NAME

List::RewriteElements - Create a new list by rewriting elements of a first list

SYNOPSIS

use List::RewriteElements;

Constructor

Simplest case: Input from array, output to STDOUT.

$lre = List::RewriteElements->new( {
    list        => \@source,
    body_rule   => sub {
                        my $record = shift;
                        $record .= q{additional field};
                   },
} );

Input from file, output to STDOUT:

$lre = List::RewriteElements->new( {
    file        => "/path/to/source/file",
    body_rule   => sub {
                        my $record = shift;
                        $record .= q{,additional field};
                   },
} );

Provide a different rule for the first element in the list:

$lre = List::RewriteElements->new( {
    file        => "/path/to/source/file",
    header_rule => sub {
                        my $record = shift;
                        $record .= q{,ADDITIONAL HEADER};
                   },
    body_rule   => sub {
                        my $record = shift;
                        $record .= q{,additional field};
                   },
} );

Input from file, output to file:

$lre = List::RewriteElements->new( {
    file        => "/path/to/source/file",
    body_rule   => sub {
                        my $record = shift;
                        $record .= q{additional field};
                   },
    output_file => "/path/to/output/file",
} );

To name output file, just provide a suffix to filename:

$lre = List::RewriteElements->new( {
    file            => "/path/to/source/file",
    body_rule       => sub {
                        my $record = shift;
                        $record .= q{additional field};
                       },
    output_suffix   => '.out',
} );

Provide criteria to suppress output of header or individual record.

$lre = List::RewriteElements->new( {
    file            => "/path/to/source/file",
    header_suppress => sub {
                        my $record = shift;
                        return if $record =~ /$somepattern/;
                    },
    body_suppress   => sub {
                        my $record = shift;
                        return if $record ne 'somestring';
                    },
    body_rule       => sub {
                        my $record = shift;
                        $record .= q{additional field};
                    },
} );

Generate Output

$lre->generate_output();

Report Output Information

$path_to_output_file    = $lre->get_output_path();

$output_file_basename   = $lre->get_output_basename();

$output_row_count       = $lre->get_total_rows();

$output_record_count    = $lre->get_total_records();

$records_changed        = $lre->get_records_changed();

$records_unchanged      = $lre->get_records_unchanged();

$records_deleted        = $lre->get_records_deleted();

$header_status          = $lre->get_header_status();

DESCRIPTION

It is common in many situations for you to receive a flat data file from someone else and have to generate a new file in which each row or record in the incoming file must either (a) be transformed according to some rule before being printing to the new file; or (b) if it meets certain criteria, not output to the new file at all.

List::RewriteElements enables you to write such rules and criteria, generate the file of transformed data records, and get back some basic statistics about the transformation.

List::RewriteElements is useful when the number of records in the incoming file may be large and you do not want to hold the entire list in memory. Similarly, the newly generated records are not held in memory but are immediately printed to STDOUT or to file.

On the other hand, if for some reason you already have an array of records in memory, you can use List::RewriteElements to apply rules and criteria to each element of the array and then print the transformed records (again, without holding the output in memory).

SUBROUTINES

`new()`

Purpose: List::RewriteElements constructor.

Arguments: Reference to a hash holding the following keys:

file or list

The hash must hold either a file element or a list element -- but not both! The value for the file key must be an absolute path to an input file. The value for list must be a reference to an array in memory.
body_rule

The hash must have a body_rule element whose value is a reference to a subroutine providing a formula for the transformation of an individual record in the incoming file to a record in the outgoing file. The first argument passed to this subroutine must be the record from the incoming file. The return value from this subroutine should be a string immediately ready for printing to the output file (though the string should not end in a newline, as printing will be handled by generate_output()).
body_suppress

Optionally, you may provide a body_suppress element whose value is a reference to a subroutine providing a criterion according to which an individual record in the incoming file should be output to the outgoing file or not output, i.e., omitted from the output entirely. The first argument to this subroutine should be the record from the incoming file. The subroutine should, at least implicitly, return a true value when the record should be output. The subroutine should simply return, <i.e.>, return an implicit undef, when the record should be omitted from the outgoing file.
header_rule

Frequently the first row in a flat data file is a header row containing, say, the names of the columns in a data table, joined by a delimiter. Because the header row is different from all subsequent rows, you may optionally provide a header_rule element whose value is a reference to a subroutine providing a formula for the transformation of the header row in the incoming file to the header in the outgoing file. The first argument passed to this subroutine must be the header row from the incoming file. The return value from this subroutine should be a string immediately ready for printing to the output file (though the string should not end in a newline, as printing will be handled by generate_output()).
header_suppress

Optionally, if you have provided a header_rule element, you may provide a header_suppress element whose value is a reference to a subroutine providing a criterion according to which an the header row from the incoming file should be output to the outgoing file or not output, i.e., omitted from the output entirely. The first argument to this subroutine should be the header from the incoming file. The subroutine should, at least implicitly, return a true value when the header should be output. The subroutine should simply return, <i.e.>, return an implicit undef, when the header should be omitted from the outgoing file.
output_file or output_suffix

It is recommended that you supply either an output_file or an output_suffix element to the constructor; otherwise, the new list generated by application of the rules and criteria will simply print to STDOUT. The value of an output_file element should be a full path to the newly created file. If you wish to create a new file name without specifying a full path but simply by tacking on a suffix to the name of the incoming file, provide an output_suffix element and the outgoing file will be created in the directory which is the current working directory as of the point where generate_output() is called. An output_suffix element will be ignored if an output_file element is provided.
Note 1

If neither a header_rule or header_suppress element is provide to the constructor, List::RewriteElements will treat the first row of the incoming file the same as any other row, i.e., it will apply the body_rule transformation formula.
Note 2

A body_suppress or header_suppress criterion, if present, will be logically applied before any body_rule or header_rule formula. We don't apply the formula to transform a record if the record should not be output at all.
Note 3

Return Value: List::RewriteElements object.

`generate_output()`

Purpose: Generates the output specified by arguments to new(), i.e., creates an output file or prints to STDOUT with records transformed as per those arguments.

Arguments: None.

Return Value: Returns true value upon success. In case of failure it will croak with some error message.

`get_output_path()`

Purpose: Get the full path to the newly created output file.

Arguments: None.

Return Value: String holding path to newly created output file.

Comment: Since use of the output_suffix attribute means that the full path to the output file will not be known until generate_output() has been called, get_output_path() will only give a meaningful result once generate_output() has been called. Otherwise, it will default to an empty string.

`get_output_basename()`

Purpose: Get only the basename of the newly created output file.

Arguments: None.

Return Value: String holding basename of newly created output file.

Comment: Since use of the output_suffix attribute means that the full path to the output file will not be known until generate_output() has been called, get_output_basename() will only give a meaningful result once generate_output() has been called. Otherwise, it will default to an empty string.

`get_total_rows()`

Purpose: Get the total number of rows in the newly created output file. This will include any header row.

Arguments: None.