magic number

magic number
—n
1.	physics any of the numbers 2, 8, 20, 28, 50, 82, and 126. Nuclides with these numbers of nucleons appear to have greater stability than other nuclides
2.	chem a number of atoms that is particularly stable in certain types of compound that have clusters of the same type of atom

magic number   (māj'ĭk)  Pronunciation Key  Any of the numbers, 2, 8, 20, 28, 50, 82, or 126, that represent the number of neutrons or protons in strongly bound and exceptionally stable atomic nuclei. The existence of such stable nuclei is explained by assuming a shell structure for nucleons, much like the shell structure of electron orbitals around the nucleus.

magic number

n. [Unix/C; common]
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 commonsense
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'. Many other kinds of files now have magic numbers somewhere; some magic numbers are, in fact, strings, like the `!' at the beginning of a Unix archive file or the `%!' leading PostScript files. 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.

magic number definition

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 pseudorandom 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. MS DOS 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)

magic number

in physics, in the shell models of both atomic and nuclear structure, any of a series of numbers that connote stable structure

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