I want to know how to speed up my for loop,it's a simple for loop.thanks

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ang lee 2019-10-22

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评论： ang lee 2019-10-23

采纳的回答： Daniel M

%% this code cost aroud 6s.

b=[]; %Establish a null matrix

N=length(ampw_1); % N=103;

M=length(amp_m); % M=512;

for j = 1:M-N+1; % j=1：1：410；

index_sm = j; %index_sm = 1:410;

index_em = j+N-1; %index_em = 103:512;

ampw_2 = abs(amp_m(index_sm:index_em))/max(abs(amp_m(index_sm :index_em))); %ampw_2 from 1 to 103; from 2 to 104; from 3 to 105....from 410 to 512

% Every loop，this segment will store 103 * 1 So,the At the end of the loop,the ampw_2 array is 103 * 410 and give b array.In my program, this statement took 4S，and i want to know how to speed it up.

b = [b ampw_2]; %give the result of every loop to b,at the end of the loop , the b array is 103*410;

end

%% then I I built an empty array，the code as follow,But it's only a few seconds shorter,just 0,4s.

N=length(ampw_1); %N=103

M=length(amp_m); %M=512

ampw_2 = zeros(N,M-N+1); %Set up a zero array of 103 * 410;

for j = 1:M-N+1; % j=1：1：410；

index_sm = j; %index_sm = 1:410;

index_em = j+N-1; %index_em = 103:512;

ampw_2(:,j) = abs(amp_m(index_sm:index_em))/max(abs(amp_m(index_sm :index_em)));

end

%% I want to know is there Any method to speed up the program,then I use the matlabpool open 2;this code cost So More time,it's 60s; Just modify:

parfor j=1:M-N+1;

....

end

if you have any method or proposal,please tell me , thank you very much,

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Daniel M 2019-10-22

You don't need to do this in a loop at all.

ang lee 2019-10-22

Can you be more specific,or for a simple example,I can't run the code you gave me.thank you again

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Answer 1

Daniel M 2019-10-22

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编辑：Daniel M 2019-10-22

在 MATLAB Online 中打开

You don't need to do this in a loop at all. Here is how to vectorize the creation of the indices from 1:103, ..., 410:512. From there you can probably figure out how to do it without using a loop at all. (Assuming amp_m is a variable and not a function).

n = 103;
m = 410;
indexMatrix = (0:n-1)'+(1:m);

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Daniel M 2019-10-22

在 MATLAB Online 中打开

clear
clc
% in a loop
b=[];                            %Establish a null matrix
N=103;    %  N=103;
M=512;     %  M=512;
amp_m = rand(1,M);
tic
for   j = 1:M-N+1            %   j=1:1:410;
    index_sm = j;                %index_sm =  1:410;
    index_em = j+N-1;        %index_em = 103:512;
    ampw_2 = abs(amp_m(index_sm:index_em))/max(abs(amp_m(index_sm :index_em)));  %ampw_2 from 1 to 103; from 2 to 104; from 3 to 105....from 410 to 512
    %  Every loop,this segment will store 103 * 1  So,the At the end of the loop,the ampw_2 array is 103 * 410 and give b array.In my program, this statement took           4S,and i want to know how to speed it up.
    b =  [b ampw_2];    %give the result of every loop to b,at the end of the loop , the b array is 103*410;
end
toc % Elapsed time is 0.015229 seconds.
%%% vectorized solution
tic
indexMatrix = (0:N-1)'+(1:(M-N+1));
absamp_m = abs(amp_m(indexMatrix));
ampw_2b = absamp_m./max(absamp_m);
b2 = ampw_2b(:)';
toc % Elapsed time is 0.000938 seconds.
isequal(b,b2) % ans = 1

The results are equal. There is a speedup of 16x using the vectorized method. (0.015229/0.000938 = 16.236)

Daniel M 2019-10-23

在 MATLAB Online 中打开

You must have an older version of MATLAB that can't do implicit expansion. Here is the fix for that line of code. Tell me if there are other errors.

indexMatrix = bsxfun(@plus,(0:N-1)',(1:(M-N+1)));

ang lee 2019-10-23

thank you, I can use this code in other version,and your answer is so good.Help me a lot.If I have other question,i hope i can ask you . thank you again,best wishes.

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