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gawk: Uninitialized Subscripts

 
 8.3 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 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 (⇒
 Options).
 
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