2000 Copyrights, Danielle S. Lahmani Foreach example = ( 3, 5, 7, 9) foreach $one ) { $one*=3; }...

2000 Copyrights, Danielle S. Lahmani

Foreach example

• Example:@numbers = ( 3, 5, 7, 9)foreach $one (@numbers ) {

$one*=3;}

# @numbers is now (9,15,21,27)


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

$hashname{key}


HASH: Examples

• Example: %carcolor = ("buick", "yellow");

• $mycolor = $carcolor{ "buick"};• $mycolor is now "yellow"• %copy = %original; # copy from

%original to %copy


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 $fred{$key} \n"; # show key and value


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");

• @lastname = values(%lastname); #grab the values


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";}


Hash function: delete

• removes hash elements, takes a hash reference as argument

• delete $lastname{"barney"}; #lastname has only one key-value pair now.


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


BASIC I/O Input from STDIN• Perl uses the variable $_ to contain the line read

from STDIN.• $a = <STDIN>; #reads the next line• @a = <STDIN>; # reads all lines until control ^D• typically

while (defined ($line = <STDIN>) {# process $line here

}when no more lines read. <STDIN> returns undef.


using the diamond operator <>

<> operates like <STDIN>, but gets data from file or files

• specified on the command line that invoked the PERL program.

• <> looks at the @ARGV array#!/usr/bin/perlwhile (<>) {

print $_;}


Output to STDOUT

• print for normal output• printf for formatted output


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 (<STDIN> = ~ /^[yY]/) {

print" what can I do for you? ";


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


Example of split function

$line = "merlyn::118:120:Randal:/home/merlyn:/usr/bin/perl";

@fields = split(/:/, $line); # split $line, using : as delimiter

# now @fields is ("merlyn,"", "118", "120", "Randal",

# "/home/merlyn", "/usr/bin/perl")


join function

• takes a list of values and glues them together with a glue string between each list element.

• Example: $outline = join(":", @fields);


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.


Function Arguments

• Subroutine invocation is followed by a list within parenthesis

• Causing the list to be automatically assigned to a special variable named @_ for the duration of the subroutine.

• $_[0] is the 1st element of the @_ array• @_ variable is private to the subroutine.


Function private variables

• using the my operatorsub add {

my ($sum); #make $sum a local variableforeach $_ ( @_) {$sum += $_; # add each element

}return $sum #last expression evaluated

}


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.


FILEHANDLES• Recommendation use all uppercase letters in your

filehandles .• 3 files handles, STDIN, STDOUT, STDERR for

standard in, out and error.open(FILEHANDLE, "filename");

>"filename">> "filename”

die: equivalent to "open that file or die."open(FILEHANDLE,>"filename") | |

die "Sorry could not create filename\n";• Perl provides -op file tests just like the shells.


Perl modules

• TBD


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:


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";

}


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


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.

2000 Copyrights, Danielle S. Lahmani Foreach example = ( 3, 5, 7, 9) foreach $one ) { $one*=3; }...

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