Hi,

I am developing some code to run on hardware and I am using the sin() and cos() functions inside the standard math.h many times, which slows the program down dramatically. I was wondering if there is a way to "approximate" the sine and cosine of a value (angle is between 0 and 2pi).
Apparently there is a way to do it using look up tables but I don't really know how to create one? I would like to do it using just basic arithmetic operators, no math.h functions. Is there a simple way to compute the sine and cosine of a value?
(I would also like to find out if this is possible for a square root, but for now just sine and cosine)
Thanks for your help!

Sam

Without seeing the code, it's hard to make decent suggestions on how to optimize it. Can you post it, and be more detailed about the +/- error that is acceptable?

I'm not clear about what has you stumped either, about creating a look up table. It's a 2D array (frequently). Your program can get precomputed/preloaded values by just referring to the right array element's value. Nothing esoteric, really.

Imagine for a moment that you are in a library. Everything you need for reference, is in the library. When you need a piece of info, you can go to the card catalog, and then to the right row of shelves, and eventually locate the book with the right info -- OR you could just look at a little pad next to you at the table, with the info you need. That's a look up table.

Thanks Adak,
The angle is given in radians and the tolerance is (+/-0.05 rad). I am using it for the FFT Function, I posted in my other post about the malloc function, and I am using it for processor on a development board.
How would I fill the array though? It would be really long to fill it manually, unless of course I use the sin() function once to fill it. My question is then, how would I access it? My angle value will be some decimal number, how do I link it to the closest angle given in the table? Would I need to round the angle to the nearest angle provided in the array?
Thanks

A lookup table is a list of values. You define the array and it's values at compile time:

float sinLookup[] = { val1, val2, val3, ... };

Each entry corresponds to one angle you want available.

Get the angle, use it as an index into the array to get the sine. Of course you need to figure out how to take the angle and convert it into an integer index. That's the main task.

Thanks Adak,
The angle is given in radians and the tolerance is (+/-0.05 rad). I am using it for the FFT Function, I posted in my other post about the malloc function, and I am using it for processor on a development board.
How would I fill the array though? It would be really long to fill it manually, unless of course I use the sin() function once to fill it. My question is then, how would I access it? My angle value will be some decimal number, how do I link it to the closest angle given in the table? Would I need to round the angle to the nearest angle provided in the array?
Thanks

You might load a look up table from a file, or have it calculated when the program first starts up, but probably the most common way is to simply assign the table with values.

No, I've never seen a look up table filled manually -- you're cracking me up here ;)
It could happen, odd though it sounds. Wouldn't advise it though.

This is an example of several look up tables, from a Sudoku program (it's not mine):

/****************************************************************\
**                                                              **
**  BB_Sudoku  Bit Based Sudoku Solver                          **
**                                                              **
**             Copyright (c) 2008, Brian Turner                 **
**             All rights reserved                              **
**                                                              **
**  BB_Sudoku_Tables.h : This file contains the various tables  **
**    used by the solver:                                       **
**                                                              **
**  General purpose tables:                                     **
**    V2B     : Value to Binary provides a lookup from a value  **
**                to its bit position.                          **
**    B2V     : Binary to Value provides a lookup from a bit    **
**                to a value, but only if a single bit is set.  **
**                If multiple bits are set, it returns 0.       ** 
**    NSet    : Number of bits set for a given possibility      **
**                value (so 0x0F0 gives 4, 0x004 gives 1)       **
**                                                              **
**  Group Tables (for Naked and Hidden Singles)                 **
**    Group   : Lists the cells that belong to each group       **
**    C2Group : Cell to Group, provides a list of groups that   **
**                contain a given cell                          **
**                                                              **
**  Locked Candidate tables (for Lock Candidates)               **
**    LC_Segment : defines related segments for the LC search   **
**                                                              **
\****************************************************************/


// V2B and B2V stand for "Binary to Value" and "Value to 
//   Binary", and provide a quick lookup between bit positions
//   and values.  This provides an easy method to convert
//   between values and bit positions for single values.
//
// NSet looks up the number of bits set (potential values)
//   for numbers 0 to 511 (9 bits).

// Values to Bit positions lookup
unsigned int V2B[17] = {0x0000, 0x0001, 0x0002, 0x0004, 0x0008,
                                0x0010, 0x0020, 0x0040, 0x0080,
                                0x0100, 0x0200, 0x0400, 0x0800,
                                0x1000, 0x2000, 0x4000, 0x8000};

// Bits to Value lookup
int B2V[512]; 

// Number of bits set in the number, up to 9 bits
int NSet[512]; 



// Groups and P2Group define the groups for a standard
//   3x3 grid and a look from a position (0-80) to the
//   three groups it is a part of.

// 
const int In_Groups[81][20] = 
  {{  1,  2,  3,  4,  5,  6,  7,  8,  9, 18, 27, 36, 45, 54, 63, 72, 10, 11, 19, 20 },
   {  0,  2,  3,  4,  5,  6,  7,  8, 10, 19, 28, 37, 46, 55, 64, 73,  9, 11, 18, 20 },
   {  0,  1,  3,  4,  5,  6,  7,  8, 11, 20, 29, 38, 47, 56, 65, 74,  9, 10, 18, 19 },
   {  0,  1,  2,  4,  5,  6,  7,  8, 12, 21, 30, 39, 48, 57, 66, 75, 13, 14, 22, 23 },
   {  0,  1,  2,  3,  5,  6,  7,  8, 13, 22, 31, 40, 49, 58, 67, 76, 12, 14, 21, 23 },
   {  0,  1,  2,  3,  4,  6,  7,  8, 14, 23, 32, 41, 50, 59, 68, 77, 12, 13, 21, 22 },
   {  0,  1,  2,  3,  4,  5,  7,  8, 15, 24, 33, 42, 51, 60, 69, 78, 16, 17, 25, 26 },
   {  0,  1,  2,  3,  4,  5,  6,  8, 16, 25, 34, 43, 52, 61, 70, 79, 15, 17, 24, 26 },
   {  0,  1,  2,  3,  4,  5,  6,  7, 17, 26, 35, 44, 53, 62, 71, 80, 15, 16, 24, 25 },
   { 10, 11, 12, 13, 14, 15, 16, 17,  0, 18, 27, 36, 45, 54, 63, 72,  1,  2, 19, 20 },
   {  9, 11, 12, 13, 14, 15, 16, 17,  1, 19, 28, 37, 46, 55, 64, 73,  0,  2, 18, 20 },
   {  9, 10, 12, 13, 14, 15, 16, 17,  2, 20, 29, 38, 47, 56, 65, 74,  0,  1, 18, 19 },
   {  9, 10, 11, 13, 14, 15, 16, 17,  3, 21, 30, 39, 48, 57, 66, 75,  4,  5, 22, 23 },
   {  9, 10, 11, 12, 14, 15, 16, 17,  4, 22, 31, 40, 49, 58, 67, 76,  3,  5, 21, 23 },
   {  9, 10, 11, 12, 13, 15, 16, 17,  5, 23, 32, 41, 50, 59, 68, 77,  3,  4, 21, 22 },
   {  9, 10, 11, 12, 13, 14, 16, 17,  6, 24, 33, 42, 51, 60, 69, 78,  7,  8, 25, 26 },
   {  9, 10, 11, 12, 13, 14, 15, 17,  7, 25, 34, 43, 52, 61, 70, 79,  6,  8, 24, 26 },
   {  9, 10, 11, 12, 13, 14, 15, 16,  8, 26, 35, 44, 53, 62, 71, 80,  6,  7, 24, 25 },
   { 19, 20, 21, 22, 23, 24, 25, 26,  0,  9, 27, 36, 45, 54, 63, 72,  1,  2, 10, 11 },
   { 18, 20, 21, 22, 23, 24, 25, 26,  1, 10, 28, 37, 46, 55, 64, 73,  0,  2,  9, 11 },
   { 18, 19, 21, 22, 23, 24, 25, 26,  2, 11, 29, 38, 47, 56, 65, 74,  0,  1,  9, 10 },
   { 18, 19, 20, 22, 23, 24, 25, 26,  3, 12, 30, 39, 48, 57, 66, 75,  4,  5, 13, 14 },
   { 18, 19, 20, 21, 23, 24, 25, 26,  4, 13, 31, 40, 49, 58, 67, 76,  3,  5, 12, 14 },
   { 18, 19, 20, 21, 22, 24, 25, 26,  5, 14, 32, 41, 50, 59, 68, 77,  3,  4, 12, 13 },
   { 18, 19, 20, 21, 22, 23, 25, 26,  6, 15, 33, 42, 51, 60, 69, 78,  7,  8, 16, 17 },
   { 18, 19, 20, 21, 22, 23, 24, 26,  7, 16, 34, 43, 52, 61, 70, 79,  6,  8, 15, 17 },
   { 18, 19, 20, 21, 22, 23, 24, 25,  8, 17, 35, 44, 53, 62, 71, 80,  6,  7, 15, 16 },
   { 28, 29, 30, 31, 32, 33, 34, 35,  0,  9, 18, 36, 45, 54, 63, 72, 37, 38, 46, 47 },
   { 27, 29, 30, 31, 32, 33, 34, 35,  1, 10, 19, 37, 46, 55, 64, 73, 36, 38, 45, 47 },
   { 27, 28, 30, 31, 32, 33, 34, 35,  2, 11, 20, 38, 47, 56, 65, 74, 36, 37, 45, 46 },
   { 27, 28, 29, 31, 32, 33, 34, 35,  3, 12, 21, 39, 48, 57, 66, 75, 40, 41, 49, 50 },
   { 27, 28, 29, 30, 32, 33, 34, 35,  4, 13, 22, 40, 49, 58, 67, 76, 39, 41, 48, 50 },
   { 27, 28, 29, 30, 31, 33, 34, 35,  5, 14, 23, 41, 50, 59, 68, 77, 39, 40, 48, 49 },
   { 27, 28, 29, 30, 31, 32, 34, 35,  6, 15, 24, 42, 51, 60, 69, 78, 43, 44, 52, 53 },
   { 27, 28, 29, 30, 31, 32, 33, 35,  7, 16, 25, 43, 52, 61, 70, 79, 42, 44, 51, 53 },
   { 27, 28, 29, 30, 31, 32, 33, 34,  8, 17, 26, 44, 53, 62, 71, 80, 42, 43, 51, 52 },
   { 37, 38, 39, 40, 41, 42, 43, 44,  0,  9, 18, 27, 45, 54, 63, 72, 28, 29, 46, 47 },
   { 36, 38, 39, 40, 41, 42, 43, 44,  1, 10, 19, 28, 46, 55, 64, 73, 27, 29, 45, 47 },
   { 36, 37, 39, 40, 41, 42, 43, 44,  2, 11, 20, 29, 47, 56, 65, 74, 27, 28, 45, 46 },
   { 36, 37, 38, 40, 41, 42, 43, 44,  3, 12, 21, 30, 48, 57, 66, 75, 31, 32, 49, 50 },
   { 36, 37, 38, 39, 41, 42, 43, 44,  4, 13, 22, 31, 49, 58, 67, 76, 30, 32, 48, 50 },
   { 36, 37, 38, 39, 40, 42, 43, 44,  5, 14, 23, 32, 50, 59, 68, 77, 30, 31, 48, 49 },
   { 36, 37, 38, 39, 40, 41, 43, 44,  6, 15, 24, 33, 51, 60, 69, 78, 34, 35, 52, 53 },
   { 36, 37, 38, 39, 40, 41, 42, 44,  7, 16, 25, 34, 52, 61, 70, 79, 33, 35, 51, 53 },
   { 36, 37, 38, 39, 40, 41, 42, 43,  8, 17, 26, 35, 53, 62, 71, 80, 33, 34, 51, 52 },
   { 46, 47, 48, 49, 50, 51, 52, 53,  0,  9, 18, 27, 36, 54, 63, 72, 28, 29, 37, 38 },
   { 45, 47, 48, 49, 50, 51, 52, 53,  1, 10, 19, 28, 37, 55, 64, 73, 27, 29, 36, 38 },
   { 45, 46, 48, 49, 50, 51, 52, 53,  2, 11, 20, 29, 38, 56, 65, 74, 27, 28, 36, 37 },
   { 45, 46, 47, 49, 50, 51, 52, 53,  3, 12, 21, 30, 39, 57, 66, 75, 31, 32, 40, 41 },
   { 45, 46, 47, 48, 50, 51, 52, 53,  4, 13, 22, 31, 40, 58, 67, 76, 30, 32, 39, 41 },
   { 45, 46, 47, 48, 49, 51, 52, 53,  5, 14, 23, 32, 41, 59, 68, 77, 30, 31, 39, 40 },
   { 45, 46, 47, 48, 49, 50, 52, 53,  6, 15, 24, 33, 42, 60, 69, 78, 34, 35, 43, 44 },
   { 45, 46, 47, 48, 49, 50, 51, 53,  7, 16, 25, 34, 43, 61, 70, 79, 33, 35, 42, 44 },
   { 45, 46, 47, 48, 49, 50, 51, 52,  8, 17, 26, 35, 44, 62, 71, 80, 33, 34, 42, 43 },
   { 55, 56, 57, 58, 59, 60, 61, 62,  0,  9, 18, 27, 36, 45, 63, 72, 64, 65, 73, 74 },
   { 54, 56, 57, 58, 59, 60, 61, 62,  1, 10, 19, 28, 37, 46, 64, 73, 63, 65, 72, 74 },
   { 54, 55, 57, 58, 59, 60, 61, 62,  2, 11, 20, 29, 38, 47, 65, 74, 63, 64, 72, 73 },
   { 54, 55, 56, 58, 59, 60, 61, 62,  3, 12, 21, 30, 39, 48, 66, 75, 67, 68, 76, 77 },
   { 54, 55, 56, 57, 59, 60, 61, 62,  4, 13, 22, 31, 40, 49, 67, 76, 66, 68, 75, 77 },
   { 54, 55, 56, 57, 58, 60, 61, 62,  5, 14, 23, 32, 41, 50, 68, 77, 66, 67, 75, 76 },
   { 54, 55, 56, 57, 58, 59, 61, 62,  6, 15, 24, 33, 42, 51, 69, 78, 70, 71, 79, 80 },
   { 54, 55, 56, 57, 58, 59, 60, 62,  7, 16, 25, 34, 43, 52, 70, 79, 69, 71, 78, 80 },
   { 54, 55, 56, 57, 58, 59, 60, 61,  8, 17, 26, 35, 44, 53, 71, 80, 69, 70, 78, 79 },
   { 64, 65, 66, 67, 68, 69, 70, 71,  0,  9, 18, 27, 36, 45, 54, 72, 55, 56, 73, 74 },
   { 63, 65, 66, 67, 68, 69, 70, 71,  1, 10, 19, 28, 37, 46, 55, 73, 54, 56, 72, 74 },
   { 63, 64, 66, 67, 68, 69, 70, 71,  2, 11, 20, 29, 38, 47, 56, 74, 54, 55, 72, 73 },
   { 63, 64, 65, 67, 68, 69, 70, 71,  3, 12, 21, 30, 39, 48, 57, 75, 58, 59, 76, 77 },
   { 63, 64, 65, 66, 68, 69, 70, 71,  4, 13, 22, 31, 40, 49, 58, 76, 57, 59, 75, 77 },
   { 63, 64, 65, 66, 67, 69, 70, 71,  5, 14, 23, 32, 41, 50, 59, 77, 57, 58, 75, 76 },
   { 63, 64, 65, 66, 67, 68, 70, 71,  6, 15, 24, 33, 42, 51, 60, 78, 61, 62, 79, 80 },
   { 63, 64, 65, 66, 67, 68, 69, 71,  7, 16, 25, 34, 43, 52, 61, 79, 60, 62, 78, 80 },
   { 63, 64, 65, 66, 67, 68, 69, 70,  8, 17, 26, 35, 44, 53, 62, 80, 60, 61, 78, 79 },
   { 73, 74, 75, 76, 77, 78, 79, 80,  0,  9, 18, 27, 36, 45, 54, 63, 55, 56, 64, 65 },
   { 72, 74, 75, 76, 77, 78, 79, 80,  1, 10, 19, 28, 37, 46, 55, 64, 54, 56, 63, 65 },
   { 72, 73, 75, 76, 77, 78, 79, 80,  2, 11, 20, 29, 38, 47, 56, 65, 54, 55, 63, 64 },
   { 72, 73, 74, 76, 77, 78, 79, 80,  3, 12, 21, 30, 39, 48, 57, 66, 58, 59, 67, 68 },
   { 72, 73, 74, 75, 77, 78, 79, 80,  4, 13, 22, 31, 40, 49, 58, 67, 57, 59, 66, 68 },
   { 72, 73, 74, 75, 76, 78, 79, 80,  5, 14, 23, 32, 41, 50, 59, 68, 57, 58, 66, 67 },
   { 72, 73, 74, 75, 76, 77, 79, 80,  6, 15, 24, 33, 42, 51, 60, 69, 61, 62, 70, 71 },
   { 72, 73, 74, 75, 76, 77, 78, 80,  7, 16, 25, 34, 43, 52, 61, 70, 60, 62, 69, 71 },
   { 72, 73, 74, 75, 76, 77, 78, 79,  8, 17, 26, 35, 44, 53, 62, 71, 60, 61, 69, 70 }};


// Defines the groups used in a standard 3x3 grid
const char Group[27][9] =
  {{  0,  1,  2,  3,  4,  5,  6,  7,  8 }, {  9, 10, 11, 12, 13, 14, 15, 16, 17 }, { 18, 19, 20, 21, 22, 23, 24, 25, 26 },
   { 27, 28, 29, 30, 31, 32, 33, 34, 35 }, { 36, 37, 38, 39, 40, 41, 42, 43, 44 }, { 45, 46, 47, 48, 49, 50, 51, 52, 53 },
   { 54, 55, 56, 57, 58, 59, 60, 61, 62 }, { 63, 64, 65, 66, 67, 68, 69, 70, 71 }, { 72, 73, 74, 75, 76, 77, 78, 79, 80 },

   {  0,  9, 18, 27, 36, 45, 54, 63, 72 }, {  1, 10, 19, 28, 37, 46, 55, 64, 73 }, {  2, 11, 20, 29, 38, 47, 56, 65, 74 },
   {  3, 12, 21, 30, 39, 48, 57, 66, 75 }, {  4, 13, 22, 31, 40, 49, 58, 67, 76 }, {  5, 14, 23, 32, 41, 50, 59, 68, 77 },
   {  6, 15, 24, 33, 42, 51, 60, 69, 78 }, {  7, 16, 25, 34, 43, 52, 61, 70, 79 }, {  8, 17, 26, 35, 44, 53, 62, 71, 80 },

   {  0,  1,  2,  9, 10, 11, 18, 19, 20 }, {  3,  4,  5, 12, 13, 14, 21, 22, 23 }, {  6,  7,  8, 15, 16, 17, 24, 25, 26 },
   { 27, 28, 29, 36, 37, 38, 45, 46, 47 }, { 30, 31, 32, 39, 40, 41, 48, 49, 50 }, { 33, 34, 35, 42, 43, 44, 51, 52, 53 },
   { 54, 55, 56, 63, 64, 65, 72, 73, 74 }, { 57, 58, 59, 66, 67, 68, 75, 76, 77 }, { 60, 61, 62, 69, 70, 71, 78, 79, 80 }};

// Defines the 3 groups each position is part of
const int C2Group[81][3] =
  {{ 0,  9, 18}, { 0, 10, 18}, { 0, 11, 18}, { 0, 12, 19}, { 0, 13, 19}, { 0, 14, 19}, { 0, 15, 20}, { 0, 16, 20}, { 0, 17, 20}, 
   { 1,  9, 18}, { 1, 10, 18}, { 1, 11, 18}, { 1, 12, 19}, { 1, 13, 19}, { 1, 14, 19}, { 1, 15, 20}, { 1, 16, 20}, { 1, 17, 20}, 
   { 2,  9, 18}, { 2, 10, 18}, { 2, 11, 18}, { 2, 12, 19}, { 2, 13, 19}, { 2, 14, 19}, { 2, 15, 20}, { 2, 16, 20}, { 2, 17, 20}, 
   { 3,  9, 21}, { 3, 10, 21}, { 3, 11, 21}, { 3, 12, 22}, { 3, 13, 22}, { 3, 14, 22}, { 3, 15, 23}, { 3, 16, 23}, { 3, 17, 23}, 
   { 4,  9, 21}, { 4, 10, 21}, { 4, 11, 21}, { 4, 12, 22}, { 4, 13, 22}, { 4, 14, 22}, { 4, 15, 23}, { 4, 16, 23}, { 4, 17, 23}, 
   { 5,  9, 21}, { 5, 10, 21}, { 5, 11, 21}, { 5, 12, 22}, { 5, 13, 22}, { 5, 14, 22}, { 5, 15, 23}, { 5, 16, 23}, { 5, 17, 23}, 
   { 6,  9, 24}, { 6, 10, 24}, { 6, 11, 24}, { 6, 12, 25}, { 6, 13, 25}, { 6, 14, 25}, { 6, 15, 26}, { 6, 16, 26}, { 6, 17, 26}, 
   { 7,  9, 24}, { 7, 10, 24}, { 7, 11, 24}, { 7, 12, 25}, { 7, 13, 25}, { 7, 14, 25}, { 7, 15, 26}, { 7, 16, 26}, { 7, 17, 26},
   { 8,  9, 24}, { 8, 10, 24}, { 8, 11, 24}, { 8, 12, 25}, { 8, 13, 25}, { 8, 14, 25}, { 8, 15, 26}, { 8, 16, 26}, { 8, 17, 26}};

// Defines the interaction of segments used for Lock Candidate searches 
const char LC_Segment[9][4] = 
  {{1, 2, 3, 6}, {0, 2, 4, 7}, {0, 1, 5, 8}, {4, 5, 0, 6}, {3, 5, 1, 7}, {3, 4, 2, 8}, {7, 8, 0, 3}, {6, 8, 1, 4}, {6, 7, 2, 5}};

This is just to remind you that look-up tables are used extensively in programming - your compiler uses them, your OS uses them. I'd be amazed if there was a single OS on the planet that did NOT use look-up tables.

Since these tables (above) are accessed many times in the course of solving the puzzle, it makes sense to use a set of precomputed tables like this.

Rounding off a float or double is straight forward. Here's one way:

double dnum = 1.510267;
int intnum = (dnum + 0.5)/1;

Play around with the above, but you'll see it always rounds appropriately. If you want to round off at the hundredths place, just change 0.5 to 0.05, and so forth, as you need.

How would you find the nearest angle in the table?

Same way you would sitting at the living room table:

/* This is a one line while loop */
while(yourNumber > table[i++]); //note the unusual semi-colon here

--i; //walking back one time, you overstepped
diff1 = yourNumber - table[i];
diff2 = table[i+1] - yourNumber;

if(diff1 < diff2)
  table[i] is the closer number to yourNumber
else
  table[i+1] is the closer number to yourNumber

I am writing the above, off the cuff, so it may need some adjustment to be just right. If you have a problem with the above code you can't fix, just let me know.

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