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(gawk.info)Non-Constant Fields

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Non-Constant Field Numbers
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   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: one in the first
record, two 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

   `awk' evaluates the expression `(2*2)' and uses its value as the
number of the field to print.  The `*' sign represents multiplication,
so the expression `2*2' evaluates to four.  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'.  (All of the `awk'
operators are listed, in order of decreasing precedence, in Note:
Operator Precedence (How Operators Nest).)

   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; trying to reference one usually terminates the program.
(The POSIX standard does not define what happens when you reference a
negative field number.  `gawk' notices this and terminates your
program.  Other `awk' implementations may behave differently.)

   As mentioned in Note: Examining Fields, `awk' stores the
current record's number of fields in the built-in variable `NF' (also
Note: 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.