Perl Lists and Arrays

1
Perl Lists and Arrays

• CSC 492 Topics in Perl
• Joe Reynoldson
• January 19, 2005

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Getting Plural in Perl

• List literals
• An unnamed collection of scalars separated by
  commas and enclosed by parenthesis
• Lists can contain any combination of scalar
  values
• Arrays
• Named variables which store lists
• The type identifier for arrays is _at_
• The array namespace is separate from the scalar
  namespace, so list and _at_list are 2 different
  variables

(5, 7, 9, 11) (string, 0, undef,
var1) () (undef)
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Accessing array elements

• Arrays are indexed by integers starting at zero
• The type identifier of an single array element is
  
• All array subscripts are converted to integers
• Subscripts can also be expressions
• The subscript of the last element in an array is
  stored in the special variable array_name

_at_name_list () create an empty array first
0 name_list0 'joe' assign the first
element name_list1.5 'molly' assign the
second element print name_listfirst,"\n"
prints joe print name_listfirst 1,"\n"
prints molly print name_listname_list,"\n"
also prints molly
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Array sizes

• Perl will automatically make an array larger as
  necessary
• There are several methods to change the size of
  an array

_at_small_list (1, 2, 3) 3 items _at_big_list
(_at_small_list, _at_small_list) 6 items big_list
499 add 494 items _at_small_list () the empty
list _at_big_list undef 1 item
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List Shortcuts

• The range operator ( x..y ) creates a list
  starting from its left-hand operand and going
  up to its right-hand operand
• Create a list of strings without all of the
  quotes using the qw operator
• Assign to a list of scalar variables in one
  statement

first_num 1 last_num 100 _at_numbers
(first_num..last_num) numbers 99
_at_sammy qw/ turkey mayo cheese lettuce tomato
/ _at_paths qw( /usr/bin /usr/local/bin
/usr/X/bin )
(fruit,vegetable,meat)qw( pineapple asparagus
beef ) (array0, array1) (array1,
array0) swap!
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Array operators

• push adds onto the end of an array
• pop removes elements from the end of an array
• unshift adds elements to the beginning of an
  array
• shift removes elements from the beginning of an
  array

count 0 while(count lt 10) push(_at_numbers,
count)
while(last_number pop(_at_numbers)) print
last_number
unshift(_at_numbers, (1 .. 10))
while(_at_numbers) print shift(_at_numbers)
Check out Llama book pp 46-47 for more examples
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Array interpolation

• arrays within double quotes will be interpolated
• printing email addresses becomes tricky (llama
  page 48)
• printing just the array results in no spaces

_at_array (1 .. 10) print "_at_array\n" 1 2 3 4 5
6 7 8 9 10
print _at_array,"\n" 12345678910
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My favorite loop is foreach

• use foreach to iterate over a list of elements
• modifying the control variable modifies the
  element of the array

foreach line (_at_file) print "line" scope
'206.176.3' foreach number (1..255) print
"nslookup scope.number\n"
foreach line (_at_file) chomp(line)
line . "\r\n"
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The Cryptic _

• the 'default variable' is _
• many Perl functions use the default variable
• How do you know if a function supports _?

foreach (20..30) if(_ 2) print
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Reversing a list

• the reverse operator returns the reverse of a
  list or array
• doesn't alter the original
• reverse alone isn't very useful, and -w lets us
  know

_at_numbers (1..5) _at_numbers reverse _at_numbers
good reverse _at_numbers still reversed print
reverse _at_numbers prints in increasing order
Useless use of reverse in void context at
scriptname line line_number
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Sorting it all out

• The sort operator returns the sorted version of
  a list or array
• It sorts 'ASCIIbetically'
• This operator also doesnt modify the original
  list or array
• The warning flag/pragma can also generate a
  useful message about useless use of sort in a
  void context

_at_sorted sort qw( quarterly_report.txt
graph.jpg johnson.eml ) print
"_at_sorted\n" graph.jpg johnson.eml
quarterly_report.txt
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Context

• Page 51 of the Llama book says this is the most
  important concept of the entire book!
• Scalar context - Perl expects a scalar value as a
  result of an expression
• List context - Perl expects a list value as a
  result of an expression

a b simple example of a scalar
assignment lista b also scalar
assignment number exponent binary
assignment is scalar context
_at_a _at_b simple list assignment (a, b, c)
qw/ ay bee see / also list context foreach
name (_at_name_list) list context without
assignment
Page 53 of the Llama book has some better
examples
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Lists in scalar context

• Using a list in scalar context is a kind of
  depends
• Assigning an array to a scalar stores its size
• The scalar function forces a list or array into
  scalar context

_at_a (1..4) create a 4 element list s _at_a
store the size of the list in s print "Size s
" print the size of the list print "Index
a\n" print the last index Size 4 Index 3
print scalar _at_a, "\n" print the size of _at_a
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Scalars in list context

• This is much more straightforward when a scalar
  is used in list context it is treated as a list
  containing one item

_at_name_list 'joe' a list of one foreach name
('joe') print name list of one again
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ltSTDINgt in list context

• The diamond operator will return everything until
  End Of File (EOF) in list context
• Each item (delimited by a newline) represents 1
  element in the resulting list
• EOF from the keyboard in UNIX is CTL-D
• removing the new lines couldn't be easier

_at_input_list ltSTDINgt list context
chomp(_at_input_list ltSTDINgt) chomp em all!
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Interesting comments from the Llama
Page 46 a beginning Perl programmer (wanting
to see how Perls speed compares to Cs) will
take, say, a sorting algorithm optimized for C
(with many array index operations), rewrite it
straightforward in Perl (again, with many index
operations) and wonder why its so slow. The
answer is that using a Stradivarius violin to
pound nails should not be considered a sound
construction technique.
Page 55 Perl will generally waste memory to
save time. This is a good tradeoff if youre
short of memory, you can buy more if youre
short on time, youre hosed.