Programming in Perl Lecture 4 Examples

File modes:

Mode Symbol       Meaning 
-----------       ---------------------
(nothing)         Open for reading
<                 Open for reading
>                 Open for writing 
                    (destroys current contents)
>>                Open for appending 
                    (preserves current contents; 
                     ready to write at end)
+<                Open for both reading & writing 
                    (preserves current contents)
+>                Open for both reading & writing 
                    (destroys current contents)
+>>               Open for both reading & writing 
                    (preserves current contents; 
                     ready to write at end)

Die examples:

       $tempfile = "temp.out";
       open(TEMP, ">$tempfile") or die("Cannot open $tempfile\n");

       o $! is the error string from the most recent operating system
       error 

       $errorfile = "i.do.not.exist";
       open(IN, "$errorfile") or die("Cannot open $errorfile : $!\n");
       # Cannot open i.do.not.exist : No such file or directory

       o if the error message to be printed does not include a
       newline, Perl will automatically give the script name and line
       number from which die was called

       $errorfile = "i.do.not.exist";
       open(IN, "$errorfile") or die("Cannot open $errorfile : $!;");
       # Cannot open i.do.not.exist : No such file or directory; at
       ./die.plx line 80.

Upcasing a file (output to second file):

print("This script stores the contents of file1 in file2\n");
print("with all letters made uppercase.\n");
print("Enter file1 (the input file): ");
chomp($file = <STDIN>);
print("Enter file2 (the output file): ");
chomp($upper_file = <STDIN>);

open(INFILE, "<$file") or die("Cannot open $file for input : $!\n");
open(UPFILE, ">$upper_file") 
  or die("Cannot open $upper_file for output : $!\n");

while(<INFILE>) {
    print UPFILE ("\U$_");
}

close(INFILE) or die("Cannot close $file : $!\n");
close(UPFILE) or die("Cannot close $upper_file : $!\n");

Upcasing a file (output to the same file):

print("This script changes the contents of the file entered\n");
print("making all letters uppercase.\n");
print("Enter a file name to be upcased: ");
chomp($file = <STDIN>);

# can't use +> because contents would be erased before we did
# anything with them
open(INOUTFILE, "+<$file") 
  or die("Cannot open $file for modification: $!\n");

@in_array = <INOUTFILE>;

for($i=0; $i < @in_array; $i++) {
    $in_array[$i] = "\U$in_array[$i]";
}


# seek(FILEHANDLE, OFFSET, WHENCE) 
# (OFFSET in bytes; WHENCE=0 means relative to beginning of file)
# positions file pointer back at start of INOUTFILE
seek(INOUTFILE, 0, 0);

print INOUTFILE (@in_array);

close(INOUTFILE) or die("Cannot close $file : $!\n");

Function example (return values):

        sub max_of_x_and_y {
          if($x > $y) {
            return $x;
          } else {
            return $y;
          }
        }

Function example (arguments):

        sub say {
          print("$_[0], $_[1]!\n");
        }

        say("hello", "world");

Function example (more arguments and pass by reference):

        sub largest {
         if (@_ == 0) {
           print("Warning no parameters passed to largest\n");
           return undef;
         }
         $max = $_[0];
         for($i = 1; $i < @_; $i++) {
           if($_[$i] > $max) {
             $max = $_[$i];
           }
         }
         return $max;
       }

       sub pass_by_ref {
        for($i = 0; $i < @_; $i++) {
          $_[$i] = "hi";
        }
        return $max;
      }

       $a = largest(10,200,300,49);
       print("$a\n");
       @g = (10,200,300,49);
       $a = largest(@g);
       print("$a\n");

       @g = (10,200,3,49);
       $a = pass_by_ref(@g);
       print("$a\n"); # 300 (global !)
       print("@g\n"); # hi hi hi hi
       $a = pass_by_ref(10,200,300,49);
       print("$a\n");
# Modification of a read-only value attempted at ./largest.plx line 39
# nothing printed

MY operator (private variable - lexically scoped):

        sub largest2 {
         # local variable to allow call by value
         my (@values) = @_;
         # local variable; any assignment will
         # not effect a global $max
         my ($max);
         if (@values == 0) {
           print("Warning no parameters passed to largest\n");
           return undef;
         }
         $max = $values[0];
         for($i = 1; $i < @values; $i++) {
           if($values[$i] > $max) {
             $max = $values[$i];
           }
         }
         return $max;
       }

LOCAL function (semiprivate variable - dynamically scoped):

        $value = "original";

        tellme();
        spoof();
        tellme();

        sub spoof {
          local ($value) = "temporary";
          tellme();
        }

        sub tellme {
          print("Value is $value.\n");
        }

        # prints
Value is original.
Value is temporary.
Value is orginal.