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gawk: Function Example

 
 9.2.2 Function Definition Examples
 ----------------------------------
 
 Here is an example of a user-defined function, called 'myprint()', that
 takes a number and prints it in a specific format:
 
      function myprint(num)
      {
           printf "%6.3g\n", num
      }
 
 To illustrate, here is an 'awk' rule that uses our 'myprint()' function:
 
      $3 > 0     { myprint($3) }
 
 This program prints, in our special format, all the third fields that
 contain a positive number in our input.  Therefore, when given the
 following input:
 
       1.2   3.4    5.6   7.8
       9.10 11.12 -13.14 15.16
      17.18 19.20  21.22 23.24
 
 this program, using our function to format the results, prints:
 
         5.6
        21.2
 
    This function deletes all the elements in an array (recall that the
 extra whitespace signifies the start of the local variable list):
 
      function delarray(a,    i)
      {
          for (i in a)
              delete a[i]
      }
 
    When working with arrays, it is often necessary to delete all the
 elements in an array and start over with a new list of elements (⇒
 Delete).  Instead of having to repeat this loop everywhere that you
 need to clear out an array, your program can just call 'delarray()'.
 (This guarantees portability.  The use of 'delete ARRAY' to delete the
 contents of an entire array is a relatively recent(1) addition to the
 POSIX standard.)
 
    The following is an example of a recursive function.  It takes a
 string as an input parameter and returns the string in reverse order.
 Recursive functions must always have a test that stops the recursion.
 In this case, the recursion terminates when the input string is already
 empty:
 
      function rev(str)
      {
          if (str == "")
              return ""
 
          return (rev(substr(str, 2)) substr(str, 1, 1))
      }
 
    If this function is in a file named 'rev.awk', it can be tested this
 way:
 
      $ echo "Don't Panic!" |
      > gawk -e '{ print rev($0) }' -f rev.awk
      -| !cinaP t'noD
 
    The C 'ctime()' function takes a timestamp and returns it as a
 string, formatted in a well-known fashion.  The following example uses
 the built-in 'strftime()' function (⇒Time Functions) to create an
 'awk' version of 'ctime()':
 
      # ctime.awk
      #
      # awk version of C ctime(3) function
 
      function ctime(ts,    format)
      {
          format = "%a %b %e %H:%M:%S %Z %Y"
 
          if (ts == 0)
              ts = systime()       # use current time as default
          return strftime(format, ts)
      }
 
    You might think that 'ctime()' could use 'PROCINFO["strftime"]' for
 its format string.  That would be a mistake, because 'ctime()' is
 supposed to return the time formatted in a standard fashion, and
 user-level code could have changed 'PROCINFO["strftime"]'.
 
    ---------- Footnotes ----------
 
    (1) Late in 2012.
 
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