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Introduction to Perl 5.10 for Unix System Administrators

(Perl 5.10 without excessive complexity)

by Dr Nikolai Bezroukov

Contents : Foreword : Ch01 : Ch02 : Ch03 : Ch04 : Ch05 : Ch06 : Ch07 : Ch08 :


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3.2. Numeric Values and Arithmetic operators

The operators in a computer language tell the computer what actions to perform. Perl has more operators than most languages. We already used many of them, for example assignment operator(=) in previous chapters relying on your knowledge of C or Java.

All operators are performed on operands. An operand is typically either a literal, or a variable, or an expression.

Precedence determines what operator in expression is evaluated first.

Places where Perl expects stings are called string context, and places where Perl expects numbers -- numeric contexts. All arithmetic operations enforce numeric context. That means that automatic converting of string values to floating point double precision numbers is performed before operation is attempted. C library function atof() is used for conversion. Any leading white space is ignored. After that the head of the string that contains a legitimate decimal numeric constant is converted. The rest of the string is ignored. If a string can't be converted its' value is assumed to be zero. For example

my $a = 0 + "25.5'"; # $a will be equal to 25.5 -- trailing suffix "'" 
                     # is ignored

Perl arithmetic operators are very similar to C. Like in C there are unary and binary operators. There is one ternary operator -- conditional assignment

$condition ? $value_if_true : $value_if_false
Unary operators are typically prefix operators. Exceptions are the post-increment (++) and post-decrement (--) operators (like in C those operators can be both pre and post). For example:
$y=! $x                # a unary operator
$z=$x * $y             # a binary operator
$z=$x ? $y : $z        # a conditional assignment
An operator's precedence in Perl is very similar to C and Java. That means that any operators borrowed from C will keep the same precedence relationship with each other as in C. Operators with higher precedence grab the arguments around them before operators with lower precedence.

The order in which two operators of the same precedence are executed depends on their associativity. Typically it is left as in

$x * 3 * 4          # means ($x * 3) * 4, left associative
but there few cases of right associativity as well, for example:
$x ** 3 ** 4        # means $x ** (3 ** 4), right associative 
In case you are not sure use brackets or, if you want the hard way analyze the table below which lists the associativity and arity of the Perl operators from highest precedence to lowest.
Associativity Arity Precedence Class
Left 2 ->
None 1 ++ --
Right 2 **
Right 1 ! ~ > and unary + and -
Left 2 =~ !~
Left 2 * / % x
Left 2 + - .
Left 2 << >>
None 2 < > <= >= lt gt le ge
None 2 == != <=> eq ne cmp
Left 2 &
Left 2 | ^
Left 2 &&
Left 2 ||
None 2 .. ...
Right 3 ?:
Right 2 = += -= *= and so on
Left 2 , =>
Right 1 not
Left 2 and
Left 2 or xor

As you can see it is easier to put in extra parentheses to avoid doubts (and possible bugs in the program).

Some Binary Arithmetic Operators

There are six binary arithmetic operators: addition, subtraction, multiplication, exponentiation, division, and modulus.
Operator Description
op1 + op2 Addition
op1 op2 Subtraction
op1 * op2 Multiplication
op1 ** op2 Exponentiation
op1 / op2 Division
op1 % op2 Modulus

The two that require some comments are exponentiation operator and modulus operator

The Exponentiation Operator

The exponentiation operator is used to raise a number to a power. For instance, 2 **4 is equivalent to 2 * 2 * 2 * 2 which equals 16. In Perl typically this operator is used for powers of 10 or 2 as it is easier to understand 2**8 then 64000. The operator is right associative: 2 ** 2 ** 4 is equivalent to 2 ** (2**4) .

The Modulus Operator

The modulus operator is used to find the remainder of the division between two integer operands. For instance, 10 % 7 equals 3 because 10 / 7 equals 1 with 3 left over.

I've found the modulus operator to be useful when my programs need to run down a list and do something every few items. This example shows you how to do something every 10 items.

for ($i = 0; $i <= 100; $i++) {
    if ($i % 10 == 0) { print("$i");
    }
}
When this program is run, the output should look like the following:
0 10 20 30 40 50 60 70 80 90 100
Notice that every 10th item is printed.

The Unary Arithmetic Operators

The unary arithmetic operators act on a single operand. They are used to change the sign of a value; to increment a value or to decrement a value. Incrementing a value means to add one to its value. Decrementing a value means to subtract one from its value.
Operator Description
Changing the sign of op1
+$a Positive operand
-$a Negative operand
Changing the value of op1 before usage
++$a Pre-increment operand by one
--$a Pre-decrement operand by one
Changing the value of op1 after usage
$a++ Post-increment operand by one
$a-- Post-decrement operand by one

Like in C the ++ and operators are valuable shorthand notation for $a=$a+1 and $a=$a-1, correspondingly. If the ++ or operators appear in front of the operand, the operand is incremented or decremented before its value is used. If the ++ or operators appear after the operand, then the value of the operand is used and then the operand is incremented or decremented as required.

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