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Thu, 12 Feb 2009

More Uzi-clubbing: a counterexample
Last year I wrote an article about iterating over a hash, searching for a certain key. Larry Wall called said this was like "clubbing someone to death with a loaded Uzi", because the whole point of a hash is that you don't have to scan all the keys to find the one you want.

I ended the article by saying:

I had already realized that you could, in principle, commit this error with a regular array instead of with a hash, but I had never seen an example until...
Just recently I saw another example, which I think is interesting because it seems to be a counterexample. It's part of a somewhat longer Java program. The crucial section is:

    ...
    LINE: while ( ( line = in.readLine()) != null ) {
        String[] fields = line.split("\t");  

        ...
        for ( int i = 0; i < fields.length; i++ ) {
            if ( ! isEmpty(fields[i]) ) {
                switch(i) {
                    case 0: citation.setCitationType(fields[i]); break;
                    case 1: setAuthors(citation,fields[i],personHome,false); break;
                    case 2: citation.setPublishYear(Integer.parseInt(fields[i])); break;
                    case 3: citation.setTitle(fields[i]); break;
                    ...
                    case 19: citation.setURL(fields[i]); break;
                    case 20: citation.setDoi(fields[i]); break;
                    default: warn("Empty field expected, found: " + fields[i] + " for line: " + line); break;
                }
            }
        }
    }
    ...
The Perlishness of this Java code might lead you to think that I wrote it, but I did not.

My temptation here was to replace the loop and the switch with code like this:

                    citation.setCitationType(fields[0]);
                    setAuthors(citation,fields[1],personHome,false);
                    citation.setPublishYear(Integer.parseInt(fields[2]));
                    citation.setTitle(fields[3]);
                    ...
                    citation.setURL(fields[19]);
                    citation.setDoi(fields[20]);
We lost the warnings, but there were only 4 of those, so we can add them back explicitly:

		    if (! isEmpty(fields[13])) warn("Empty field expected...");
This might have been an improvement, except that we also lost the isEmpty tests on the nonempty fields. To get them back we must spend at least all our gains, possibly more:

                    if (! isEmpty(fields[0])) citation.setCitationType(fields[0]);
                    if (! isEmpty(fields[1])) setAuthors(citation,fields[1],personHome,false);
                    if (! isEmpty(fields[2])) citation.setPublishYear(Integer.parseInt(fields[2]));
                    if (! isEmpty(fields[3])) citation.setTitle(fields[3]);
                    ...
		    if (! isEmpty(fields[13])) warn("Empty field expected...");
                    ...
                    if (! isEmpty(fields[19])) citation.setURL(fields[19]);
                    if (! isEmpty(fields[20])) citation.setDoi(fields[20]);
So at least in this case, my instinct to eliminate the loop-switch was not helpful. There are plenty of Java-esque techniques for cutting up the complexity and sweeping each little piece underneath its own little carpet ("Replace fields with an object! Or with a series of 20 objects!") but nothing that actually reduces the entia multiplicantis. There may be ways to easily improve this code, but I have not been able to think of any.


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