Suppose it’s necessary to write a program to print the input data in reverse order. A reasonable attempt to do so (with some test data) might look like this:
$ echo 'line 1
> line 2
> line 3' | awk '{ l[lines] = $0; ++lines }
> END {
> for (i = lines - 1; i >= 0; i--)
> print l[i]
> }'
-| line 3
-| line 2
Unfortunately, the very first line of input data did not appear in the output!
Upon first glance, we would think that this program should have worked.
The variable lines
is uninitialized, and uninitialized variables have the numeric value zero.
So, awk should have printed the value of l[0].
The issue here is that subscripts for awk arrays are always
strings. Uninitialized variables, when used as strings, have the
value "", not zero. Thus, ‘line 1’ ends up stored in
l[""].
The following version of the program works correctly:
{ l[lines++] = $0 }
END {
for (i = lines - 1; i >= 0; i--)
print l[i]
}
Here, the ‘++’ forces lines to be numeric, thus making
the “old value” numeric zero. This is then converted to "0"
as the array subscript.
Even though it is somewhat unusual, the null string
("") is a valid array subscript.
(d.c.)
gawk warns about the use of the null string as a subscript
if --lint is provided
on the command line (see Command-Line Options).