Perl Functions

1
Perl Functions

• Learning Objectives
• To learn how to create functions in a Perls
  program how to call them
• To learn how to pass structured arguments into
  functions return value from a function
• To understand the scope of a Perls program

2
Defining a Function (1)

• A user-defined function or subroutine is defined
  in Perl as follows
• sub subname
• statement1
• statement2
• statement3
• Simple Example
• sub hello
• print "hello world!\n"

3
Defining a Function (2)

• Subroutine definitions can be anywhere in your
  program text (they are skipped on execution)
• Within the subroutine body, you may use any
  variable from the main program (variables in Perl
  are global by default).
• !/usr/local/bin/perl5 -w
• user "horner"
• hello()
• print "goodbye user!\n"
• sub hello
• print "hello user!\n"

4
Global Variable

• You can also use variables from the subroutine
  back in the main program (it is the same global
  variable)
• cat sum1
• !/usr/local/bin/perl5 -w
• a 1 b 2
• sum 0
• sum_a_and_b()
• print "sum of a plus b sum\n"
• sub sum_a_and_b
• sum a b
• sum1
• sum of 1 plus 2 3

5
Sorting Question Revisited

• Consider a studentsIDCGA from last
  lecture
• Print out the students IDs and CGAs in order
  of student CGA.
• Modify sort function by order of students CGA

foreach ID ( sort by_CGA keys(students) )
sub by_CGA (studentsaCGA'
ltgt studentsbCGA') (a cmp b)
6
Returning Values (1)

• You can return a value from a function, and use
  it any expression as in C
• cat sum2
• !/usr/local/bin/perl5 -w
• a 1
• b 2
• c sum_a_and_b() 1
• print "value of c c\n"
• sub sum_a_and_b
• return a b
• sum2
• value of c 4

7
Returning Values (2)

• A subroutine can also return a list of values
• cat list1
• !/usr/local/bin/perl5 -w
• a 1
• b 2
• _at_c list_of_a_and_b()
• print "list of c _at_c\n"
• sub list_of_a_and_b
• return (a,b)
• list1
• list of c 1 2

8
Returning Values (3)

• Another example
• cat max1
• !/usr/local/bin/perl5 -w
• a 1
• b 2
• max max_of_a_and_b()
• print "max max\n"
• sub max_of_a_and_b
• if(a gt b) return a
• else return b
• max1
• max 2

9
Arguments (1)

• You can also pass arguments to a subroutine.
• The arguments are assigned to a list in a special
  variable _at__ for the duration of the subroutine.
• cat max2
• !/usr/local/bin/perl5 -w
• a 1
• b 2
• max max(a, b)
• print "max max\n"
• sub max
• if(_0 gt _1) return _0
• else return _1
• max2
• max 2

10
Arguments (2)

• A more general way to write max() with no limit
  on the number of arguments
• cat max3
• !/usr/local/bin/perl5 -w
• a 1
• b 2
• max max(a, b, 5)
• print "max max\n"
• sub max
• max 0
• foreach n (_at__)
• if(n gt max) max n
• return max
• max3
• max 5

11
Arguments (3)

• Dont confuse _ and _at__, they are unrelated.
• Excess parameters are ignored if you dont use
  them.
• Insufficient parameters simply return undef if
  you look beyond the end of the _at__ array.
• _at__ is local to the subroutine.

12
Local Variables (1)

• You can create local versions of scalar, array
  and hash variables with the my() operator.
• cat max4
• !/usr/local/bin/perl5 -w
• a 1 b 2
• max 0
• max1 max(a, b, 5)
• print "max1 max1\n"
• print "max max\n"
• sub max
• my(max,n) local variables
• max 0
• foreach n (_at__)
• if(n gt max) max n
• return max
• max4
• max1 5
• max 0

13
Local Variables (2)

• You can initialize local variables
• cat max4
• !/usr/local/bin/perl5 -w
• a 1 b 2
• max 0
• max1 max(a, b, 5)
• print "max1 max1\n"
• print "max max\n"
• sub max
• my(max,n) (0,0) initialized locals
• foreach n (_at__)
• if(n gt max) max n
• return max
• max4
• max1 5
• max 0

14
Local Variables (3)

• You can also load local variables directly from
  _at__
• cat max5
• !/usr/local/bin/perl5 -w
• a 1
• b 2
• max max(a, b)
• print "max max\n"
• sub max
• my(n1, n2) _at__
• if(n1 gt n2) return n1
• else return n2
• max5
• max 2

15
Keyword use strict (1)

• You can force all variables to require
  declaration with my() by starting your program
  with use strict
• cat max5
• !/usr/local/bin/perl5 -w
• use strict
• my a 1 declare and initialize a
• my b 2 declare and initialize b
• my max max(a, b) declare and
  initialize
• print "max max\n"
• sub max
• my(n1, n2) _at__ declare locals from _at__
• if(n1 gt n2) return n1
• else return n2
• max5
• max 2

16
Keyword use strict (2)

• use strict effectively makes all variables local.
  
• Typing mistakes are easier to catch with use
  strict, because you can no longer accidentally
  reference billl instead of bill.
• Programs also run a bit faster with use strict.
• For these reasons, many Perl programmers
  automatically begin every Perl program with use
  strict.
• It is up to you which style you prefer.

17
More on Parameters

• Variables are passed by name in Perl, i.e.,
  parameters from the caller can be changed.
• cat swap
• !/usr/bin/perl -w
• sub swap
• (_0,_1)(_1,_0)
• (a,b)(1,2) swap(a, b)
• print "a, b \n"
• _at_x(1,2)
• swap(_at_x) print "_at_x \n"
• swap
• 2, 1
• 2 1

18
Structures as Parameters

• To pass arrays, hashes or other structures, use
  pointers
• cat vsum
• !/usr/bin/perl -w
• sub vsum
• return undef if (_at__0 ! _at__1)
• for (i0 i lt _at__0 i )
• sumi _0i _1i
• return _at_sum
• _at_a(1,2) _at_b(3,4) _at_sum vsum(\_at_a, \_at_b)
• print "Sum1 _at_sum "
• _at_sum2vsum(2,3,4,5)
• print "Sum2 _at_sum2\n"
• vsum
• Sum1 4 6 Sum2 6 8

_at__0 first array, _at__1 second array _at_
gives element count in a scalar context
_0i i-th element (cf, 2D array)
returns reference of an anonymous array We
cannot use ( ) here
19
Recursive Directory Processing

• Here is the skeleton of a program for recursive
  directory processing
• !/usr/local/bin/perl5 -w
• sub dirtree
• my _at_files lt_0/gt local variable
  required
• print "Directory listing for _0\n"
• foreach (_at_files)
• print "_\n"
• dirtree(_) if (-d _) recursion
• dirtree(ARGV0)