High Level Languages

1
High Level Languages

• Perl
• Lecture 2

Chris Coleman cqc_at_cs.nott.ac.uk
2
Lecture 1 Summary

• Last time we did
• Intro to Perl
• Hello World
• Running Perl programs
• Scalars
• Numerical operations
• String operations
• Conditionals

3
Lecture 2 Summary

• Now onwards to
• Loops
• Arrays
• Hashes

4
Loops

• Perl supports six kinds of loop
• while
• do-while
• until
• do-until
• for
• foreach
• The condition part of each loop has the same
  rules as for if statements

5
Loops While

• A while loop runs the following block of code
  until the condition is no longer true
• If the condition was never true the code will not
  be executed even once

6
Loops While

• will print out value from 0 to 9
• not including 10
• a 0
• while (a lt 10)
• print "a is ",a," \n"
• a

7
Loops While

• nothing will get printed here
• a 5
• while (a gt 10)
• print "a is ", a, "\n"
• a--

8
Loops Do-while

• A do-while loop has the test at the end of the
  block of code
• This means the code will get executed at least
  once

9
Loops Do-while

• this will print out "a is 5"
• a 5
• do
• print "a is ", a, "\n"
• a--
• while (a gt 10)

10
Loops Until, Do-until

• until and do-until behave like while and do-while
  except with the tests reversed
• print 0 to 9
• a 0
• until (a 10)
• print "a is ",a ,"\n"
• a

11
Loops For

• A for loop is like a while loop except the set-up
  command, test condition and iteration command are
  all on the one line
• The format is
• for (setup test iteration)
• code goes here

12
Loops For

• prints from 0 to 9
• for (a 0 a lt 10 a)
• print "a is ", a, "\n"

13
Loops - Backwards

• Like if statements, Perl will let you write the
  loop after the command
• print "hello ",a until (a 10)
• print "world",a while (a lt 10)
• This makes more sense when you learn about
  sub-routines

14
Loops Foreach

• foreach loops are special versions of for loops
• Used to iterate through each entry in an array
• Arrays? We're getting to that...

15
Loops Controlling

• In any of these loops you can use the next and
  last commands
• next means skip the rest of the loop block but
  carry on running the loop (similar to continue in
  Java)
• last will exit the loop completely (break in Java)

16
Loops Next

• print even numbers between 1 and 10
• for (a 1 a lt 10 a)
• test if this number is odd
• skip if it is
• if ((a 2) 1)
• next
• print a," is even\n"

17
Loops Last

• print numbers between 50 and 100
• abort if number is divisible by 17
• for (a 50 a lt 100 a)
• if ((a 17) 0)
• last
• print "number ", a, "\n"

18
Loops Labels

• If you inside a nested loop next and last will
  only affect the inner loop
• If you want to affect the outer loop you can use
  labels
• A label goes before the start of the loop and
  after the next or last

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Loops Labels

• print out products until
• result goes over 200
• OUTER for (a 0 a lt 100 a)
• for (b 0 b lt 100 b)
• total a b
• last OUTER if (total gt 200)
• print a, "x", b, "", total, "\n"

20
Arrays

• The concept of arrays is the same as for other
  languages
• Whole arrays in Perl are identified by the _at_
  symbol
• No need to declare size of array before you use
  it, arrays will increase or decrease in size as
  they need to
• First entry is number 0
• An array can store a mix of data types

21
Arrays

• Individual entries in the array are scalars, so
  referenced by a with square brackets after,
  e.g.
• First item is number 0 (not 1 like in Pascal)
• set the first entry in name array
• to a string
• name0 "Duncan" right
• _at_name0 "Duncan" wrong

22
Lists

• Lists are not arrays, but it's best to mention
  them now
• Lists are a collection of data between round
  brackets, separated by a comma
• You can assign to an array from a list, e.g.
• _at_names ("Duncan", "Jim", "Saffron")
• print names0 prints Duncan

23
Lists

• Lists can contain arrays, so
• _at_extra ("Saffron", "Liz")
• _at_names ("Duncan","Jim", _at_extra, "Mark")
• print names0 Duncan
• print names1 Jim
• print names2 Saffron
• print names3 Liz
• print names4 Mark

24
Lists

• To avoid all the quotes and commas you can use
  the qw (quote words) function
• one way
• _at_names ("Duncan", "Jim", "Saffron"
• another way
• _at_names qw(Duncan Jim Saffron)

25
Arrays Functions

• push _at_array, list
• Adds the contents of list to the end of _at_array
• _at_names qw(Duncan Jim)
• push _at_names, "Saffron"
• print names2 Saffron
• push _at_names, "Mark", "Liz"
• print names4 Liz

26
Arrays Functions

• pop _at_array
• Removes and returns the last element in the array
• _at_names qw(Duncan Jim Saffron)
• onename pop _at_names
• print onename Saffron
• remove last entry, ignore the value
• pop _at_names
• pop and print all in one line
• print pop _at_names Duncan

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Arrays Functions

• unshift _at_array, list
• Same as push but operates on the start of the
  array
• shift _at_array
• Same as pop but operates on the start of the
  array
• sort _at_array
• Returns an alphabetically sorted version of the
  array

28
Arrays Length

• You can get the length of an array by using the
  array in scalar context
• _at_names qw(Duncan Jim Saffron)
• length _at_names length is 3

29
Arrays Length

• You might have to force scalar context with the
  scalar keyword
• _at_names qw(Duncan Jim Saffron)
• print _at_names will print the
• contents of the array
• print scalar _at_names will print 3

30
Arrays Foreach

• Remember foreach?
• You use this to iterate through all the entries
  in an array
• So, to print out all the contents of an array
• _at_names qw(Saffron Duncan Jim)
• foreach item (_at_names)
• print item,"\n"

31
Arrays Foreach

• Since sort takes an array and returns an array
  you can use it to sort the array whilst printing
• _at_names qw(Saffron Duncan Jim)
• foreach item (sort _at_names)
• print item,"\n"

32
Arrays Slices

• A slice is a subset of an array, built up form
  the specified indexes
• The indexes can be specified individually or by
  using a range
• _at_names3..5 entries 3 to 5
• _at_names1, 4, 6 entries 1, 4 and 6
• _at_names0..5, 9 entries 0 to 5, and 9

33
Arrays Slices

• You can use slices in a number of ways
• set _at_little to be the first 100 entries
• of the array _at_big
• _at_little _at_big0..99
• replace entries 4 to 6 in array _at_names
• with new values
• _at_names4..6 qw(Kev Sally Sarah)

34
Arrays Slices

• When using a slice to replace values in an array,
  you don't need the same number of values on the
  right hand side
• replace 10 names with just 2
• _at_names5..14 qw(Jon Harry)

35
Arrays Java baiting

• Consider the amount of code you'd need in Java
  (or another language) to have an array type
  structure that can accept any data type and then
  print them all out.
• I was going to write the Java code out but I got
  bored, it involved multiple Vector objects
  though...

36
Arrays Java baiting

• The Perl version
• push _at_list, "some string", 23, 3.14
• foreach item (_at_list)
• print "item ", item, "\n"

37
Hashes

• Hashes (aka Associate arrays) are very usefull
• Like an array, but instead of referencing an item
  by number you reference it by a string
• Whole hashes are identified with a , individual
  values are scalars so use before and after.

38
Hashes Example

• age'Duncan' 24
• age'Jim' 28
• print age'Duncan' prints 24

39
Hashes Assigning to

• You can assign to a hash from a list or array
• The keys will be the 1st, 3rd, 5th.. entries
• The values will be the 2nd, 4th, 6th entries
• ages ('Duncan',24,'Jim',28)
• print ages'Jim' prints 28
• You can use gt as a sugar when assigning
• ages ('Duncan' gt 24, 'Jim' gt 28)

40
Hashes Functions

• exists
• Returns true if a key exists
• age('Duncan' gt 24, 'Jim' gt 28)
• unless (exists age'Saffron')
• print "Age not known\n"
• delete
• Deletes a key from the hash
• delete age'Duncan'

41
Hashes Functions

• keys
• Returns a list of all the keys in the hash
• The keys will be in no particular order
• ages ('Duncan' gt 24, 'Jim' gt 28)
• _at_mykeys keys ages
• print "1st key ", mykeys0, "\n"
• print "2nd key ", mykeys1, "\n"

42
Hashes Looping

• To loop through a hash (say to print it) use keys
  to get a list of keys and then a foreach loop.
• ages ('Duncan' gt 24, 'Jim' gt 28)
• _at_allnames keys ages
• foreach name (_at_allnames)
• print name, " is "
• print agesname," years old\n"

43
Hashes Looping

• There's no need to store the result of keys in an
  array, as we can put the expression straight into
  the foreach loop
• ages ('Duncan' gt 24, 'Jim' gt 28)
• foreach name (keys ages)
• print name, " is "
• print agesname," years old\n"

44
Hashes Sorting

• Hashes are unordered whilst stored, you can only
  sort them when you come to work with them
• Sorting the keys into alphabetical order is
  pretty easy
• Remember the sort function takes an array and
  returns a sorted version of that array
• So...

45
Hashes Sorting

• ages ('Duncan' gt 24, 'Jim' gt 28)
• foreach name (sort keys ages)
• print name, " is "
• print agesname," years old\n"

46
Data structures Advanced

• Hashes and arrays aren't only capable of storing
  scalars, they can store hashes and arrays as well
• So you can have
• Arrays of arrays
• Arrays of hashes
• Hashes of hashes
• Hashes of arrays
• We won't be covering this...

47
Data Structures Warning

• You're allowed to have different data types with
  the same name
• camel ("humps" gt 2)
• _at_camel ("bob", "fred")
• camel 4
• print camel'humps' 2
• print camel1 "fred"
• print camel 4
• It's not a good idea though

48
Lecture 2 Summary

• We just did
• Loops
• Arrays
• Hashes