The
Bitwise Operators
The
bitwise operators are similar to the logical operators, except
that they work on a smaller scale  binary representations of data.
The
following operators are available:
 op1
& op2  The AND operator compares two bits and generates
a result of 1 if both bits are 1; otherwise, it returns 0.
 op1
 op2  The OR operator compares two bits and generates
a result of 1 if the bits are complementary; otherwise, it returns
0.
 op1^
op2  The EXCLUSIVEOR operator compares two bits
and generates a result of 1 if either or both bits are 1; otherwise,
it returns 0.
~ op1
 The COMPLEMENT operator is used to invert all of the bits of
the operand.
 op1
>> op2  The SHIFT RIGHT operator moves the bits to
the right, discards the far right bit, and assigns the leftmost
bit a value of 0. Each move to the right effectively divides op1
in half.
 op1
<< op2  The SHIFT LEFT operator moves the bits to
the left, discards the far left bit, and assigns the rightmost
bit a value of 0. Each move to the left effectively multiplies
op1 by 2.
Note
Both operands associated with the bitwise operator must be integers.
Bitwise
operators are used to change individual bits in an operand. A single
byte of computer memorywhen viewed as 8 bitscan signify the true/false
status of 8 flags because each bit can be used as a boolean variable
that can hold one of two values: true or false. A flag variable
is typically used to indicate the status of something. For instance,
computer files can be marked as readonly. So you might have a $fReadOnly
variable whose job would be to hold the readonly status of a file.
This variable is called a flag variable because when $fReadOnly
has a true value, it's equivalent to a football referee throwing
a flag. The variable says, "Whoa! Don't modify this file."
When
you have more than one flag variable, it might be more efficient
to use a single variable to indicate the value of more than one
flag. The next example shows you how to do this.
Example:
Using the &, , and ^ Operators
The
first step to using bitwise operators to indicate more than one
flag in a single variable is to define the meaning of the bits that
you'd like to use. Figure 5.1 shows
an example of 8 bits that could be used to control the attributes
of text on a display.
The
bit definition of a text attribute control variable
If
you assume that $textAttr is used to control the text attributes,
then you could set the italic attribute by setting $textAttr
equal to 128 like this:
$textAttr = 128;
because
the bit pattern of 128 is 10000000. The bit that is turned on corresponds
to the italic position in $textAttr.
Now
let's set both the italic and underline attributes on at the same
time. The underline value is 16, which has a bit pattern of 00010000.
You already know the value for italic is 128. So we call on the
OR operator to combine the two values.
$textAttr = 128  16;
or
using the bit patterns (this is just an exampleyou can't do this
in Perl)
$textAttr = 10000000  00010000;
You
will see that $textAttr gets assigned a value of 144 (or
10010000 as a bit pattern). This will set both italic and underline
attributes on.
The
next step might be to turn the italic attribute off. This is done
with the EXCLUSIVEOR operator, like so:
$textAttr
= $textAttr ^ 128;
Example:
Using the >> and << Operators
The
bitwise shift operators are used to move all of the bits
in the operand left or right a given number of times. They come
in quite handy when you need to divide or multiply integer values.
This
example will divide by 4 using the >> operator.
$firstVar = 128;
$secondVar = $firstVar >> 2;
print("$secondVar\n");
Here
we
 Assign
a value of 128 to the $firstVar variable.
 Shift
the bits inside $firstVar two places to the right and
 assign
the new value to $secondVar .
 Print
the $secondVart variable.
The
program produces the following output:
32
Let's
look at the bit patterns of the variables before and after the shift
operation. First, $firstVar is assigned 128 or 10000000.
Then, the value in $firstVar is shifted left by two places.
So the new value is 00100000 or 32, which is assigned to $secondVar.
The
rightmost bit of a value is lost when the bits are shifted right.
You can see this in the next example.
The
next example will multiply 128 by 8.
$firstVar = 128;
$secondVar = $firstVar << 3;
print $secondVar;
The program
produces the following output:
1024
The
value of 1024 is beyond the bounds of the 8 bits that the other
examples used. This was done to show you that the number of bits
available for your use is not limited to one byte. You are really
limited by however many bytes Perl uses for one scalar variable
(probably 4). You'll need to read the Perl documentation that came
with the interpreter to determine how many bytes your scalar variables
use.
