10-23-2012
I believe that 'sort -u' saves just the first occurrance of the unique key, so you sort first non-unique to get the right first record saved.
However, I agree it is a bit of a shame to use so much storage and processing when you could just tuck them in an associative array and overwrite any old values, especially in cases where the data starts out sorted in some relevant way. The only drawback is that the speed of shell operations might be a drag on big volume. You can scale up sort in parallel using pipes and sort -m, but for the unsorted lookup solution at machine speed, C++ or at least JAVA can work a hash table faster, and you can pre-size the hash table big enough to get good use of RAM and VM in even a 32 bit app. I like big powers of 2, since a modulus of the hash becomes a lower bit mask. Empty hash table entries are just pointers in an array, 4 or 8 bytes cost each, which is pretty cheap, and does no harm for smaller data sets! Hash beats tree for query and churn speed, but tree does provide sorted output and scales more automatically. Linear hash (tables in power of two sizes that can double the hash table size for congested buckets) has a better dynamic scaling, but slower query and churn than straight hash. I have not found a lot of hash implementations that reveal they are linear.
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LEARN ABOUT REDHAT
tie::substrhash
Tie::SubstrHash(3pm) Perl Programmers Reference Guide Tie::SubstrHash(3pm)
NAME
Tie::SubstrHash - Fixed-table-size, fixed-key-length hashing
SYNOPSIS
require Tie::SubstrHash;
tie %myhash, 'Tie::SubstrHash', $key_len, $value_len, $table_size;
DESCRIPTION
The Tie::SubstrHash package provides a hash-table-like interface to an array of determinate size, with constant key size and record size.
Upon tying a new hash to this package, the developer must specify the size of the keys that will be used, the size of the value fields that
the keys will index, and the size of the overall table (in terms of key-value pairs, not size in hard memory). These values will not change
for the duration of the tied hash. The newly-allocated hash table may now have data stored and retrieved. Efforts to store more than $ta-
ble_size elements will result in a fatal error, as will efforts to store a value not exactly $value_len characters in length, or reference
through a key not exactly $key_len characters in length. While these constraints may seem excessive, the result is a hash table using much
less internal memory than an equivalent freely-allocated hash table.
CAVEATS
Because the current implementation uses the table and key sizes for the hashing algorithm, there is no means by which to dynamically change
the value of any of the initialization parameters.
The hash does not support exists().
perl v5.8.0 2002-06-01 Tie::SubstrHash(3pm)