Can I make this function faster?

Question

lianne 2014-2-7

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回答： J 2014-2-8

I need the following function in a program, but it takes a lot of time to run. Does anyone know how I can make it faster? "meting" can be as large as 25000x1 cell, every cell consists of (max, mostly less) 1x2000 char.

if true
  % code
function [meetwaarde,power]=databewerking(meting,multiplier,offset)
string=cell2mat(meting);
hexa=regexp(string(1,:),'\w{1,2}','match');
lengtekolom=size(meting,1);
for i=2:lengtekolom                               
  var=regexp(string(i,:),'\w{1,2}','match');
  hexa=cat(1,hexa,var);
end
meetaantal=size(meting{1},2)/2;
meetwaarde=zeros(lengtekolom,meetaantal);
deca=hex2dec(hexa);
dec=reshape(deca,lengtekolom,meetaantal);   
for i=1:lengtekolom
  for j=1:meetaantal
      if dec(i,j)==0
          meetwaarde(i,j)=0;
      else
          meetwaarde(i,j)=(multiplier*dec(i,j))-offset;  
      end
  end
end
power=sum(meetwaarde,2);
end
end

Thanks a lot!!

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Patrik Ek 2014-2-7

编辑：Patrik Ek 2014-2-7

在 MATLAB Online 中打开

And what about the code above. Does that not give you an error? Anyway in MATLAB you should always have the function definition

function out = myFunction(in)

on the top. Alternally you could have subfunctions below that is called by the "main function" (the first function defined on row 1). However, you cannot call a function written earlier in the code from a row below. Also you can then only call the "main function" from another .m file.

lianne 2014-2-7

If true isn't in the actual code, it is just to show the code correctly (if you click on {}Code, you get this ;) )

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Answer 1

J 2014-2-8

1
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在 MATLAB Online 中打开

Your code is slow because you continuously grow a cell array within this line:

hexa=cat(1,hexa,var);

Since the cell-array is growing every loop, Matlab has to re-alloacate a new memory location for the cell array within each loop. Using vectorized code for this would significantly speed up your code. I think the following code gives the exact same result as your code, but is much faster. For a 5000x1 cell array with 1x2000 char I obtain a speed up of 500x on my laptop.

function [meetwaarde,power]=databewerking(meting,multiplier,offset)
lengtekolom=size(meting,1);
meetaantal=size(meting{1},2)/2;
string=cell2mat(meting);
string = reshape(string.', 2, numel(string)/2).';
deca=hex2dec(string);
dec=reshape(deca,meetaantal,lengtekolom).'; 
meetwaarde=zeros(lengtekolom,meetaantal);
meetwaarde(dec ~= 0) = (multiplier*dec(dec ~= 0))-offset; 
power=sum(meetwaarde,2);

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Answer 2

Dina Irofti 2014-2-7

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Have you tried parallel for loop? See parfor .

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Patrik Ek 2014-2-7

Parfor seems to be a bit of overkill in this case, since it seems the most of the code could be vectorized. Also, the code takes much time to run and no matlabpools are available, eg if the code is run on the personal laptop, then you may not want to occupy all cores at the same time.

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