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Error Basics in Perl

Perl Basics Part 18

Perl Course

Foreword: In this part of the series, I talk about the basics of errors in Perl.

By: Chrysanthus Date Published: 29 Mar 2015


This is part 18 of my series, Perl Basics. In this part of the series, I talk about the basics of errors in Perl. You should have read the previous parts of the series before reaching here, as this is a continuation.

Programming Errors
There are three types of programming errors. In other words, there are three types of errors that can occur in a program. You have Syntax Errors, Logic Errors and Runtime Errors.

Syntax Errors
This is the wrong use of syntax. These errors are wrong statements. When you type a statement, which is wrong, that is a syntax error. Such a statement cannot be executed. For example, in a statement you can type a variable without the $ sign. Under this condition, your program does not work. Depending on how you configure your Perl installation, such an error might be indicated by Perl to the output device just before the program is to be executed, when you give a command to run the program. With a syntax error, the program is not executed.

Logic Errors
In this case, Perl understands your program very well; the program is executed. However, the program will not do what you wanted it to do. It will do something slightly different or completely different. The fault is yours. For example, a loop that is required to do 10 iterations might do 5 iterations, because you coded it mistakenly to do 5 iterations. Another example is that a loop might iterate infinitely, because the condition you gave for the loop made it that way. Logic Errors occur when the program is being executed. The only way to solve this problem is to test your program very well before you hand it to the customer (who asked for it).

Runtime Errors
Runtime errors occur when the program is being executed as a result of the fact that you did not take certain factor into consideration when coding. For example, let us say your code is to divide 8 by some denominator that the user inputs. If the user inputs 2, the division will work, giving you 4 as answer. If the user inputs zero, the division will not work, because 8/0 is undefined. When a runtime error occurs your program normally crashes (and stops). To solve runtime errors, you have to write extra code that will prevent the execution of the particular code segment or statement from taking place, under certain conditions. In this division example, you have to write code that will prevent division by zero from taking place, and possibly informing the user of the mistake he made by inputting zero as a denominator.

Preventing Runtime Errors
The following code illustrates how to prevent the above error (division by zero).

use strict;

my $numerator = 8;
my $denominator = 2;

        if ($denominator != 0 )
                my $answer = $numerator/$denominator;
                print $answer;
                print "Division by zero is not allowed!";

The particular code segment here is that of the if-block. Read and try the above code if you have not done so. The code should be self-explanatory. Change the value of the denominator from 2 to 0 and try the code again.

Let us stop here for this part of the series. We continue in the next part.


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