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The most common control statement used in CGI
scripts is the "if" test. The if test checks to
see if some expression is true, and if so, executes the
routines in the statement block. Perl uses a simple binary
comparison as a test of truth. If the result of some
operation is true, the operation returns a one and the
statement block is executed. If the result is false, it
returns a zero, and the statement block is not executed.
For example, consider the following code:
if ($name eq "Selena Sol")
{
print "Hello Selena.\n";
}
In this example, Perl checks to see if the scalar variable
$name has the value of "Selena Sol". If the patterns
match, the matching operation will return true and the
script will execute the print statement within the statement
block. If Perl discovers that $name is not equal to
"Selena Sol" however, the print will not be executed.
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Be careful with your usage of "eq" versus
"=". Within an "if" test, if you write $name =
"Selena Sol", you will actually be assigning "Selena
Sol" to the variable $name rather than comparing it
to the value "Selena Sol". Since this action will be
performed successfully, the if test will always test to
true and the statement block will always be
performed even if $name did not initially equal
"Selena Sol".
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The if test also provides for alternatives: the "else" and
the "elsif" control statements. The elsif alternative adds a
second check for truth and the else alternative defines a
final course of action for every case of failed if or elsif
tests. The following code snippet demonstrates the usage
of if, elsif, and else.
if ($name eq "Selena Sol")
{
print "Hi, Selena.\n";
}
elsif ($name eq "Gunther Birznieks")
{
print "Hi, Gunther\n";
}
else
{
print "Who are you?\n";
}
Obviously, the else need not perform a match since it is a
catch-all control statement.
The "unless" control statement works like an inverse "if"
control statement. Essentially it says, "execute some
statement block unless some condition is true". The
unless control statement is exemplified in the code below:
unless ($name eq "Selena")
{
print "You are NOT Selena!\n";
}
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