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gawk: Arrays Summary

 8.7 Summary
    * Standard 'awk' provides one-dimensional associative arrays (arrays
      indexed by string values).  All arrays are associative; numeric
      indices are converted automatically to strings.
    * Array elements are referenced as 'ARRAY[INDX]'.  Referencing an
      element creates it if it did not exist previously.
    * The proper way to see if an array has an element with a given index
      is to use the 'in' operator: 'INDX in ARRAY'.
    * Use 'for (INDX in ARRAY) ...' to scan through all the individual
      elements of an array.  In the body of the loop, INDX takes on the
      value of each element's index in turn.
    * The order in which a 'for (INDX in ARRAY)' loop traverses an array
      is undefined in POSIX 'awk' and varies among implementations.
      'gawk' lets you control the order by assigning special predefined
      values to 'PROCINFO["sorted_in"]'.
    * Use 'delete ARRAY[INDX]' to delete an individual element.  To
      delete all of the elements in an array, use 'delete ARRAY'.  This
      latter feature has been a common extension for many years and is
      now standard, but may not be supported by all commercial versions
      of 'awk'.
    * Standard 'awk' simulates multidimensional arrays by separating
      subscript values with commas.  The values are concatenated into a
      single string, separated by the value of 'SUBSEP'.  The fact that
      such a subscript was created in this way is not retained; thus,
      changing 'SUBSEP' may have unexpected consequences.  You can use
      '(SUB1, SUB2, ...) in ARRAY' to see if such a multidimensional
      subscript exists in ARRAY.
    * 'gawk' provides true arrays of arrays.  You use a separate set of
      square brackets for each dimension in such an array:
      'data[row][col]', for example.  Array elements may thus be either
      scalar values (number or string) or other arrays.
    * Use the 'isarray()' built-in function to determine if an array
      element is itself a subarray.
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