Re: Array Of Constant Strings?

[Dr Nancy's Sweetie <kilroy@elvis.rowan.edu>]

Last February, I wrote about setting arrays of constant strings that
were always the same length and never changed.  I got some useful
help on the subject, and a few people expressed concerns such as:

  > If all the strings are always the same length, then using fixed
  > length strings is even simpler. However, I would tend to be
  > suspicious of such a requirement; things like that are almost
  > always changed later.

and

  > Well, we don't know why the OP wants same-length strings, but
  > I agree -- such a restriction seems suspicious.

I wanted to reply at the time, but couldn't, because the question went
to my version of the sample solution of a problem for a programming
contest.  Keeping the problem a secret before the contest is obviously
necessary.  As we've had the contest (last Saturday), I can explain.

The problem involved parsing out the 95 bits encoded by a UPC-A bar
code, with the bits represented as a string of 1s and 0s.  You can read
the whole thing at <http://elvis.rowan.edu/hspc/>, but the short version
is that each digit in the UPC code is represented by 7 bits, and so the
student programs have to split the input string into 7-character chunks
and then check each substring against a table to see which digit it's
supposed to be.  Such a limitation is usually a bad idea, but (a) the
UPC-A code is not likely to change much any time soon, given the huge
investment in equipment which currently supports it, and (b) for the
contest problem, I get to define any limits I like.  8-)

The relevant snippets of code are here:

   ---------------
   -- Constants -- values from problem description
   ---------------
   Bit_String_Size : constant Integer :=  7;

   [...]

   subtype Bit_String        is String(1..Bit_String_Size);

   [...]

      -- type for bitstring lookup table
      type Encoding_Array is array(0..9) of Bit_String;

      -- Lookup tables for extracted bit strings
      -- (encodings taken from problem description)
      Left_Encoding  : constant Encoding_Array :=
           ("0001101", "0011001", "0010011", "0111101", "0100011",
            "0110001", "0101111", "0111011", "0110111", "0001011");
      Right_Encoding : constant Encoding_Array :=
           ("1110010", "1100110", "1101100", "1000010", "1011100",
            "1001110", "1010000", "1000100", "1001000", "1110100");


There are other ways to attack this, of course (make a 128-entry
table and interpret the strings as binary numbers to give a direct
lookup, eg), but I wanted a program whose operation would be
understandable to high school students.


Thanks for all your help!


Darren Provine ! kilroy@elvis.rowan.edu ! http://www.rowan.edu/~kilroy
"Fred Rogers was not only the nicest man I ever met, he was the nicest
 man ANYONE ever met." -- Robert X. Cringely