Foreach example Example numbers 3 5 7 9

Foreach example • Example: @numbers = ( 3, 5, 7, 9) foreach $one (@numbers ) { $one*=3; } # @numbers is now (9, 15, 21, 27) 2000 Copyrights, Danielle S. Lahmani

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} 2000 Copyrights, Danielle S. Lahmani

HASH: Examples • Example: %carcolor = ("buick", "yellow"); • $mycolor = $carcolor{ "buick"}; • $mycolor is now "yellow" • %copy = %original; # copy from %original to %copy 2000 Copyrights, Danielle S. Lahmani

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 2000 Copyrights, Danielle S. Lahmani

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 2000 Copyrights, Danielle S. Lahmani

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 $lastn"; } 2000 Copyrights, Danielle S. Lahmani

Hash function: delete • removes hash elements, takes a hash reference as argument • delete $lastname{"barney"}; #lastname has only one key-value pair now. 2000 Copyrights, Danielle S. Lahmani

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 2000 Copyrights, Danielle S. Lahmani

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. 2000 Copyrights, Danielle S. Lahmani

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/perl while (<>) { print $_; } 2000 Copyrights, Danielle S. Lahmani

Output to STDOUT • print for normal output • printf formatted output 2000 Copyrights, Danielle S. Lahmani

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> = ~ /^[y. Y]/) { print" what can I do for you? "; 2000 Copyrights, Danielle S. Lahmani

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 2000 Copyrights, Danielle S. Lahmani

Example of split function $line = "merlyn: : 118: 120: Randal: /home/merlyn: /usr/bi n/perl"; @fields = split(/: /, $line); # split $line, using : as delimiter # now @fields is ("merlyn, "", "118", "120", "Randal", # "/home/merlyn", "/usr/bin/perl") 2000 Copyrights, Danielle S. Lahmani

join function • takes a list of values and glues them together with a glue string between each list element. • Example: $outline = join(": ", @fields); 2000 Copyrights, Danielle S. Lahmani

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. 2000 Copyrights, Danielle S. Lahmani

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 1 st element of the @_ array • @_ variable is private to the subroutine. 2000 Copyrights, Danielle S. Lahmani

Function private variables • using the my operator sub add { my ($sum); #make $sum a local variable foreach $_ ( @_) { $sum += $_; # add each element } return $sum #last expression evaluated } 2000 Copyrights, Danielle S. Lahmani

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. 2000 Copyrights, Danielle S. Lahmani

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 filenamen"; • Perl provides -op file tests just like the shells. 2000 Copyrights, Danielle S. Lahmani

Perl modules • TBD 2000 Copyrights, Danielle S. Lahmani

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: 2000 Copyrights, Danielle S. Lahmani

Database interface example dbmopen(%ARRAYNAME, "dbmfilename", $mode); dbmopen(%FRED, . "mydatabase", 0644); delete $FRED{"barney"} while (($key, $value) = each(%FRED)) { print "$key has value of $valuen"; } 2000 Copyrights, Danielle S. Lahmani

perl debugging • • perl -d h print out a help message T stack trace s insgle step n next f finish c continue 2000 Copyrights, Danielle S. q quit Lahmani

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