Exercise 3 : Reading and writing files

Laboratory Exercise for Introduction to Perl

This assumes you have already set up your initial directory with the init-perllab command (instructions).

---

Reading from a file (cabeza.pl)

1. Change to the Lab3 directory:
   
   cd /scratch/mylogin/PerlLab/Lab3
   
   The directory path may be different if you installed the files in another location.
2. List the directory contents:
   
   ls -l
   
   You should see the files: INSTRUCTIONS README Solutions cabeza.pl pairs.pl
3. Look at the cabeza.pl file:
   
   cat cabeza.pl
   
   The contents of this file are as follows (note the line numbers have been added for clarity and are not in the actual file):

   cabeza.pl

   #!/usr/bin/perl
    
   # cabeza.pl - print the first 10 lines of a given file
    
   use warnings;
   use strict;
    
   # This is the first argument on the command line
   my $filename = shift(@ARGV);
    
   # Check that file exists and is readable
   unless (-r $filename)
   {
       die "could not read file ($filename)";
   }
    
   # Open the file, read up to 10 lines or to the end of file, and
   # output the lines to standard out.  This should operate like the
   # unix command "head".

4. Edit the cabeza.pl program so it will open the file specified in the variable $filename, read up to 10 lines, and output them to standard out. For example, to print the first 10 lines of the file "INSTRUCTIONS", it will look like:
   

   command

   ./cabeza.pl INSTRUCTIONS

   
   

   output

   http://sc.tamu.edu/shortcourses/SC-perl/lab/ex2/
   
     Exercise 3 : Reading and writing files
   
   
       Laboratory Exercise for Introduction to Perl
   
   This assumes you have already set up your initial directory with the
   init-perllab command.
   
   

   
   
5. For reference, read the perl documentation on open() or the tutorial on using open(). You can also view these from eos by typing:
   
   perldoc -f open
   man perlopentut
   

SOLUTION

---

Writing to a file (pairs.pl)

1. Look at the pairs.pl file:
   
   cat pairs.pl
   
   The contents of this file are as follows (note the line numbers have been added for clarity and are not in the actual file):

   pairs.pl

   #!/usr/bin/perl
    
   # pairs.pl - save a list of numeric pairs to a file
   #
   #  Write to the file "squares.dat" a list.  Each line
   #  will contain a number (i) and the square of that number.
   #  The starting and ending number will be specified as user
   #  input.
    
   use warnings;
   use strict;
    
   my $out_filename = 'squares.dat';
    
   my ($start, $end);
    
   print "Enter starting value: ";
   $start = <>;
    
   print "Enter ending value: ";
   $end = <>;
    
   # check that $start <= $end
    
   # open $out_filename for writing
    
   # make a loop ($i) from $start to $end
   #  write $i and $i*$i to the file
    
   # close the file

2. Edit the program so it will open the file "squares.dat" and write a line for each value from the value in $start to the value in $end. On each line, print the value and the square of the value. For example, an input of 3 for start and 11 for end would result in the file "squares.dat" containing:

   output

     3   9
     4  16
     5  25
     6  36
     7  49
     8  64
     9  81
    10 100
    11 121
   

   
   
3. For reference, read the perl documentation on open(), printf(), and die(). You can also view these from eos by typing:
   
   perldoc -f open
   perldoc -f printf
   perldoc -f die
   

SOLUTION

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