CODEWITHSUNDEEP
matlabperformance
user3051460
user3051460

Reputation: 1485

How to fastly access the variable in a function?

I have a function cal_power that called many times (3000times) in my main project. It uses the loop-up table way to perform computing the power of a scale number (follows some rule to make the table). The TABLE is a vector 1x510.

In my current solution, for each time the function cal_power is called, the TABLE is initialization again, thus it takes some waste time. We know that the value in TABLE is fixed, do we have anyway in MATLAB to initialize the value of TABLE just one time, and it can access anywhere? I tried to use a global variable but it spends more time than my current solution. Thanks

function p = cal_power( ii )
% Input: ii: Integer in {0,255} (forced to be, if not)
% Output: 
%     p = TABLE( mod( ii, 255) + 1 );
% TABLE : look-up table

TABLE =  [1, 2, 4, 8, 16, 32, 64, 128, 29, 58, 116, 232, 205, 135, 19, 38, 76,...
   152, 45, 90, 180, 117, 234, 201, 143, 3, 6, 12, 24, 48, 96, 192, 157,...
   39, 78, 156, 37, 74, 148, 53, 106, 212, 181, 119, 238, 193, 159, 35,...
   70, 140, 5, 10, 20, 40, 80, 160, 93, 186, 105, 210, 185, 111, 222,...
   161, 95, 190, 97, 194, 153, 47, 94, 188, 101, 202, 137, 15, 30, 60,...
   120, 240, 253, 231, 211, 187, 107, 214, 177, 127, 254, 225, 223, 163,...
   91, 182, 113, 226, 217, 175, 67, 134, 17, 34, 68, 136, 13, 26, 52,...
   104, 208, 189, 103, 206, 129, 31, 62, 124, 248, 237, 199, 147, 59,...
   118, 236, 197, 151, 51, 102, 204, 133, 23, 46, 92, 184, 109, 218,...
   169, 79, 158, 33, 66, 132, 21, 42, 84, 168, 77, 154, 41, 82, 164, 85,...
   170, 73, 146, 57, 114, 228, 213, 183, 115, 230, 209, 191, 99, 198,...
   145, 63, 126, 252, 229, 215, 179, 123, 246, 241, 255, 227, 219, 171,...
   75, 150, 49, 98, 196, 149, 55, 110, 220, 165, 87, 174, 65, 130, 25,...
   50, 100, 200, 141, 7, 14, 28, 56, 112, 224, 221, 167, 83, 166, 81,...
   162, 89, 178, 121, 242, 249, 239, 195, 155, 43, 86, 172, 69, 138, 9,...
   18, 36, 72, 144, 61, 122, 244, 245, 247, 243, 251, 235, 203, 139, 11,...
   22, 44, 88, 176, 125, 250, 233, 207, 131, 27, 54, 108, 216, 173, 71,...
   142, 1, 2, 4, 8, 16, 32, 64, 128, 29, 58, 116, 232, 205, 135, 19, 38,...
   76, 152, 45, 90, 180, 117, 234, 201, 143, 3, 6, 12, 24, 48, 96, 192,...
   157, 39, 78, 156, 37, 74, 148, 53, 106, 212, 181, 119, 238, 193, 159,...
   35, 70, 140, 5, 10, 20, 40, 80, 160, 93, 186, 105, 210, 185, 111,...
   222, 161, 95, 190, 97, 194, 153, 47, 94, 188, 101, 202, 137, 15, 30,...
   60, 120, 240, 253, 231, 211, 187, 107, 214, 177, 127, 254, 225, 223,...
   163, 91, 182, 113, 226, 217, 175, 67, 134, 17, 34, 68, 136, 13, 26,...
   52, 104, 208, 189, 103, 206, 129, 31, 62, 124, 248, 237, 199, 147,...
   59, 118, 236, 197, 151, 51, 102, 204, 133, 23, 46, 92, 184, 109, 218,...
   169, 79, 158, 33, 66, 132, 21, 42, 84, 168, 77, 154, 41, 82, 164, 85,...
   170, 73, 146, 57, 114, 228, 213, 183, 115, 230, 209, 191, 99, 198,...
   145, 63, 126, 252, 229, 215, 179, 123, 246, 241, 255, 227, 219, 171,...
   75, 150, 49, 98, 196, 149, 55, 110, 220, 165, 87, 174, 65, 130, 25,...
   50, 100, 200, 141, 7, 14, 28, 56, 112, 224, 221, 167, 83, 166, 81,...
   162, 89, 178, 121, 242, 249, 239, 195, 155, 43, 86, 172, 69, 138, 9,...
   18, 36, 72, 144, 61, 122, 244, 245, 247, 243, 251, 235, 203, 139, 11,...
   22, 44, 88, 176, 125, 250, 233, 207, 131, 27, 54, 108, 216, 173, 71,...
   142];

p = TABLE( mod( ii, 255) + 1 );

Upvotes: 0

Views: 70

Answers (1)

Marcus
Marcus

Reputation: 450

Just use Matlab Persistent Variable: 

function p = cal_power( ii )
% Input: ii: Integer in {0,255} (forced to be, if not)
% Output: 
%     p = TABLE( mod( ii, 255) + 1 );
% TABLE : look-up table
persistent TABLE;

if isempty(TABLE)
    TABLE =  [1, 2, 4, 8, 16, 32, 64, 128, 29, 58, 116, 232, 205, 135, 19, 38, 76,...
    152, 45, 90, 180, 117, 234, 201, 143, 3, 6, 12, 24, 48, 96, 192, 157,...
    39, 78, 156, 37, 74, 148, 53, 106, 212, 181, 119, 238, 193, 159, 35,...
    70, 140, 5, 10, 20, 40, 80, 160, 93, 186, 105, 210, 185, 111, 222,...
end

UPDATE:

The following Code featuring the persistent variable took 0.034 seconds on my machine compared to 0.039 seconds without persistent:

tic
for i=1:3000
    ii = round(rand*254)+1;
    p=cal_power(ii);
end
toc

So I don't see that it is slower at all. Especially not by a factor 5.

If you still need a different solution, initialize the TABLE in the beginning of your main-function and just pass it to cal_power as additional parameter. 

function p = cal_power( ii , TABLE)
% don't initialize TABLE here anymore
...
end

This way you make sure that TABLE only gets initialized once. Passing the variable as a reference should not consume much computation time too.

Upvotes: 1

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