Foreach example

1
Foreach example

• Example
• _at_numbers ( 3, 5, 7, 9)
• foreach one (_at_numbers )
• one3
• _at_numbers is now (9,15,21,27)

2
PERL HASHES

• A hash is a collection of scalar data with
  individual elements selected by some index.
• Index values are arbitrary scalars called keys
  They are used to retrieve values from the array.
• Elements of a hash have no special order
• A hash is denoted by "" sign
• Elements of a hash are referenced by
  hashnamekey

3
HASH Examples

• Example carcolor ("buick", "yellow")
• mycolor carcolor "buick"
• mycolor is now "yellow"
• copy original copy from original to
  copy

4
HASH FUNCTIONS

• keys functions
• keys(hashname) yields a list of the current
  keys in the hash hashname.
• Example keys(hashname) keys hashname
• once for each key of fred
• foreach key (keys (fred))
• print "at key we have fredkey \n"
• show key and value

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HASH FUNCTIONS (CONT')

• values function
• values(hashname) yields a list of the current
  valuesof hashname in the same order as keys
  returned by keys(hashname)
• lastname ("barney", "flinstone", "gerry",
  "smith")
• _at_lastname values(lastname) grab the
  values

6
HASH FUNCTIONS (each)

• each function
• each(hashname) returns a key-value pair as a
  two element list.
• Used to iterate over an entire hash (examining
  every element of ).
• Example
• while ((first, last)) each(lastname))
• print "the last name of first is last\n"

7
Hash function delete

• removes hash elements, takes a hash reference as
  argument
• delete lastname"barney" lastname has only
  one key-value pair now.

8
CONTROL STRUCTURES

• Perl supports "if", "for" while" similar than
  those in C.
• "foreach" constructs is from the C shell
• foreach example
• If the list we are iterating over is made of real
  variables Rather than some functions returning a
  list value,
• Then the variable being used for iteration is in
  fact An alias for each variable in the list
  instead of being A merely copy of the values

9
BASIC I/O

• Input from STDIN
• Perl uses the variable _ to contain the line
  read from STDIN.
• a ltSTDINgt reads the next line
• _at_a ltSTDINgt reads all lines until control D
• typically
• while (defined (line ltSTDINgt)
• process line here
• when no more lines read. ltSTDINgt returns undef.

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using the diamond operator ltgt

• ltgt operates like ltSTDINgt, but gets data from
  file or files
• specified on the command line that invoked the
  PERL program.
• ltgt looks at the _at_ARGV array
• !/usr/bin/perl
• while (ltgt)
• print _

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Output to STDOUT

• print for normal output
• printf for formatted output

12
REGULAR EXPRESSIONS

• PERL supports the same regular expressions as in
  SED.
• match operator
• It takes a regular expression operator on the
  right side and changes the target of the operator
  to some value.
• The target of the operator can be any
  expression that yields some scalar string value.
• Example if (ltSTDINgt /yY/)
• print" what can I do for you? "

13
Regular expressions split function

• split function takes a regular expression and a
  string, and looks for all occurrences of the
  regular expression withinthat string.
• Parts of the string that don't match the regular
  expression are returned in sequence as a list of
  values

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Example of split function

• line "merlyn118120Randal/home/merlyn/usr/
  bin/perl"
• _at_fields split(//, line) split line,
  using as delimiter
• now _at_fields is ("merlyn,"", "118", "120",
  "Randal",
• "/home/merlyn", "/usr/bin/perl")

15
join function

• takes a list of values and glues them together
  with a glue string between each list element.
• Example outline join("", _at_fields)

16
PERL FUNCTIONS

• Defining a user function
• Sub subname
• Statement_1
• Statement_2
• Statement_3
• return value is the value of the return statement
  or of the last expression evaluated in the
  subroutine.

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Function Arguments

• Subroutine invocation is followed by a list
  within parenthesis
• Causing the list to be automatically assigned to
  a special variable named _at__ for the duration of
  the subroutine.
• _0 is the 1st element of the _at__ array
• _at__ variable is private to the subroutine.

18
Function private variables

• using the my operator
• sub add
• my (sum) make sum a local variable
• foreach _ ( _at__)
• sum _ add each element
• return sum last expression evaluated

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Functions Semi-private variables

• semi-private variables using local
• local variables are visible to functions called
  from within the block in which those variables
  are declared.

20
FILEHANDLES

• Recommendation use all uppercase letters in your
  filehandles .
• 3 files handles, STDIN, STDOUT, STDERR for
  standard in, out and error.
• open(FILEHANDLE, "filename")
• gt"filename"
• gtgt "filename
• die equivalent to "open that file or die."
• open(FILEHANDLE,gt"filename")
• die "Sorry could not create filename\n"
• Perl provides -op file tests just like the shells.

21
Perl modules

• TBD

22
USER DATABASE MANIPULATION

• Most UNIX systems have a standard library called
  DBM,
• Which allows programs to store a collection of
  key-value pairs
• Into a pair of disk files.
• In Perl, a hash may be associated with a DBM
  through a process
• Similar to opening a file.
• dbmopen function associates a DBM database with a
  DBM array

23
Database interface example

• dbmopen(ARRAYNAME, "dbmfilename", mode)
• dbmopen(FRED, ."mydatabase", 0644)
• delete FRED"barney"
• while ((key, value) each(FRED))
• print "key has value of value\n"

24
perl debugging

• perl -d
• h print out a help message
• T stack trace
• s insgle step
• n next
• f finish
• c continue
• q quit
• D delete all breakpoints

25
SYSTEM CALLS

• Perl provides an interface to many UNIX system
  calls.
• Interface is via Perl functions, not directly
  through the system call library.
• The interface use is dependent on the
  implementation and version of Perl being used.