Perl IO

1
Software Tools

• Perl I/O

2
Input from STDIN

• Reading from STDIN is easy, and we have done it
  many times.
• a ltSTDINgt
• In a scalar context, this gives the next line of
  input, or undef if there are are no more lines
  (the user has hit CTRL-d).
• In a list context, this gives all remaining lines
  (all the lines until the user hits CTRL-d).
• _at_a ltSTDINgt
• Each element is one line, and includes the
  terminating newline (the last line may or may not
  have a newline).

3
Input from STDIN

• Typically, a program will read in one line at a
  time and process the line
• while(defined(line ltSTDINgt))
• process line
• print "line"
• In the above example, defined() returns true
  unless its argument is undef.
• As long as a line has been read, ltSTDINgt returns
  a defined value, and the loop continues.
• When ltSTDINgt has no more lines to read (it hits
  CTRL-d), it returns undef, terminating the loop.

4
Input from STDIN

• Here is an example of program input and output.
• White text is typed by the user, green text is
  printed by the program or shell.
• cat line1
• !/usr/local/bin/perl5 -w
• while(defined(line ltSTDINgt))
• process line
• print "line"
• line1
• hi
• hi
• test
• test
• CTRL-d

5
Input from STDIN

• We can also read and print the lines altogether.
• Again, white text is typed by the user, green
  text is printed by the program or shell.
• cat line2
• !/usr/local/bin/perl5 -w
• _at_lines ltSTDINgt
• process lines
• print "_at_lines"
• line3
• hi
• test
• CTRL-d hi
• test

6
Input from STDIN

• Perl has a shortcut for reading a value from
  ltSTDINgt into _.
• Whenever a condition consists solely of ltSTDINgt,
  Perl automatically copies the line into _.
• while(ltSTDINgt)
• same as while(defined(_ ltSTDINgt))
• chomp same as chomp "_"
• print "_\n"
• _ is the default for many operations, such as
  chomp and print.

7
Input from ltgt

• Another way to read input is with ltgt.
• ltgt returns a single line in a scalar context, or
  all the remaining lines in a list context.
• However, unlike ltSTDINgt, ltgt gets its data from
  the file (or files) specified on the command
  line.
• cat f1
• 1
• cat f2
• 2
• cat mycat
• !/usr/local/bin/perl5 -w
• while(ltgt)
• print
• mycat f1 f2
• 1
• 2

8
Input from ltgt

• If you do not specify any filenames on the
  command line, ltgt reads from standard input
  automatically.
• cat mycat
• !/usr/local/bin/perl5 -w
• while(ltgt)
• print
• mycat
• hi
• hi
• test
• test
• CTRL-d
• ltgt is the easiest way to read files in Perl (we
  will learn more advanced ways later).

9
Output to STDOUT

• We have also often used print.
• The print function takes a list of strings, and
  sends each to STDOUT, without adding any
  characters between them.
• print "Hi Bill!\n"
• print "Hi ", "Bill!", "\n"
• print("Hi ", "Bill!", "\n")
• Sometimes, you will need to add parentheses to
  print, especially when the first thing starts
  with a left parenthesis
• print (11), "cheap Bill\n" problem!
• print ((11), "cheap Bill\n") ok
• print 11, "cheap Bill\n" also ok

10
printf Formatted Output

• You may wish to have more control over your
  output than print provides.
• The printf function takes a list of arguments,
  where the first argument is a format control
  string, which defines how to print the remaining
  arguments (like printf in C).
• printf "s d f\n", s, n, real
• Format types
• strings s
• integer numbers d
• floating-point numbers f

11
printf Formatted Output

• cat printf1
• !/usr/local/bin/perl5 -w
• s "1234567890"
• n 12345
• real 1234567.123
• printf "15s 5d 10.2f\n", s, n, real
• printf1
• 1234567890 12345 1234567.12
• This example prints s in a 15-character field,
  then space, then n as a decimal integer in a
  5-character field, then another space, then real
  as a floating-point value with 2 decimal places
  in a 10-character field, and finally a newline.