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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'. (See section Arithmetic Operators.) 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 section Conversion of Strings and Numbers.

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; see section Examining Fields), and by a feature that has not been discussed yet, the output field separator, OFS, which is used to separate the fields (see section 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"
    print "everything is normal"

should print `everything is normal', because NF+1 is certain to be out of range. (See section The if-else Statement, for more information about awk's if-else statements. See section Variable Typing and Comparison Expressions, 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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