珀尔 | undef 和定义的函数

undef用于那些没有任何赋值的变量。可以将其与 NULL（在Java、 PHP等中）和 Nil（在 Ruby 中）进行比较。所以基本上当程序员声明一个标量变量并且不给它赋值时，那么变量应该包含undef值。在 Perl 中，如果变量的初始值是 undef，那么它将不打印任何内容。
例子：

Perl

# Perl program for undef variable
# variable which declared without
# initial value
 
# variable x without initial value
my $x;
 
# printing its value
print "The value of x is = ${x}";

输出：

The value of x is = 

undef()函数： undef用于重置任何变量的值。它可以带或不带括号使用。这意味着在使用此函数时括号是可选的。

• 例子：
  

Perl

# Perl program to illustrate
# the undef() function
 
# a variable with initial value 10
$k = 10;
 
# displaying its initial value
print "The value of variable k is ${k}\n";
 
 
# undef the variable
undef $k;
 
# value after the undef function
print "The value of variable k is ${x}\n";
 
# a variable with initial value 20
$m = 20;
 
# displaying its initial value
print "The value of variable m is ${m}\n";
 
# undef the variable
# you can also use
# $m = undef;
$m = undef();
 
# value after the undef function
print "The value of variable m is ${m}\n";

• 输出：
  

The value of variable k is 10
The value of variable k is 
The value of variable m is 20
The value of variable m is 

defined()函数：该函数用于检查是否为变量赋值。如果将值分配给变量，它将返回 True，否则将返回 false。

• 句法：
  

defined $variable_name

• 例子：
  

Perl

# Perl program to illustrate
# the defined() function.
 
# a variable assigned value 15
$k = 15;
 
# To check if variable is defined or not
if (defined $k)
{
     print "k is defined\n";
}
else
{
    print "k is not defined\n";
}
 
 
# undef the variable
$k = undef;
 
# To check if the variable is defined or not
if (defined $k)
{
    print "k is defined\n";
}
else
{
    print "k is not defined\n";
}

• 输出：
  

k is defined
k is not defined

注意：虽然未定义的变量没有定义的值，但它们在运行过程中可以取空值。例如，如果用户添加两个变量 x 和 y，其中 x 为 5，y 未定义，则结果将保持为 5，因为 y 将占用值 0。类似地，如果用户将两个字符串strx 和 stry 与 strx 连接为值“GGF”并且stry是undef，结果将是“GFG”，因为stry取空字符串“”的值。

• 例子：
  

Perl

# Perl program to demonstrate behaviour
# of undef variables, during operation
# like arithmetic, concatenation etc.
 
# first variable assigned with value
$x = 125;
 
# second variable with no value
my $y;
 
# arithmetic operation
$z = $x + $y;
 
# displaying sum
print "The sum of x and y is ${z}\n";
 
# declaring strx and assigned a value to it
$strx = "GFG";
 
# declaring stry without assigned value
my $stry;
 
# string concatenation
$strz = $strx.$stry;
 
# after concatenation
print "After concatenation of strx and stry  ${strz}\n";

• 输出：
  

The sum of x and y is 125
After concatenation of strx and stry  GFG