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gawk: Join Function

 10.2.6 Merging an Array into a String
 When doing string processing, it is often useful to be able to join all
 the strings in an array into one long string.  The following function,
 'join()', accomplishes this task.  It is used later in several of the
 application programs (⇒Sample Programs).
    Good function design is important; this function needs to be general,
 but it should also have a reasonable default behavior.  It is called
 with an array as well as the beginning and ending indices of the
 elements in the array to be merged.  This assumes that the array indices
 are numeric--a reasonable assumption, as the array was likely created
 with 'split()' (⇒String Functions):
      # join.awk --- join an array into a string
      function join(array, start, end, sep,    result, i)
          if (sep == "")
             sep = " "
          else if (sep == SUBSEP) # magic value
             sep = ""
          result = array[start]
          for (i = start + 1; i <= end; i++)
              result = result sep array[i]
          return result
    An optional additional argument is the separator to use when joining
 the strings back together.  If the caller supplies a nonempty value,
 'join()' uses it; if it is not supplied, it has a null value.  In this
 case, 'join()' uses a single space as a default separator for the
 strings.  If the value is equal to 'SUBSEP', then 'join()' joins the
 strings with no separator between them.  'SUBSEP' serves as a "magic"
 value to indicate that there should be no separation between the
 component strings.(1)
    ---------- Footnotes ----------
    (1) It would be nice if 'awk' had an assignment operator for
 concatenation.  The lack of an explicit operator for concatenation makes
 string operations more difficult than they really need to be.
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