Path: utzoo!attcan!uunet!mcvax!philmds!leo
From: leo@philmds.UUCP (Leo de Wit)
Newsgroups: comp.lang.c
Subject: Re: Table lookups
Message-ID: <547@philmds.UUCP>
Date: 3 Jul 88 07:37:54 GMT
References: <1802@loral.UUCP>
Reply-To: leo@philmds.UUCP (Leo de Wit)
Organization: Philips I&E DTS Eindhoven
Lines: 52

In article <1802@loral.UUCP> jlh@loral.UUCP (The Mad Merkin Hunter) writes:
>Lets say I have a value that can range from 0-15 and I want to print
>an ascii string based on that value.  Normally I create an array of
>strings and index into it.  K&R does this with the names of the months
>when they talk about arrays of pointers.  Now lets say that instead of
>index values from 0-15 I have bit positions, i.e., the values are
>0x1, 0x2, 0x4, ..., 0x80.  I'd like to do a table lookup on these values
>also, as opposed to building a switch statement.  The only methods I
>can come up with take a lot longer and are much less clear than the
>switch statement (for example, logarithms).  Anyone got any ideas?

A general solution is a function that maps values to natural numbers;
for instance if you have a function log2(n) that returns the base 2 logarithm
of a number (leaving in the middle what should be returned if the number
is not a power of 2) then you have your problem solved. For instance:

      answer = table[log2(n)];

(You probably have to check the range of the function before using it).

This scheme is also useful as an alternative to switch cases; you could do
something like:

some_type firstf(), secondf(), thirdf(); 

some_type (*funcarr[])() = { firstf, secondf, thirdf };

some_type some_var;

    /* now use it ... */

    some_var = (*funcarr[log2(n)])(param);

especially when you have much code in the switch cases, this can be a real
nice alternative.

Consider a single key command editor; the mapping function degenerates
to a simple cast to unsigned char (if the input character value hadn't
already this type) - possibly preceded by a simple range check -, the
array of functions are the diverse actions to be performed for each input
(including 'invalid key' actions: sounding the bell), a parameter is
the 'current count' for instance, the return value is a boolean indicated
whether there was a problem.
Another nice example is a disassembler that uses the (first part of
the) opcode to index into an array of functions that handle moving,
shifting, logic, control flow, arithmetic etc. I've done that for a 68K
disassembler (hint:  use the first 4 bits of the instruction word to
index into an array of 16 functions).

As for the log2 function itself, some suggestion has already been made.

       Leo.