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Introduction to Perl 5.10 for Unix System Administrators

(Perl 5.10 without excessive complexity)

by Dr Nikolai Bezroukov

Contents : Foreword : Ch01 : Ch02 : Ch03 : Ch04 : Ch05 : Ch06 : Ch07 : Ch08 :


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3.5. Typical errors

Perl automatically converts operands or the particular operator to strings or numbers depending on operation used (string context or numeric context). It is very important to remember that strings and numbers compare using different operators. Using for example "==" for string comparison is a nasty but very common bug in Perl scripts.  It is common even for seasoned Perl programmers, especially if they also use the second language like C. This also bedevil novice Perl programmers who are still thinking in term of their previous language, such as Java or C.

That means that you need to put special efforts to avoid this. Remember that == is used for string equality and eq for number equality (letter operators are used for comparing strings). After writing script is make sense to browse it line by line trying to find this type of errors manually.

 

The most typical problem that lead to large number of errors in Perl scripts is that you need to use different comparison operators for numbers and strings. For example to compare numbers for equality "==" should be used while for comparison of strings "eq" should be used. This is the source of tremendous number of errors in Perl programs. In Perl 5.10 and later you can use ~~ to ask Perl to determine type of operation automatically, but in complex cases this "automatic" way produces errors too as compiler can guess wrong.

Accidentally omitting leading $ for scalar variables or using @ when accessing or assigning a value to array elements (which are considered to be scalars in Perl) is another frequent error

While in many way a very innovative language Perl suffers from some design decisions connected with using separate sets of build-in functions for strings and arrays:

The main problem with Perl that I see is the although it provides a rich set of functions for string and arrays they are not generalized in such a way that similar functions are applicable to both without renaming, although this is entirely possible. So instead of substr functions (that is defined on strings) you need to use splice function for arrays and remember idiosyncrasies of two functions instead of one. Here Larry Wall made a very bad decision, but we can do nothing about it. You will see the problem yourself after you will face the assortment of functions that are available in Perl for arrays and strings. Actually you are forced to use a subset as full assortment is beyond human capacities to master (starving in the food store effect ;-)

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