Perl recipies

1
Perl recipies

• Problem
• You want to print out the number represented by a
  given ASCII character, or you want to print out
  an ASCII character given a number.
• Solution
• Use ord to convert a character to a number, or
  use chr to convert a number to a character

2

• num ord(char) char chr(num) The c
  format used in printf and sprintf also converts a
  number to a character
• char sprintf("c", num) slower than
  chr(num)
• printf("Number d is character c\n", num,
  num)
• Number 101 is character e
• A C template used with pack and unpack can
  quickly convert many characters.
• _at_ASCII unpack("C", string)
• STRING pack("C", _at_ascii)

3
Case controlling

• use locale
• big uc(little)
• little lc(big)
• big \Ulittle
• little \Lbig
• \u, \l - just one character

4

• 1.12. Reformatting Paragraphs
• Problem
• Your string is too big to fit the screen, and you
  want to break it up into lines of words, without
  splitting a word between lines. For instance, a
  style correction script might read a text file a
  paragraph at a time, replacing bad phrases with
  good ones. Replacing a phrase like utilizes the
  inherent functionality of with uses will change
  the length of lines, so it must somehow reformat
  the paragraphs when they're output.
• Solution
• Use the standard TextWrap module to put line
  breaks at the right place.

5

• use TextWrap
• _at_OUTPUT wrap(LEADTAB, NEXTTAB, _at_PARA)
• LEADTAB is the tab for the 1st line and
• NEXTTAB is the tab for the subsequent lines

6

• You want to check whether a string represents a
  valid number. This is a common problem when
  validating input, as in a CGI script.
• Solution
• Compare it against a regular expression that
  matches the kinds of numbers you're interested
  in.
• if (string /PATTERN/) is a number else
  is not

7

• warn "has nondigits" if /\D/
• warn "not a natural number" unless /\d/
  rejects -3
• warn "not an integer" unless /-?\d/
  rejects 3
• warn "not an integer" unless /-?\d/
• warn "not a decimal number" unless
  /-?\d\.?\d/ rejects .2
• warn "not a decimal number" unless
  /-?(?\d(?\.\d)?\.\d)/
• warn "not a C float" unless /(-?)(?\d\.\d)\d
  (\.\d)?(Ee(-?\d))?/

8
Operating on a series of integers

• foreach (X .. Y) _ is set to every integer
  from X to Y, inclusive
• foreach i (X .. Y) i is set to every
  integer from X to Y, inclusive
• for (i X i every integer from X to Y, inclusive

9
Arabic and Roman numeral

• use Roman
• roman roman(arabic) convert to roman
  numerals
• arabic arabic(roman) if isroman(roman)
  convert from roman numerals

10
Random number

• Problem
• You want to make random numbers in a given range,
  inclusive, from x to Y.
• Solution
• Use Perl's rand function.
• random int( rand( Y-X1 ) ) X

11
Multiplying matrices

• Problem
• You want to multiply a pair of two-dimensional
  arrays. Mathematicians and engineers often need
  this.
• Solution
• Use the PDL modules, available from CPAN. PDL is
  the Perl Data Language  - modules that give fast
  access to compact matrix and mathematical
  functions

12
Example)

• use PDL
• a pdl 3, 2, 3 , 5, 9, 8 ,
• b pdl 4, 7 , 9, 3 , 8, 1 ,
• c a x b x overload

13
Perl grep function

• Ex) _at_matching grep /gnat / who
• Ex) _at_persons grep _-income 26000
  _-income
• Unlike unix grep function, perl grep function
  can have test conditions inside the bracket.

14
Testing for the Presence of a Key in a Hash

• Problem
• You need to know whether a hash has a particular
  key, regardless of any possible associated value.
• Solution
• Use the exists function.

15
exists functions

• does HASH have a value for KEY ?
• if (exists(HASHKEY)) it exists else
  it doesn't

16
Retrieving from a Hash in Insertion Order

• Problem
• The keys and each functions give you the hash
  elements in a strange order, and you want them in
  the order in which you inserted them.
• Solution
• Use the TieIxHash module.

17
Example)

• use TieIxHash
• tie HASH, "TieIxHash" manipulate HASH
• _at_keys keys HASH _at_keys is in insertion
  order

18
Inverting a hash

• Problem
• Hashes map keys to values. You have a hash and a
  value for which you want to find the
  corresponding key.
• Solution
• Use reverse to create an inverted hash whose
  values are the original hash's keys and vice
  versa.
• LOOKUP maps keys to values
• REVERSE reverse LOOKUP

19
Sorting a hash

• The following code simply uses sort to order the
  keys alphabetically
• foreach food (sort keys food_color) print
  "food is food_colorfood.\n"
• This sorts the keys by their associated values
• foreach food (sort food_colora cmp
  food_colorb keys food_color) print
  "food is food_colorfood.\n"

20

• This sorts by length of the values
• _at_foods sort length(food_colora)
  length(food_colorb) keys food_color
  foreach food (_at_foods) print "food is
  food_colorfood.\n"

21
Presizing a Hash

• Problem
• You want to preallocate memory for a hash to
  speed up your program so Perl won't have to
  incrementally allocate memory each time a new
  entry is added to the hash.
• Solution
• Assign the number of key-value pairs your hash
  will have to keys HASH.
• presize hash to num
• keys(hash) num
• keys(users) 512

22
Representing Relationships Between Data

• Problem
• You want to represent relationships between
  elements of data - for instance, the mother of
  relationship in a family tree or parent process
  for a process table.
• Solution
• Use a hash to represent the relationship.

23
Example) part of the family tree from the Bible

• father ( 'Cain' 'Adam',
• 'Abel' 'Adam',
• 'Seth' 'Adam',
• 'Enoch' 'Cain',
• 'Irad' 'Enoch',
• 'Mehujael' 'Irad',
• 'Methusael' 'Mehujael',
• 'Lamech' 'Methusael',
• 'Jabal' 'Lamech')

24
Example)

• This lets us, for instance, easily trace a
  person's lineage
• while () chomp
• do print "_ " print the current name
• _ father_ set _ to _'s father
  while defined until we run out of fathers
  print "\n"

25
Whom did Lamech beget?"

• while ( (k,v) each father ) push( _at_
  childrenv , k )
• " ', ' separate output with commas
• while () chomp
• if (children_) _at_children
  _at_children_ else _at_children "nobody"
  
• print "_ begat _at_children.\n"

26
Copying and Substituting Simultaneously

• Problem
• You're tired of constantly using two separate
  statements with redundant information, one to
  copy and another to substitute.
• Solution
• Instead of
• dst src dst s/this/that/
• use
• (dst src) s/this/that/