How Can i speed up "for" loop ?

Hello to everyone, I need to speed up my for loop. I use this loop for filling a vector with a temperature profile (that changes over time). I don't know if this is the fastest way so I am asking for your help. Thanks in advance.

 T=25 + 273; 
 Tmax= 1100+273;
 h=0.001;
 temporaggiungimento=50;
 tmax=100;
 passo=( Tmax / temporaggiungimento)*h;
 profilo1=linspace(25+273,1100+273,temporaggiungimento/h);
 tempo=zeros(1,tmax/h);
 profilo=zeros(1,tmax/h);
 profilo(1)=25+273;
 for j=1:(temporaggiungimento/h-1)
    j=j+1
    %profilo(j)=profilo(j-1)+passo;
    profilo(j)=profilo1(j);
    for j=temporaggiungimento/h:(tmax/h)-1
        j=j+1;
        profilo(j)=1100+273;
    end;
  end;

Answer 1

[Stephen23]

MATLAB's most basic data type is the array, and one can perform many operations on the whole array at once:

https://www.mathworks.com/help/matlab/matlab_prog/vectorization.html

Try something like this instead of those loops:

profilo2 = zeros(1,tmax/h);
idx = 1:temporaggiungimento/h;
profilo2(idx) = profilo1;
idy = 1+temporaggiungimento/h:tmax/h;
profilo2(idy) = 1100+273;

And the time difference:

Elapsed time is 0.007594 seconds.   % my code
Elapsed time is 108.509334 seconds. % your code

Yet the output is exactly the same:

>> isequal(profilo,profilo2)
ans =
     1

More than ten thousand times faster with effective use of arrays:

https://www.mathworks.com/help/matlab/matlab_prog/techniques-for-improving-performance.html

https://www.mathworks.com/matlabcentral/answers/228557-experts-of-matlab-how-did-you-learn-any-advice-for-beginner-intermediate-users

[Giuseppe Napoli]

Thanks for your reply. Infact vectorizing is the fastest way to fill an array. I have to change my way of thinking.

[Stephen23]

@Giuseppe Napoli: I am glad to be able to help. If my answer helped you (e.g. sped up your code more than ten thousand times), then please accept my answer. On this forum accepting the best answer is an easy way for you to show that you appreciate our effort (we are volunteers).

Answer 2

[Mina Mino]

I have the same problem and I need to speed up my code in the following form. I would be grateful if anyone can help me. it is so urgent and important for me. Thanks.

%begining of the Code%

clc

clear all

close all

format long g

fid=fopen('list2.txt','r');

if fid==-1

disp('there is an error')

else

end

S = textscan(fid,'%s');

fclose(fid);

i=1;

fnames= S{i,i};

tic

NA=[];

for fiile=1:700

varargout = readhgt(fnames{fiile});

LAT=varargout.lat;LAT(end)=[];

LON=varargout.lon;LON(end)=[];

Z=varargout.z;Z(:,end)=[];

Z(end,:)=[];

% Z_vec=reshape(Z,[12967201,1]);

ROUGH=[];

for row=1:3:3597

for col=1:3:3597

A=[Z(row,col) Z(row,col+1) Z(row,col+2);Z(row+1,col) Z(row+1,col+1) Z(row+1,col+2);Z(row+2,col) Z(row+2,col+1) Z(row+2,col+2)];

Dif=(double(A)-mean(mean(A))).^2;

rough=sqrt(mean(mean(Dif)));

ROUGH=[ROUGH;mean([LAT(row) LAT(row+1) LAT(row+2)]) mean([LON(col) LON(col+1) LON(col+2)]) rough];

end

end

NA=[NA;ROUGH];

end

toc

%end of code