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Learning Perl, 5th Edition
book

Learning Perl, 5th Edition

by Randal L. Schwartz, Tom Phoenix, brian d foy
June 2008
Beginner content levelBeginner
352 pages
11h 16m
English
O'Reilly Media, Inc.
Content preview from Learning Perl, 5th Edition

Slices

It often happens that we need to work with only a few elements from a given list. For example, the Bedrock Library keeps information about their patrons in a large file.[*] Each line in the file describes one patron with six colon-separated fields: a person’s name, library card number, home address, home phone number, work phone number, and number of items currently checked out. A little bit of the file looks something like this:

fred flintstone:2168:301 Cobblestone Way:555-1212:555-2121:3
barney rubble:709918:3128 Granite Blvd:555-3333:555-3438:0

One of the library’s applications needs only the card numbers and number of items checked out; it doesn’t use any of the other data. It could use code something like this to get only the fields it needs:

while (<FILE>) {
  chomp;
  my @items = split /:/;
  my($card_num, $count) = ($items[1], $items[5]);
  ...  # now work with those two variables
}

But the array @items isn’t needed for anything else; it seems like a waste.[*] Maybe it would be better to assign the result of split to a list of scalars, like this:

my($name, $card_num, $addr, $home, $work, $count) = split /:/;

Well, that avoids the unneeded array @items—but now we have four scalar variables that we didn’t really need. For this situation, some people used to make up a number of dummy variable names, like $dummy_1, that showed that they really didn’t care about that element from the split. But Larry thought that that was too much trouble, so he added a special use of undef. If an item ...

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Publisher Resources

ISBN: 9780596520106Supplemental ContentErrata Page