Perl Cheatsheet 1

August 30, 2014

Perl w1

  • a little thought on Perl
    • make sure you know the context when writing perl program
    • make sure you understand which type of var you are using
      • scalar always start with $
      • list always start with @
      • dict always start with %

I/O

  • input from stdin, one line at a time: while (<STDIN>) {…}, this is a shorthand for while (defined(<STDIN>) )
  • input from a handler, store each line as one element in a array, my @ar = <file>
  • input from command-line argument. while (<>) {…} e.g. ./read.pl file1 file2 … will get input from file1, then file2, then…
  • special symbol: all arguments from command-line stores in @ARGV, a special array.
  • get the output from external unix command: open my $file, "ls -al |" open a pipe
  • send the handler to external unix command: open $file, "| lpr"
  • Idiom
    • read file word by word
      • read entire file as string use File::Slurp;
      • read entire file as string my $file = read_file($ARGV[0])
      • split the string into words my @words = split /[ \t\r\n]+/, $file
      • iterate thru for (@words) …
    • read file char by char
      • read entire file as string use File::Slurp
      • split the string into characters my @chars = split //, $file
      • iterate thru for (@chars) …
      • more efficient you can split char line by line
        • while <$file> {@chars = split //}

File handles

  • Filehandler is actually a pointer

  • three default file handlers: STDIN STDOUT STDERR

  • the open tells Perl to ask os to open connection between your program and the outside world
  • open can open any connection to real world, e.g. socket, process, file, pipe…
    • meaning, if you want to input from other text file rather than STDIN, use open HANDLERNAME, "<", "textfile" first, then treat HANDLERNAME just as normal stdin.
    • meaning, if you want to output to other text file rancher than STDOUT, use open HANDLER, ">", "textfile" first, then treat HANDLER just as STDOUT.
    • e.g. open IN, "<", "textfile"; for (@IN) {…} read from file
    • e.g. open OUT, ">", "textfile"; select OUT; print…; write to file
    • e.g. open OUT, ">", "textfile"; print OUT, "…"; write to file just use print
    • open will return true if successfully opened, otherwise false
  • special symbol: $! print the essential information about system error. e.g. print "bad, $!"
    • Bad FileHandle: my $ok = open LOG, ">>", "log.txt"; if(!ok) {…}

Subroutine

  • invoke: &name(parameter1, parameter2,…): e.g. $n = &max(1,2)

  • mutable parameters, all vars get passed into sub are reference, meaning all mutable !

  • special symbol: @_, all parameters passed into subroutine stored in special array @_, you can retrieve any value like normal. e.g. $_[0]
    • important: these $_[2] values has nothing to do with $_, don’t get confused.
    • give name to default parameters: ($name1, $name2) = ($_[0], $_[1]) or ($name1, $name2, $name3…) = @_
  • return value: last expression evaluated will be return value!
    • the return keyword isn’t really necessary, for simplicity, you can omit, because always the last expression gets evaluated return its value
  • prototype: sub add($$) tell perl this sub need exactly two scalar. sub divide($@) tell perl this sub need exactly one scalar and one array

Hash

  • a little thought on Perl
    • make sure you know the context when writing perl program
    • make sure you understand which type of var you are using
      • scalar always start with $
      • list always start with @
      • dict always start with %
  • what is dict
    • key-value pair.
    • key: must be string.
    • value: must be scalar var.
    • random access, no order guarantee !
  • dict assignment
    • my %dict; $dict{"s344"} = "alex";
    • my %dict = qw(s344 alex s345 pcy);
    • my %dict = ("s344", 2, "s355", 1);
    • my %dict = ("s344"=>2, "s444"=>1);
    • the %dict accepts a list, and convert elements to key-value pair, the example above may be converted like “(key,value,key,value)”
  • dict as a whole
    • my %dict; %rev = reverse %dict; will get “(value=>key)” pairs
    • @list = %dict; print "@list" will print a “(key, value)” list
  • dict operation
    • keys: yield a list of all keys in hash. my @k = keys %dict;
    • values: yield a list of all values in hash. my @v = values %dict;
    • iterate thru
      • each: return next key-value pair. ` while( my($k, $v) = each %hash ) {…}` more efficient
      • for (keys %dict) { my $v = $dict{$_} …} less efficient
      • you should not delete a key out of a hash (or add keys) while iterating over it using each(), because this may confuse Perl: instead, creating a new flittered array or hash with push() or dict assignment while iterated thru SRC
    • delete key
      • delete: delete $books{$person} delete key-value pair, tricky is the return value is that key!
    • tips: my $count = keys %dict; return # of keys !
    • tips: print %dict will print content of hash in an array.