PS_03_Perl - Basic_I/O_&_Operators

Basic I/O

The easiest means to get operator input to your program is using the “diamond” operator:

$input = <>;

The input from the diamond operator includes a newline (\n). To get rid of this pesky character, use either chop() or chomp(). chop() removes the last character of the string, while chomp() removes any line-ending characters (defined in the special variable $/). If no argument is given, these functions operate on the $_ variable. To do the converse, simply use Perl’s print function:

print $output.”\n”;

Basic Operators

Arithmetic

Example Name Result

$a + $b Addition Sum of $a and $b

$a * $b Multiplication Product of $a and $b

$a % $b Modulus Remainder of $a divided by $b

$a ** $b Exponentiation $a to the power of $b

String

Example Name Result

$a . “string” Concatenation String built from pieces

“$a string” Interpolation String incorporating the value of $a

$a x $b Repeat String in which $a is repeated $b times

Assignment

The basic assignment operator is “=”: $a = $b.

Perl conforms to the C idiom that lvalue operator= expression is evaluated as: lvalue = lvalue operator expression

So that $a *= $b is equivalent to $a = $a * $b

$a += $b $a = $a + $b

This also works for the string concatenation operator: $a .= “\n”

Autoincrement and Autodecrement

The autoincrement and autodecrement operators are special cases of the assignment operators, which add or subtract 1 from the value of a variable:

++$a, $a++ Autoincrement Add 1 to $a

--$a, $a-- Autodecrement Subtract 1 from $a

Logical

Conditions for truth: Any string is true except for “” and “0” Any number is true except for 0 Any reference is true Any undefined value is false

Example Name Result

$a && $b And True if both $a and $b are true

$a || $b Or $a if $a is true; $b otherwise

!$a Not True if $a is not true

$a and $b And True if both $a and $b are true

$a or $b Or $a if $a is true; $b otherwise

not $a Not True if $a is not true

Logical operators are often used to “short circuit” expressions, as in:

open(FILE,”< input.dat”) or die “Can’t open file”;

Comparison

Comparison Numeric String Result

Equal == eq True if $a equal to $b

Not equal != ne True if $a not equal to $b

Less than < lt True if $a less than $b

Greater than > gt True if $a greater than $b

Less than or equal <= le True if $a not greater than $b

Comparison <=> cmp 0 if $a and $b equal

1 if $a greater

-1 if $b greater

Operator Precedence

Perl operators have the following precedence, listed from the highest to the lowest, where operators at the same precedence level resolve according to associativity:

Associativity Operators Description

Left Terms and

list operators

Left -> Infix dereference operator

++

--

Auto-increment

Auto-decrement

Right

Right

Right

\

! ~

+ -

Reference to an object (unary)

Unary negation, bitwise complement

Unary plus, minus

Left

Left

=~

!~

Binds scalar to a match pattern

Same, but negates the result

Left * / % x Multiplication, Division, Modulo, Repeat

Left + - . Addition, Subtraction, Concatenation

Left >> << Bitwise shift right, left

< > <= >=

lt gt le ge

Numerical relational operators

String relational operators

== != <=>

eq ne cmp

Numerical comparison operators

String comparison operators

Left & Bitwise AND

Left | ^ Bitwise OR, Exclusive OR

Left && Logical AND

Left || Logical OR

.. In scalar context, range operator

In array context, enumeration

Right ?: Conditional (if ? then : else) operator

Right = += -= etc Assignment operators

Left ,

=>

Comma operator, also list element

separator

Same, enforces left operand to be string

Right not Low precedence logical NOT

Right and Low precedence logical AND

Right or xor Low precedence logical OR

Parentheses can be used to group an expression into a term. A list consists of expressions, variables, or lists, separated by commas. An array variable or an array slice many always be used instead of a list.