Perl Boolean and Relational Operators

The following section discusses more about the boolean and relational operators used in Perl.

• String Relational Operators

  Operator	Description
  gt	greater than
  lt	less than
  ge	greater than or equal to
  le	less than or equal to

  
  Strings are evaluated based on their ASCII value (so a is greater than A) and spaces are significant. These operators test the relative value of string data. It is an error to use these operators with numbers. The string relational operators are used to test relationships between strings. For example,
  

  if ($s1 gt $s2) { 
    some code here
  }
  else { 
    some other code here
  }
  

Operator	Description
==	equal to
!=	not equal to

The numeric equality operators are used to test equality between numbers. For example,

while($x == 42) { 
some code here 
}

These operators work with numeric data.
It is an error to use these operators with strings.

Assignment Operators
Perl offers a wide variety of assignment operators, including many shortcut operators to handle common tasks. Table 3-7 lists these operators. The lvalue is the left hand side of the operator and the rvalue is the expression on the right.
Table3-7: Perl Assignment Operators

Operator	Equivalent Expression
=	Assign rvalue to lvalue
+=	lvalue = lvalue + rvalue
-=	lvalue = lvalue - rvalue
*=	lvalue = lvalue * rvalue
/=	lvalue = lvalue / rvalue
||=	lvalue = rvalue if !lvalue
//=	lvalue = rvalue if !defined lvalue
&&=	lvalue = lvalue && rvalue
|=	lvalue = lvalue | rvalue
&=	lvalue = lvalue & rvalue
**=	lvalue = lvalue ** rvalue
x=	lvalue = lvalue x rvalue
<<=	lvalue = lvalue << rvalue
>>=	lvalue = lvalue >> rvalue
^=	lvalue = lvalue ^ rvalue

In computer programming, the following notation ?: is a ternary operator that is part of the syntax for basic conditional expressions. It is commonly referred to as the conditional operator or ternary if.

Operator	Description
? :	if then else

The ternary operator uses three expressions to do its work:

$x = $a ? $b : $c

This operator evaluates from left to right:

1. $a is evaluated.
2. If $a is nonzero, $b is evaluated and returned.
3. Otherwise, $c is evaluated and returned.

So, given this:

$a = 1; $b = 2; $c = 3;
$x = $a ? $b : $c;

The value of $b (2) is assigned to $x. If $a were zero or undefined, the value of $c would be assigned to $x. Note that if $a evaluates to true (non-zero), then $b will be evaluated, but $c will not. If $a evaluates to false (zero), then $c will be evaluated but $b will not. Thus you should not write code using the ternary operator that depends on both $b and $c being evaluated.

Operator	Description
eq	equal to
ne	not equal to

The string equality operators are used to test equality between strings. For example,

while($s ne 'error') 
{ some code here }

These operators work with string data.
It is an error to use these operators with numbers.
Understanding Boolean Operators You use Boolean operators to determine if a value or expression is true or false. Because Perl lets you assign strings and numbers to variables, the Boolean operators are separated into string and numeric versions. You learn the string versions first. you see the if/else statement now just so you can understand how they work. The if statement takes an expression in parentheses and, if it evaluates as true, executes the code in the block following it. If an else block follows the if block, the else block executes only if the if expression evaluates as false. For example:

my ( $num1, $num2 ) = ( 7, 5 );
if ( $num1 < $num2 ) {
 print "$num1 is less than $num2\n";
}
else {
 print "$num1 is not less than $num2\n";
}

That code prints 7 is not less than 5. The < Boolean operator is the Boolean less than operator and returns true if the left operand is less than the right operand. Now that you have this small example out of the way, the following sections discuss the boolean operators.
Note: In order to complete this lesson's exercise, you must be familiar with these operators.

Click the Exercise link below to practice writing conditional code.
Conditional Operators - Exercise