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Shifting Things Around - Perl

This week's article teaches you how to use Perl to interact withfiles on your system, and also provides you with a quick crash course invarious array functions.

TABLE OF CONTENTS:
  1. Perl 101 (Part 4) - Mind Games
  2. Handle With Care
  3. Different Strokes
  4. A Little Brainwashing
  5. Die! Die! Die!
  6. Testing Times
  7. Popguns And Pushpins
  8. Shifting Things Around
  9. The Real World
  10. Miscellaneous Stuff
By: Vikram Vaswani and Harish Kamath, (c) Melonfire
Rating: starstarstarstarstar / 11
June 29, 2000

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pop() and push() work on the last element of the array. If you'd prefer to add something to the beginning of the array list, you need to use unshift():
#!/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 "How about a title?\n";
$title = <STDIN>;
# add title to beginning of array
unshift (@file, $title);
# 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);

And the file "thoughts.txt" will now contain a title, as entered by the user.

Obviously, removing the first element of an array requires you to use the shift() function - we'll leave you to experiment with that one yourself.

This article copyright Melonfire 2000. All rights reserved.{mospagebreak title=The Greatest Things Since Sliced() Bread} Next, two of Perl's most frequently-used functions - split() and join(). split() is used to split a string value into sub-sections, and place each of these sections into an array, while join() does exactly the opposite - it takes the various elements of an array, and joins them together into a single string, which is then assigned to a scalar variable.

Let's take a simple example:
#!/usr/bin/perl
# set up a variable
$string = "This is a string";
# split on spaces and store in array
@dummy = split(" ", $string);
# print the array
foreach $word (@dummy)
{
print "$word\n";
}

Here's the output:
This
is
a
string

In this case, we're splitting the string using a space as the separator - each element of the split string is then stored in an array.

You can also join array elements into a single string:
#!/usr/bin/perl
# set up a variable
$string = "This is a string";
# split on spaces and store in array
@dummy = split(" ", $string);
# join the words back with a different separator
$newstring = join(":", @dummy);
# print the result
print "$newstring\n";

And the output is:
This:is:a:string

And finally, we have the splice() function, which is used to extract contiguous sections of an array [if you don't know what contiguous means, this is a good time to find out!]. Here's an example which uses the splice() function to extract a section of an array, and assign it to a new array variable.
#!/usr/bin/perl
# set up a variable
$string = "Why did the fox jump over the moon?";
# split on spaces and store in array
@dummy = split(" ", $string);
# extract three words
@newdummy = splice(@dummy, 1, 4);
# join them together and print
$newstring = join(" ", @newdummy);
print "$newstring\n";

The splice() function takes three arguments - the name of the array variable from which to extract elements, the starting index, and the number of elements to extract. In this case, the output would be:
did the fox jump

You should note that splice() alters the original array as well - in the example above, the original array would now only contain
@dummy = ("Why", "over", "the", "moon?");



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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