UNDEF numeric and defined functions in Perl

1.undef

UNDEF is the default value when variables are not initialized in Perl. When this uninitialized variable is used as an integer, then undef is 0, and when the variable is used as a string, then Undef is an empty string. So when you use a non-initialized variable in Perl, the program runs without problems.

For example:

$sum + = 1;   #该语句第一次被执行时, the sum variable defaults to 0

2.defined function

Determines whether a variable is undef, that is, whether it is initialized. How to use:

Defined ($var);

Note, typically, is used to determine whether a string is an empty string or a undef

3. In addition

You can think of specifying a variable as undef, which means that the variable is initialized. How to use:

$var = undef;