Question

我有一个函数cal_power，在我的主项目中多次调用（3000次）。它使用循环表方式来执行计算比例数的功效（遵循一些规则来制作表）。 TABLE是向量1x510。

在我目前的解决方案中，每次调用函数cal_power时，TABLE都会再次初始化，因此需要一些浪费时间。我们知道TABLE中的值是固定的，我们在MATLAB中是否只有一次初始化TABLE的值，它可以在任何地方访问？我试图使用全局变量，但它花费的时间比我当前的解决方案多。感谢

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 );

Answer 1

只需使用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

更新：

以下使用持久变量的代码在我的机器上花了0.034秒而没有持久性的0.039秒：

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

所以我没有看到它慢一点。特别是不是因子5.

如果您仍需要不同的解决方案，请在main函数的开头初始化TABLE，并将其作为附加参数传递给cal_power。

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

这样可以确保TABLE只被初始化一次。将变量作为参考传递也不会消耗太多的计算时间。

如何快速访问函数中的变量？

1 个答案: