Non-constant Field Numbers
==========================

   The number of a field does not need to be a constant.  Any
expression in the `awk' language can be used after a `$' to refer to a
field.  The value of the expression specifies the field number.  If the
value is a string, rather than a number, it is converted to a number.
Consider this example:

     awk '{ print $NR }'

Recall that `NR' is the number of records read so far: 1 in the first
record, 2 in the second, etc.  So this example prints the first field
of the first record, the second field of the second record, and so on.
For the twentieth record, field number 20 is printed; most likely, the
record has fewer than 20 fields, so this prints a blank line.

   Here is another example of using expressions as field numbers:

     awk '{ print $(2*2) }' BBS-list

   The `awk' language must evaluate the expression `(2*2)' and use its
value as the number of the field to print.  The `*' sign represents
multiplication, so the expression `2*2' evaluates to 4.  The
parentheses are used so that the multiplication is done before the `$'
operation; they are necessary whenever there is a binary operator in
the field-number expression.  This example, then, prints the hours of
operation (the fourth field) for every line of the file `BBS-list'.

   If the field number you compute is zero, you get the entire record.
Thus, `$(2-2)' has the same value as `$0'.  Negative field numbers are
not allowed.

   The number of fields in the current record is stored in the built-in
variable `NF' (see Built-in Variables.).  The expression `$NF' is
not a special feature: it is the direct consequence of evaluating `NF'
and using its value as a field number.