Perl

1
Perl

• Lists and Arrays

2
Lists

• List is a sequence of scalars
• Surrounded by parentheses items separated by
  commas
• (a, 5, alfa, _, the item)
• Usual variable substitution rules apply

3
List Shortcuts

• qw(the speed color fox)
• Creates list from each word with single quotes
• qq(the speed color fox)
• Creates list from each word with double quotes
• (1 .. 9, 11..15)
• Range of numbers

4
Arrays

• Lists occur in code but arrays variables are
  needed to store and manipulate list values
• _at_names qw(Joe, Jill, Mary, Bill)
• _at_empty ()
• _at_double (_at_names, _at_names)
• Array is flattened to list

5
Arrays (cont.)

• _at_nums (1..10)
• (first, _at_rest, last) _at_nums
• first 1 _at_rest (2..10) last undef
• Arrays are greedy
• (f,n) _at_nums
• f1 n2 ignore rest
• (a, b, c) qq(a b)
• aa bb c undef

6
Array Subscripts

• Zero-based subscripts
• _at_nums (1..10)
• nums0
• gives or takes the first element
• _at_nums1,2,5,7,9 is array slice
• nums-1
• Last element -2 is next to last, etc.

7
Array Size

• array
• Returns highest subscript of array
• Setting it changes size of array
• size _at_array
• Gives size of array

8
Context

• Determines effect of operations
• Scalar context
• Operations use scalars
• List context
• Operations use lists (or arrays)
• In assignment, left side determines context

9
Context (cont.)

• ab scalar context
• _at_m_at_n list context
• b_at_list scalar context
• _at_ab list context, one element list
• lineltSTDINgt scalar so one line
• _at_inputltSTDINgt list so whole file
• (x)ltSTDINgt list, but what?

10
Context (cont.)

• Context can have significant effects
• _at_start() x 100 array size 100 of
• which(a, b, c) scalar comma expr
• localtime function
• Scalar formatted date string
• List array of date/time components

11
Stepping through an array

• foreach i (1..10)
• foreach (_at_name)
• Uses _
• Modifying the control variable alters the
  corresponding entry in the array

12
Split and join

• split(/ /, words)
• Splits string words on blanks
• First argument is a pattern, a regular expression
  to match between items
• //, the empty string splits each letter
• join(, , (1..5))
• Inserts comma space between numbers

13
Sorting

• _at_alist sort _at_names
• Alphabetic sort
• _at_snum sort a ltgt b (5, 3, 7, -1)
• Can specify any code block or function to
  determine sort order. Must return -1, 0, or 1 to
  indicate order

14
Arrays as Stacks

• Operations on arrays
• push array, new_list on back
• pop array returns last entry
• shift array, new_list on front
• unshift array returns first element
• How can you make a queue?