(gawk) Changing Fields

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 Changing the Contents of a Field
 ================================
 
    You can change the contents of a field as seen by `awk' 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 this example and its output:
 
      $ awk '{ $3 = $2 - 10; print $2, $3 }' inventory-shipped
      -| 13 3
      -| 15 5
      -| 15 5
      ...
 
 The `-' sign represents subtraction, so this program reassigns field
 three, `$3', to be the value of field two minus ten, `$2 - 10'.  (
 Arithmetic Operators Arithmetic Ops.)  Then field two, and the new
 value for field three, are printed.
 
    In order for this to work, the text in field `$2' must make sense as
 a number; the string of characters must be converted to a number in
 order for the computer to do arithmetic on it.  The number resulting
 from the subtraction is converted back to a string of characters which
 then becomes field three.   Conversion of Strings and Numbers
 Conversion.
 
    When you change the value of a field (as perceived by `awk'), the
 text of the input record is recalculated to contain the new field where
 the old one was.  Therefore, `$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
      ...
 
    You can also 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--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;  Examining Fields Fields.), and by a feature that has not
 been discussed yet, the "output field separator", `OFS', which is used
 to separate the fields ( Output Separators).  For example, the
 value of `NF' is set to the number of the highest field you create.
 
    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.  ( The `if'-`else' Statement If Statement, for more
 information about `awk''s `if-else' statements.   Variable Typing
 and Comparison Expressions Typing and Comparison, for more information
 about the `!=' operator.)
 
    It is important to note that making an assignment to an existing
 field will change the value of `$0', but will 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.  You can tell
 because there are two colons in a row.
 
    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.
 
    Finally, decrementing `NF' will lose the values of the fields after
 the new value of `NF', and `$0' will be recomputed.  Here is an example:
 
      $ echo a b c d e f | ../gawk '{ print "NF =", NF;
      >                               NF = 3; print $0 }'
      -| NF = 6
      -| a b c
 

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(gawk) Non-Constant Fields

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