Notes: Perl Lab 07

Outline

  1. Hash Syntax
  2. Assigning Values
  3. Key Values
  4. %ENV hash

  1. Hash Syntax

     Hashes, also referred to as "associative arrays", are another type of list variable, similar to arrays. Unlike arrays, the inidividual scalar elements of a hash are accessed by a string value rather than a numeric index. In list context, the hash will begin with the percent (%) character in the same way that arrays are designated by the (@) character. In scalar context (single values) the valuess of the hash will be accessed by the scalar ($) with the subscript string enclosed in curly braces in the same way that arrays have their index number subscripted in square braces.
    %hash; # a hash
    @array; # an array
    $hash{'key_string'} = 'some value';
    The following code shows the very basic methods of assigning a value to a hash and accessing those scalar values by their keys.

      
    #!/usr/bin/perl -w
# Name: Mark Tucker
# Assignment: Lab07 Example 00
# Description: Very Basic hash syntax example
#==========================================================================

$fname{'Tucker'} = 'Mark';

$total{Jan} = 12.75;
$total{Feb} = 42;

print "Tucker's first name is $fname{'Tucker'}\n";

$month = 'Jan';
print "The total for $month is $total{$month}.\n";

exit;
# DONE

    When executed, the script above produces the following output:
      
    [mark@iguana perl] ./lab07_0.pl 
Tucker's first name is Mark
The total for Jan is 12.75.
[mark@iguana perl]

    Above, the keys for the hash %total are 'Jan' and 'Feb' with values of 12.75 and 42, respectively. The second print statement shows the use of variable substitution in the hash subscript.

  2. Assigning Values

     Since hashes are just a list of name value pairs, they may be assigned values in the same way that arrays are. The code example below shows that a hash can be assigned it's keys and values as a list.

      
    #!/usr/bin/perl -w

# Name: Mark Tucker
# Assignment: Lab07 Example 01
# Description: Assigning values to a hash as a list.
#==========================================================================

# assigning values to the hash as a list
%station_temps = ( 'BTV', 32, '1V4', 28, 'MPV', 30,
                   'RUT', 35, 'CXX', 33,
                   'DDH', 40,
                   'JAY', 25
                   );

# sample output accessing the hash values
$station = 'RUT';
print "Temp in $station is $station_temps{$station}\n";

# create an array from the hash
@some_array = %station_temps;

# list the values of the new array.
$count = 0;
foreach $k (@some_array) {
    print "element $count is $k\n";
    $count++;
}

# some examples of what you cannot do
print "Printing some_array as a hash $some_array{DDH} ??\n";
print "Printing station_temps as an array: $station_temps[3]\n";

exit;
# DONE

    When executed, the script above produces the following output:
      
    [mark@iguana perl] ./lab07_1.pl
Temp in RUT is 35
element 0 is JAY
element 1 is 25
element 2 is DDH
element 3 is 40
element 4 is CXX
element 5 is 33
element 6 is RUT
element 7 is 35
element 8 is BTV
element 9 is 32
element 10 is MPV
element 11 is 30
element 12 is 1V4
element 13 is 28
Use of uninitialized value in concatenation (.) or string at lab07_1.pl line 30.
Printing some_array as a hash  ??
Use of uninitialized value in concatenation (.) or string at lab07_1.pl line 31.
Printing station_temps as an array: 

[mark@iguana perl]

    Notice that the array @some_array can be assigned the values of the hash on line 20 of the script above. When listed as an array, the key values all have odd indexes and the odd indexed elements contain the values of the hash. Also, you cannot access a hash as an array nor can an array be accessed as a hash. The two are NOT interchangeable.

    Below shows another syntax for assigning key/values pairs in a hash

      
    #!/usr/bin/perl -w

# Name: Mark Tucker
# Assignment: Lab07 Example 02
# Description: Assigning values to a hash as a list.
#==========================================================================

# assigning values to the hash as a list
%station_temps = ( 'BTV' => 32,
                   '1V4' => 28,
                   'MPV' => 30,
                   'RUT' => 35, 
                   'CXX' => 33,
                   'DDH' => 40,
                   'JAY' => 25
                   );

# sample output accessing the hash values
$station = 'MPV';
print "Temp in $station is $station_temps{$station}\n";

exit;
# DONE

    When executed, the script above produces the following output:
      
    [mark@iguana perl] ./lab07_2.pl 
Temp in MPV is 30
[mark@iguana perl]

    Values of a hash can be assigned based on external data. In the case below:

      
    #!/usr/bin/perl -w

# Name: Mark Tucker
# Assignment: Lab07 Example 03
# Description: Assigning values to a hash from external data
#==========================================================================

$datafile = '/mnt/homes/CLASSES/MET4720/LAB07/months.list';

# check and open the data file
if( -r $datafile) {
    open(DF, "<$datafile") || die "cannot open file: $!";
}else{
    print "Cannot read file: $datafile\n";
    exit;
}

while($line = <DF>) {
    chomp($line);

    # split the line into values
    ($month,$days,$end_julian) = split(/,/, $line);

    # assign hash values with the month as the key
    $num_days{$month} = $days;
    $ending_julian{$month} = $end_julian;
}

close(DF);

# Ask the user what month to use
print "Enter a month (3 characters): ";
chomp($enter_mon = <STDIN>);

# user output
print "$enter_mon has $num_days{$enter_mon} ending with julian day ".
    "$ending_julian{$enter_mon}\n";

exit;
# DONE

    When executed, the script above produces the following output:
      
    [mark@iguana perl] cat months.list 
Jan,31,31
Feb,28,59
Mar,31,90
Apr,30,120
May,31,151
Jun,30,181
Jul,31,212
Aug,31,243
Sep,30,273
Oct,31,304
Nov,30,334
Dec,31,365
[mark@iguana perl] 
[mark@iguana perl] ./lab07_3.pl
Enter a month (3 characters): Jul
Jul has 31 ending with julian day 212
[mark@iguana perl]

  3. Hash Keys

     The keys to a hash can be accessed with the keys function. The keys function returns all the key strings for the hash in a list (think array).

      
    #!/usr/bin/perl -w

# Name: Mark Tucker
# Assignment: Lab07 Example 04
# Description: working with hash keys
#==========================================================================

# assigning values to the hash as a list
%station_temps = ( 'BTV' => 32,
                   '1V4' => 28,
                   'MPV' => 30,
                   'RUT' => 35, 
                   'CXX' => 33,
                   'DDH' => 40,
                   'JAY' => 25
                   );

# store all the key values in an array.
@hash_keys = keys(%station_temps);

print "@hash_keys\n";

# dump all the values and keys to the screen
foreach $k (sort keys %station_temps) {
    print "$k -> $station_temps{$k}\n";
}

exit;
# DONE

    When executed, the script above produces the following output:
      
    [mark@iguana perl] ./lab07_4.pl
JAY DDH CXX RUT BTV MPV 1V4
1V4 -> 28
BTV -> 32
CXX -> 33
DDH -> 40
JAY -> 25
MPV -> 30
RUT -> 35
[mark@iguana perl]

  4. The %ENV Hash

     One of the built in variables available in perl is the %ENV hash. It contains all the environment variable names and their values. This is the data that can be viewed using the env command on Unix systems or with the set command under windows.

      
    #!/usr/bin/perl -w

# Name: Mark Tucker
# Assignment: Lab07 Example 05
# Description: Example of the %ENV hash
#==========================================================================

# dump all the values and keys to the screen
foreach $k ( keys %ENV) {
    print "$k -> $ENV{$k}\n";
}

exit;
# DONE

    When executed, the script above produces the following output:
      
    [tuckerm@apollo LAB07] ./lab07_5.pl |head -5
SSH_CLIENT -> 216.114.179.103 33612 22
HOST -> apollo
TSTRM_WARN -> /mnt/data/gempak/nwx/watch_warn/tstrm_warn
TEXT_DATA -> /mnt/data/gempak/nwx
DORADE_DIR -> /mnt/data/gempak/rawnids
[tuckerm@apollo LAB07] 
[tuckerm@apollo LAB07] env |head -5
USER=tuckerm
LOGNAME=tuckerm
HOME=/mnt/homes/tuckerm
PATH=/software/mcidas/bin:/usr/local/sbin:/usr/sbin:/sbin:/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/usr/games:/usr/lib/j2re1.4.1_02/bin:.:/software/gempak/GEMPAK5.6k/bin/linux:/software/gempak/GEMPAK5.6k/bin/scripts:/opt/kde/bin:/software/vmware/bin
MAIL=/var/mail/tuckerm
[tuckerm@apollo LAB07]

last updated: 18 Mar 2012 13:06