| slovo | definícia |
magic number
(encz) | magic number,magické číslo n: [fráz.] Ivan Masár |
magic number
(wn) | magic number
n 1: the atomic number of an extra stable strongly bound atomic
nucleus: 2, 8, 20, 28, 50, 82 or 126 |
magic number
(foldoc) | magic number
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)
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magic number
(jargon) | 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 common sense
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!
4. An input that leads to a computational boundary condition, where
algorithm behavior becomes discontinuous. Numeric overflows (particularly
with signed data types) and run-time errors (divide by zero, stack
overflows) are indications of magic numbers. The Y2K scare was probably the
most notorious magic number non-incident.
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.
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