magic number

3 definitions found

magic number - WordNet (r) 2.1 (2005) :

  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 - Free On-line Dictionary of Computing (26 May 2007) :

  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.  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 - Jargon File (4.4.4, 14 Aug 2003) :

  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 !<arch> 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.