magic number
[UNIX/C] n. 1. In source code, some non-obvious
constant whose value is significant to the operation of a program
and that is inserted inconspicuously in-line (hardcoded),
rather than expanded in by a symbol set by a commented
`#define'. Magic numbers in this sense are bad style.
2. A
number that encodes critical information used in an algorithm in
some opaque way. The classic examples of these are the numbers
used in hash or CRC functions, or the coefficients in a linear
congruential generator for pseudo-random numbers. This sense
actually predates and was ancestral to the more common sense 1.
3. Special data located at the beginning of a binary data file to
indicate its type to a utility. Under UNIX, the system and various
applications programs (especially the linker) distinguish between
types of executable file by looking for a magic number. Once upon
a time, these magic numbers were PDP-11 branch instructions that
skipped over header data to the start of executable code; 0407,
for example, was octal for `branch 16 bytes relative'. Nowadays
only a wizard knows the spells to create magic numbers. How do
you choose a fresh magic number of your own? Simple --- you pick
one at random. See? It's magic!
The magic number, on the other hand, is 7+/-2. See
"The magical number seven, plus or minus two: some limits on
our capacity for processing information" by George Miller, in the
"Psychological Review" 63:81-97 (1956). This classic paper
established the number of distinct items (such as numeric digits)
that humans can hold in short-term memory. Among other things,
this strongly influenced the interface design of the phone system.
