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gawk: Changing Fields

 
 4.4 Changing the Contents of a Field
 ====================================
 
 The contents of a field, as seen by 'awk', can be changed within an
 'awk' program; this changes what 'awk' perceives as the current input
 record.  (The actual input is untouched; 'awk' _never_ modifies the
 input file.)  Consider the following example and its output:
 
      $ awk '{ nboxes = $3 ; $3 = $3 - 10
      >        print nboxes, $3 }' inventory-shipped
      -| 25 15
      -| 32 22
      -| 24 14
      ...
 
 The program first saves the original value of field three in the
 variable 'nboxes'.  The '-' sign represents subtraction, so this program
 reassigns field three, '$3', as the original value of field three minus
 ten: '$3 - 10'.  (⇒Arithmetic Ops.)  Then it prints the original
 and new values for field three.  (Someone in the warehouse made a
 consistent mistake while inventorying the red boxes.)
 
    For this to work, the text in '$3' must make sense as a number; the
 string of characters must be converted to a number for the computer to
 do arithmetic on it.  The number resulting from the subtraction is
 converted back to a string of characters that then becomes field three.
 ⇒Conversion.
 
    When the value of a field is changed (as perceived by 'awk'), the
 text of the input record is recalculated to contain the new field where
 the old one was.  In other words, '$0' changes to reflect the altered
 field.  Thus, this program prints a copy of the input file, with 10
 subtracted from the second field of each line:
 
      $ awk '{ $2 = $2 - 10; print $0 }' inventory-shipped
      -| Jan 3 25 15 115
      -| Feb 5 32 24 226
      -| Mar 5 24 34 228
      ...
 
    It is also possible to assign contents to fields that are out of
 range.  For example:
 
      $ awk '{ $6 = ($5 + $4 + $3 + $2)
      >        print $6 }' inventory-shipped
      -| 168
      -| 297
      -| 301
      ...
 
 We've just created '$6', whose value is the sum of fields '$2', '$3',
 '$4', and '$5'.  The '+' sign represents addition.  For the file
 'inventory-shipped', '$6' represents the total number of parcels shipped
 for a particular month.
 
    Creating a new field changes 'awk''s internal copy of the current
 input record, which is the value of '$0'.  Thus, if you do 'print $0'
 after adding a field, the record printed includes the new field, with
 the appropriate number of field separators between it and the previously
 existing fields.
 
    This recomputation affects and is affected by 'NF' (the number of
 fields; ⇒Fields).  For example, the value of 'NF' is set to the
 number of the highest field you create.  The exact format of '$0' is
 also affected by a feature that has not been discussed yet: the "output
 field separator", 'OFS', used to separate the fields (⇒Output
 Separators).
 
    Note, however, that merely _referencing_ an out-of-range field does
 _not_ change the value of either '$0' or 'NF'.  Referencing an
 out-of-range field only produces an empty string.  For example:
 
      if ($(NF+1) != "")
          print "can't happen"
      else
          print "everything is normal"
 
 should print 'everything is normal', because 'NF+1' is certain to be out
 of range.  (⇒If Statement for more information about 'awk''s
 'if-else' statements.  ⇒Typing and Comparison for more
 information about the '!=' operator.)
 
    It is important to note that making an assignment to an existing
 field changes the value of '$0' but does not change the value of 'NF',
 even when you assign the empty string to a field.  For example:
 
      $ echo a b c d | awk '{ OFS = ":"; $2 = ""
      >                       print $0; print NF }'
      -| a::c:d
      -| 4
 
 The field is still there; it just has an empty value, delimited by the
 two colons between 'a' and 'c'.  This example shows what happens if you
 create a new field:
 
      $ echo a b c d | awk '{ OFS = ":"; $2 = ""; $6 = "new"
      >                       print $0; print NF }'
      -| a::c:d::new
      -| 6
 
 The intervening field, '$5', is created with an empty value (indicated
 by the second pair of adjacent colons), and 'NF' is updated with the
 value six.
 
    Decrementing 'NF' throws away the values of the fields after the new
 value of 'NF' and recomputes '$0'.  (d.c.)  Here is an example:
 
      $ echo a b c d e f | awk '{ print "NF =", NF;
      >                           NF = 3; print $0 }'
      -| NF = 6
      -| a b c
 
      CAUTION: Some versions of 'awk' don't rebuild '$0' when 'NF' is
      decremented.
 
    Finally, there are times when it is convenient to force 'awk' to
 rebuild the entire record, using the current values of the fields and
 'OFS'.  To do this, use the seemingly innocuous assignment:
 
      $1 = $1   # force record to be reconstituted
      print $0  # or whatever else with $0
 
 This forces 'awk' to rebuild the record.  It does help to add a comment,
 as we've shown here.
 
    There is a flip side to the relationship between '$0' and the fields.
 Any assignment to '$0' causes the record to be reparsed into fields
 using the _current_ value of 'FS'.  This also applies to any built-in
 function that updates '$0', such as 'sub()' and 'gsub()' (⇒String
 Functions).
 
                           Understanding '$0'
 
    It is important to remember that '$0' is the _full_ record, exactly
 as it was read from the input.  This includes any leading or trailing
 whitespace, and the exact whitespace (or other characters) that
 separates the fields.
 
    It is a common error to try to change the field separators in a
 record simply by setting 'FS' and 'OFS', and then expecting a plain
 'print' or 'print $0' to print the modified record.
 
    But this does not work, because nothing was done to change the record
 itself.  Instead, you must force the record to be rebuilt, typically
 with a statement such as '$1 = $1', as described earlier.
 
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