How Can i speed up "for" loop ?

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Giuseppe Napoli il 3 Feb 2017
Risposto: Mina Mino il 7 Apr 2023
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;
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Stephen23 il 3 Feb 2017
Modificato: Stephen23 il 4 Feb 2021
MATLAB's most basic data type is the array, and one can perform many operations on the whole array at once:
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:
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Giuseppe Napoli il 3 Feb 2017
Thanks for your reply. Infact vectorizing is the fastest way to fill an array. I have to change my way of thinking.
Stephen23 il 3 Feb 2017
Modificato: Stephen23 il 3 Feb 2017
@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).

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Mina Mino il 7 Apr 2023
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
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
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