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(gawk.info)Uninitialized Subscripts

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Using Uninitialized Variables as Subscripts
===========================================

   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 come out in
the output!

   At first glance, 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
(Note: Command-Line Options.).