File: gawk.info,  Node: Changing Fields,  Next: Field Separators,  Prev: Nonconstant Fields,  Up: Reading Files

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'.  (*Note 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.
*Note 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; *note 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 (*note 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.  (*Note If Statement:: for more information about 'awk''s
'if-else' statements.  *Note 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.  Until August, 2018, this included BWK 'awk';
     fortunately his version now handles this correctly.

   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()' (*note 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.