Matlab函数性能 - 循​​环太多 [英] Matlab Function Performance - Too many loops

问题描述

我必须在很多行中写入多个信息的txt文件。结果是一个文件，如：

I have to write in a txt file several informations in a LOT of rows. The result is a file like:

result.txt:
RED;12;7;0;2;1;4;7;0.0140
RED;12;7;0;2;2;9;7;0.1484
RED;12;7;0;2;3;7;4;0.1787
RED;12;7;0;2;4;2;6;0.7891
RED;12;7;0;2;5;9;6;0.1160
RED;12;7;0;2;6;9;1;0.9893
...

通过下面的代码（缩小尺寸）：

Which is build by the code below (with some reduced dimensions):

/* the variables 'str1', 'num1', 'day', 'vect1', 'vect2' and 'MD' are inputs of this function
/* str1 is a string 1x1
/* num1 is a integer 1x1 
/* day is a vector 10x1
/* vect1 is a vector 7x1
/* vect2 is a vector 180x1
/* MD is a 4D matrix (7x180x10x15)*/

fid = fopen(path_result, 'Wt');    
for i1 = 1:15   
    for i2 = 1:10    
        for i3 = 1:7           
           for i4= 1:180
            /* print all the values */
                fprintf(fid,'%s%s%d%s%d%s%d%s%d%s%d%s%d%s%d%s%.4f \n',...
                str1,';',num1,';',i1,';',0,';',2,';',...
                day(i2,1),';',vect1(i3),';',...
                vect2(i4),';',MD(i3,i4,i2,i1));
            end
        end
    end
end

I在这里看到了一些向量化（stackoverflow后），但我不认为可以在这里申请。有任何想法吗？

预先感谢

I saw some vectorization in here (stackoverflow post), but I don't think it's possible to apply in here. Any ideas?
Thanks in advance

推荐答案

可应用于任何情况的第一个优化是生成一个格式字符串，称它为 fmt ：

A first optimization which can be applied to any situation is to generate a format string, let's call it fmt:

fmt       = sprintf('%s;%d;%%d;%d;%d;%%d;%%d;%%d;%%.4f \\n',str1,num1,0,2)
fmt =
RED;4;%d;0;2;%d;%d;%d;%.4f \n

之后循环中的代码变成：

After which the code inside the loops become:

x = sprintf(fmt, i1, day(i2,1), vect1(i3), vect2(i4), MD(i3,i4,i2,i1));

现在，一个完全的矢量化一个数量级的加速，在我的设置~10.8x从9.61到0.89秒。

Now, a fully vectorized solution, which trades off on RAM but achieves an order of magnitude speedup, on my setup ~10.8x from 9.61 to 0.89 seconds.

tic
[a,b,c,d] = ndgrid(vect2,vect1,day,1:15);
out       = sprintf(fmt, [d(:), c(:), b(:), a(:), reshape(permute(MD,[2,1,3,4]),[],1)]'); 
toc

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