magic number

jargon, programming 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

(hard-coded), 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. MSDOS executables begin with the magic string "MZ".

*The* magic number, on the other hand, is 7+/-2. The paper

cited below 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.

["The magical number seven, plus or minus two: some limits on

our capacity for processing information", George Miller, in

the "Psychological Review" 63:81-97, 1956].

[Jargon File]

(2003-07-02)