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Posts Tagged ‘psychology’

Relative spacing of operands affects perception of operator precedence

January 22nd, 2012 1 comment

What I found most intriguing about Google Code Search (shutdown Nov 2011) was how quickly searches involving regular expressions returned matches. A few days ago Russ Cox, the implementor of Code Search not only explained how it worked but also released the source and some precompiled binaries. Google’s database of source code did not include the source of R, so I decided to install CodeSearch on my local machine and run some of my previous searches against the latest (v2.14.1) R source.

In 2007 I ran an experiment that showed developers made use of variable names when making binary operator precedence decisions. At about the same time two cognitive psychologists, David Landy and Robert Goldstone, were investigating the impact of spacing on operator precedence decisions (they found that readers showed a tendency to pair together the operands that were visibly closer to each other, e.g., a with b in a+b * c rather than b with c).

As somebody very interested in finding faults in code the psychologists research findings on spacing immediately suggested to me the possibility that ‘incorrectly’ spaced expressions were a sign of failure to write code that had the intended behavior. Feeding some rather complicated regular expressions into Google’s CodeSearch threw up a number of ‘incorrectly’ spaced expressions. However, this finding went no further than an interesting email exchange with Landy and Goldstone.

Time to find out whether there are any ‘incorrectly’ spaced expressions in the R source. cindex (the tool that builds the database used by csearch) took 3 seconds on a not very fast machine to process all of the R source (56M byte) and build the search database (10M byte; the Linux database is a factor of 5.5 smaller than the sources).

The search:

csearch "\w(\+|\-)\w +(\*|\/) +\w"

returned a few interesting matches:

...
modules/internet/nanohttp.c:       used += tv_save.tv_sec + 1e-6 * tv_save.tv_usec;
modules/lapack/dlapack0.f:     $          ( T*( ONE+SQRT( ONE+S / T ) ) ) )
modules/lapack/dlapack2.f:               S = Z( 3 )*( Z( 2 ) / ( T*( ONE+SQRT( ONE+S / T ) ) ) )
modules/lapack/dlapack4.f:     $          ( T*( ONE+SQRT( ONE+S / T ) ) ) )

There were around 15 matches of code like 1e-6 * var (because the pattern \w is for alphanumeric sequences and that is not a superset of the syntax of floating-point literals).

The subexpression ONE+S / T is just the sort of thing I was looking for. The three instances all involved code that processed tridiagonal matrices in various special cases. Google search combined with my knowledge of numerical analysis was not up to the task of figuring out whether the intended usage was (ONE+S)/T or ONE+(S/T).

Searches based on various other combination of operator pairs failed to match anything that looked suspicious.

There was an order of magnitude performance difference for csearch vs. grep -R -e (real 0m0.167s vs. real 0m2.208s). A very worthwhile improvement when searching much larger code bases with more complicated patterns.

Christmas books for 2009

December 7th, 2009 No comments

I thought it would be useful to list the books that gripped me one way or another this year (and may be last year since I don’t usually track such things closely); perhaps they will give you some ideas to add to your Christmas present wish list (please make your own suggestions in the Comments). Most of the books were published a few years ago, I maintain piles of books ordered by when I plan to read them and books migrate between piles until eventually read. Looking at the list I don’t seem to have read many good books this year, perhaps I am spending too much time reading blogs.

These books contain plenty of facts backed up by numbers and an analytic approach and are ordered by physical size.

The New Science of Strong Materials by J. E. Gordon. Ideal for train journeys since it is a small book that can be read in small chunks and is not too taxing. Offers lots of insight into those properties of various materials that are needed to build things (‘new’ here means postwar).

Europe at War 1939-1945 by Norman Davies. A fascinating analysis of the war from a numbers perspective. It is hard to escape the conclusion that in the grand scheme of things us plucky Brits made a rather small contribution, although subsequent Hollywood output has suggested otherwise. Also a contender for a train book.

Japanese English language and culture contact by James Stanlaw. If you are into Japanese culture you will love this, otherwise avoid.

Evolutionary Dynamics by Martin A. Nowak. For the more mathematical folk and plenty of thought power needed. Some very powerful general results from simple processes.

Analytic Combinatorics by Philippe Flajolet and Robert Sedgewick. Probably the toughest mathematical book I have kept at (yet to get close to the end) in a few years. If number sequences fascinate you then give it a try (a pdf is available).

Probability and Computing by Michael Mitzenmacher and Eli Upfal. For the more mathematical folk and plenty of thought power needed. Don’t let the density of Theorems put you off, the approach is broad brush. Plenty of interesting results with applications to solving problems using algorithms containing a randomizing component.

Network Algorithmics by George Varghese. A real hackers book. Not so much a book about algorithms used to solve networking problems but a book about making engineering trade-offs and using every ounce of computing functionality to solve problems having severe resource and real-time constraints.

Virtual Machines by James E. Smith and Ravi Nair. Everything you every wanted to know about virtual machines and more.

Biological Psychology by James W. Kalat. This might be a coffee table book for scientists. Great illustrations, concise explanations, the nuts and bolts of how our bodies runs at the protein/DNA level.