What are subroutines and why would you use them?The limitations of “single block code” You won’t be the first person in the world to want to :

  • be able to read options from the command line
  • interpret form input in a CGI script –
  • pluralize words in English

But it doesn’t stop there, lets choose a few other seemingly simple but useful tasks that your code may need to accomplish.  You won’t be the first person in your organisation to want to

  • output your organisation’s copyright statement
  • validate an employee code
  • automatically contact a resource on your web site

These are the sort of tasks that may need to happen again and again, both in the same piece of codes or perhaps across different programs. You may need to handle the same data in several programs, or to handle in your programs the same data that your colleagues handle in theirs. And you may want to perform the same series of instructions at several places within the same program. Almost all programming languages, at least the high level ones can handle these operation including things like Perl. Even the beginners who start off with all your code has been in a single file and indeed has “flowed” from top to bottom.

You can use these subroutines to perform tasks that need to be repeated over and over again. In the context of network programming you could use a specific subroutine to assign a British IP address to a client or hardware device,
You have not been able to call the same code in two different places ‘ You have not been able to share code between programs — copying is not normally an option as it gives maintenance problems ~ You have not used your colleague’s code, nor code that’s available for everyone on the CPAN, nor additional code that’s so often needed that it’s shipped with the Perl distribution. First use of subroutines The first computer programs were written rather like the ones that we’ve written so far.

Each one for its own specific task. In time, programmers (said to be naturally lazy people) noticed that they could save effort by placing commonly used sections of code into separate blocks which could be called whenever and wherever they were needed. Such separate blocks were variously known as functions, procedures or subroutines.

We’ll use the word “subroutine” because Perl does! Structured programming The subroutine approach was then taken to extreme so that all the code was put into separate blocks, each of which could be described as performing a single task. For example, the program I run might be described as performing the task of “reporting on all towns with names matching a pattern”.  You could then split that task into multiple tasks for example creating multiple network connections to different servers.  On a multimedia server you could call the relevant subroutines depending on which channel was to be displayed e’g one for English channel, one for commercial ITV channel abroad  and another for a French variant.  All of these could be separate subroutines called from within the main code when the user presses a button.

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