Perl undef FunctionNEW!!! Check my new resource: Perl How To Tutorial eBooks Perl undef function returns an undefined value, freeing up the memory used by the variable arguments passed to it. You can use this function with scalars, lists or arrays, hashes, functions or typeglobs. In this short free tutorial I’ll show you some opportunities about using this built-in function in your Perl scripts. For this function two syntax forms are available, as shown below: You can omit the expression when you use undef and in this case undef is a lvalue, the undefined value being a special data type, a special case of a scalar value. If you’ll use it without EXPR, undef will return the undefined value. For numeric scalar variables the undef value is 0, meanwhile for string variables it indicates an empty string. To distinguish between 0 / empty strings and the undef value, Perl provides the defined function that allows you to check up if a variable has the undef value or not. Next, I’ll show you some ways to use this function.
See the following snippet code: and the output for this code:
If you want to free up the memory resources occupied by a big array, you can use the undef function like in the following lines of code: In the above code, we read from standard input some lines of data and we store them in the @array. After performing some tasks against the elements of the array, we free up the memory usage by that array variable. If you execute this code, finally the message: "@array is undefined" will be printed. However, please note that if you have a multidimensional array, which has some references to other structures, it is not enough to free the initial array only. To illustrate, please see the next code: If you want to free the usage memory for an array and for all its references too, you must consider to step down in a recursive way through all the array references and free the usage memory until no more references are found. To see if an element of an array is a reference to some other data type structure, you can use the ref function, something like in the next lines of code: and the output: You can use undef against certain elements of an array: The second element of the array will have the undef value, but the dimension of the array doesn’t change.
The below code will show you how to use Perl undef upon an entire hash (free up the memory usage by it) or to undef certain elements of the hash. If you want to delete an entry from a hash, you must use the delete function instead. Using Perl undef with a $key element of a hash (using either undef $hash{$key} or $hash{$key} = undef) will only have as effect to undefine the respective value associated with the $key. The following code speaks by itself:
There are cases when you want to write a subroutine that returns an undef value if some errors happen. The next example will show you how to return undef if you divide some numbers by 0. NEW!!! Do you want more information about the basic Perl topics? Check my new "Perl How To" Tutorial eBooks page where I'll answer the most frequent questions regarding some topics : Table of Contents: A Perl Script Install Perl Running Perl Perl Data Types Perl Variables Perl Operators Perl Lists Perl Arrays Array Size Array Length Perl Hashes Perl Statements Perl if Perl unless Perl switch Perl while Perl do-while Perl until Perl do-until Perl for Perl foreach Built-in Perl Functions Functions by Category String Functions Array Functions Functions in alphabetical order chomp chop chr crypt defined grep hex index join lc lcfirst length oct ord pack pop push reverse rindex shift split substr uc undef unshift return from Perl undef Function to Perl Basics Hit the Alarm Clock! 
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