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11.10. Reading and Writing Hash Records to Text Files

11.10.1. Problem

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

11.10.2. Solution

Use a simple file format with one field per line:

FieldName: Value

and separate records with blank lines.

11.10.3. Discussion

If you have an array of records that you'd like to store into 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 whitespace. 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 produce 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 and writing a plain text file, so you can
use related recipes for additional components. You could use Recipe 7.18 to ensure that you have clean,
concurrent access; Recipe 1.18 to store
colons and newlines in keys and values; and Recipe 11.3 to store more complex structures.

If you are willing to sacrifice the elegance of a plain textfile for
a quick, random-access database of records, use a DBM file, as
described in Recipe 11.14.

11.10.4. See Also

The split function in
perlfunc(1) and Chapter 29 of
Programming Perl; Recipe 11.9; Recipe 11.13; Recipe 11.14