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:

     awk '{ $3 = $2 - 10; print $2, $3 }' inventory-shipped

The `-' sign represents subtraction, so this program reassigns field
three, `$3', to be the value of field two minus ten, `$2 - 10'.  (
See 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.  See 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,

     awk '{ $2 = $2 - 10; print $0 }' inventory-shipped

prints a copy of the input file, with 10 subtracted from the second
field of each line.

   You can also assign contents to fields that are out of range.  For
example:

     awk '{ $6 = ($5 + $4 + $3 + $2) ; print $6 }' inventory-shipped

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 the internal `awk' 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 several features not
yet discussed, in particular, the "output field separator", `OFS',
which is used to separate the fields (see Output Separators.), and
`NF' (the number of fields; see Examining Fields: Fields.).  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 merely produces a null 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.  (See The `if' Statement: If Statement, for more
information about `awk''s `if-else' statements.)

   It is important to note that assigning to a field will change the
value of `$0', but will not change the value of `NF', even when you
assign the null string to a field.  For example:

     echo a b c d | awk '{ OFS = ":"; $2 = "" ; print ; print NF }'

prints

     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.