Random Bits of Perl

1
Random Bits of Perl

• None of this stuff is worthy of its own lecture,
  but its all a bunch of things you should learn
  to use Perl well

2
eval

• From Camel eval is the way to do all exception
  handling in Perl.
• Evaluate Perl code
• make divide-by-zero non-fatal
• eval answer a / b warn _at_ if _at_
• trap errors
• using block notation, if there are any run-time
  errors, eval returns undef and sets error in _at_

3
Back-ticks

• Third kind of quoted string
• Executes an external program and returns the
  output
• _at_files ls .html
• Your script will run ls .html from the command
  prompt, and store the output of that program in
  _at_files
• Back-ticks do interpolation just like double
  quotes
• Note that this is not the same as the system
  function
• system returns return value of the called
  program, not the output

4
Quoting operators

• You may get strings that end up looking messy
  because of all the quotes
• s He said \She said \They said \This is
  bad.\\\
• can use the quoting operator qq// to choose your
  own quotes.
• Much like choosing your own delimiters in pattern
  matching
• s He said qq/She said qq(They said qqThis is
  bad)/
• You could argue this doesnt look much better
• You could be right.

5
Many quoting operators

• In each case below, you can use any
  non-alpha-numeric character for the delimiter,
  just as you can with m// and s///
• q// - single quoted string
• qq// - double quoted string
• does interpolation
• qx// back-ticked string
• qw// - quote words
• qw/Mon Tue Wed/ ? (Mon, Tue, Wed)
• qr// - quote regular expression
• evaluate string as though its a RegExp
• Then use result in an actual pattern match
• mostly for efficiency

6
map

• map expression list
• evaluate expression (or a block) for each value
  of list. Sets _ to each value of list, much
  like a foreach loop
• Returns a list of all results of the expression
• _at_words map split _at_lines
• Set _ to first member of _at_lines, run split
  (split acts on _ if no arg provided), push
  results into _at_words, set _ to next member of
  _at_lines, repeat.

7
grep

• Similar to map
• returns a list of all members of the original
  list for which evaluation was true
• (map returns list of all return values of each
  evaluation)
• Typically used to pick out lines you want to keep
• _at_code grep !/\s/, _at_all_lines
• removes all lines beginning with comments
• Assigns _ to each member of _at_all_lines, then
  evaluates the pattern match. If pattern match is
  true, the _ is added to _at_code

8
do

• Similar to C/C do/while construct.
• Execute entire block following do once. If block
  followed by a while modifier, check the
  conditional, and then redo the loop.
• Major Difference in Perl, do is not a looping
  structure.
• Cannot use next, last, or redo.

9
Here Documents (heredocs)

• My favorite Perl construct..
• Simple form
• print ltltEOF
• print this string
• EOF
• ltlt is operator no space between ltltEOF
• EOF is identifier must be at start of ending line

10
heredocs

• Less simple, no semi-colon after identifier
• print ltltNUMBER
• 1024
• NUMBER
• 1024 prints 2048

11
heredocs

• With backticks around identifier
• print ltltEOC
• echo something
• EOC
• Will execute command echo and print the results

12
heredocs

• Double-Quoted identifier same as no quotes,
  single quoted identifier escapes interpolating
• print ltltEOF
• do not print this string not interpolated
• EOF
• The type of quotes used determines how Perl
  treats text

13
heredocs

• Lather, rinse and repeat
• Print ltlt x 10
• repeat this string ten times!
• The blank line at end is identifier, signified by
  , which are of course optional, but deprecated.

14
heredoc

• Sending heredoc string somewhere else besides
  STDOUT
• print ltltEOF
• equivalent to
• print STDOUT ltltEOF
• So replacing with a file descriptor is obvious
• print ltltOUTPUT
• send this output to a file
• OUTPUT

15
Final heredoc example

• Heredocs VERY useful in CGI (or javascripting,
  etc)
• my title this is the title
• my x_title ltltEOF
• lttitlegttitlelt/titlegt
• EOF
• print x_title
• Output lttitlegtthis is the titlelt/titlegt
• Eliminates print lttitlegttitlelt/titlegt\n