Popguns And Pushpins - Perl

If you've been paying attention, you should now know the basics of reading and writing files in Perl. And as you've seen, reading a file into an array is a simple and efficient way of getting data from an external file into your Perl program. But once you've got it in, what do you do with it?

Well, over the next couple of pages, we're going to be taking a look at some of Perl's most useful array functions - these babies will let you split, join and re-arrange array elements to your heart - followed by a real-life example that should help you put it all in context.

Let's start with a basic example:

---

#!/usr/bin/perl
# open file and define a handle for it
open(MIND,"thoughts.txt") || die "Unable to open file!\n";
# suck the file into an array
@file = <MIND>;
# close file when done
close(MIND);
# use a loop to keep reading the file
# until it reaches the end
foreach $line (@file)
{
print $line;
}

---

At this point, the contents of file "thoughts.txt" are stored in the array @file. Now, let's add some code that asks the user for his thoughts on what he's just seen, adds those comments to the end of the array, and writes the whole shebang back to the file "thoughts.txt"

---

#!/usr/bin/perl
# open file and define a handle for it
open(MIND,"thoughts.txt") || die("Unable to open file!\n");
# suck the file into an array
@file = <MIND>;
# close file when done
close(MIND);
# use a loop to keep reading the file
# until it reaches the end
foreach $line (@file)
{
print $line;
}
# ask for input and process it
print "What do you think?\n";
$comment = <STDIN>;
# add comment to end of array
push (@file, $comment);
# open file for writing
open(MIND,">thoughts.txt") || die("Unable to open file!\n");
# print array back into file
foreach $line (@file)
{
print MIND $line;
}
# close file when done
close(MIND);

---

The important thing to note here is the push() function, which adds an element - the user's input - to the end of an array. The entire array is then written back to the file "thoughts.txt", destroying the original contents in the process. So, if you were to run

---

$ cat thoughts.txt

---

at your shell, you'd see

---

We're running out of space on planet Earth.
Scientists are attempting to colonize Mars.
I have a huge amount of empty real estate in my mind.
Imagine if I could rent it to the citizens of Earth for a nominal monthly
fee.
Would I be rich? Or just crazy?
This idea sucks!

---

Now, how about removing that last line? Simple - use the pop() function to remove the last item from the array:

---

#!/usr/bin/perl
# open file and define a handle for it
open(MIND,"thoughts.txt") || die("Unable to open file!\n");
# suck the file into an array
@file = <MIND>;
# close file when done
close(MIND);
# remove last element of array
pop @file;
# open file for writing
open(MIND,">thoughts.txt") || die("Unable to open file!\n");
# print array back into file
foreach $line (@file)
{
print MIND $line;
}
# close file when done
close (MIND);

---

The pop() function removes the last element of the array specified. And when you "cat" the file again, you'll see that the last line has been removed.

This article copyright Melonfire 2000. All rights reserved.

>>> More Perl Programming Articles          >>> More By Vikram Vaswani and Harish Kamath, (c) Melonfire

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