator. Thus, the following two expressions are equivalent and yield the identical results when entered at the command line:

set /a total=total+1
set /a total+=1

Understanding Operator Precedence

One thing you should understand when working with mathematic operations is operator precedence. Operator precedence determines what happens when the command shell must evaluate an expression that involves more than one operator. For example:

set /a total=8+3*4 

If evaluated from left to right, this expression equals 44 (8+3=11, 11*4=44). But as in standard mathematics, that’s not how the command line evaluates the expression. Instead, the command shell evaluates the expression as 20 (3*4=12, 8+12=20) because the precedence of operations is the following:

1. Modulus

2. Multiplication and division

3. Addition and subtraction

Note

When an expression contains multiple operations at the same precedence level, these operations are performed from left to right. Hence set /a total=10-4+2 equals 8 (10-4=6, 6+2=8).

However, as with standard mathematics, you can use parenthetical grouping to ensure numbers are processed in a certain way. This means you can use the expression

set /a total=(8+3)*4

to ensure that the command-line interprets the expression as (8+3=11, 11*4=44).

Simulating Exponents

Although you can perform many mathematical operations at the command line, there is no way to raise values to exponents. You can, however, perform these operations manually. For example, the easiest way to get a value for 2³ is to enter

set /a total=2*2*2

The result is 8. Similarly, you can get a value for 10⁵ by entering

set /a total=10*10*10*10*10

The result is 100,000.

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