Perl Cookbook

Reading and Writing Hash Records to Text Files

Problem

You want to read or write hash records to text files.

Solution

Use a simple file format with one field per line:

FieldName: Value

and separate records with blank lines.

Discussion

If you have an array of records that you’d like to store and retrieve from a text file, you can use a simple format based on mail headers. The format’s simplicity requires that the keys have neither colons nor newlines, and the values not have newlines.

This code writes them out:

foreach $record (@Array_of_Records) { 
    for $key (sort keys %$record) {
        print "$key: $record->{$key}\n";
    } 
    print "\n";
}

Reading them in is easy, too.

$/ = "";                # paragraph read mode
while (<>) {
    my @fields = split /^([^:]+):\s*/m;
    shift @fields;      # for leading null field
    push(@Array_of_Records, { map /(.*)/, @fields });
}

The split acts upon $_, its default second argument, which contains a full paragraph. The pattern looks for start of line (not just start of record, thanks to the /m) followed by one or more non-colons, followed by a colon and optional white space. When split’s pattern contains parentheses, these are returned along with the values. The return values placed in @fields are in key-value order, with a leading null field we shift off. The braces in the call to push produces a reference to a new anonymous hash, which we copy @fields into. Since that array was stored in order of the needed key-value pairing, this makes for well-ordered hash contents.

All you’re doing is reading ...