How can I make this loop faster?

2020 4 3
1 个回答
更新时间：2020 4 3
4 次查看（30 天）

0 个投票

Why is this simple loop taking me so much time?
n=2^13;
idx=cell(2*n-1,1);
for i=1:2*n-1
temp=zeros(i,2);
for j=1:i
if j>n
idx1=0;
else
idx1=j;
end
if i-j+1>n
idx2=0;
else
idx2=i-j+1;
end
temp(j,:)=[idx1,idx2];
end
idx{i}=temp;
end

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回答（1 个）

Ameer Hamza
Ameer Hamza 2020-4-3

0 个投票

The matrices generated by your code just contain a linear array of integers. If you can tell, why do you want to create them in the first place, maybe I can suggest a better solution. Anyway, for the current code, try the following
n=2^13;
idxA=cell(2*n-1,1);
for i=1:2*n-1
if i <= n
temp = (1:i)';
else
temp = [(1:n)';zeros(i-n,1)];
end
idxA{i} = [temp flipud(temp)];
end
On my system, the execution time improved by about 8x.
tic
n=2^13;
idxA=cell(2*n-1,1);
for i=1:2*n-1
if i <= n
temp = (1:i)';
else
temp = [(1:n)';zeros(i-n,1)];
end
idxA{i} = [temp flipud(temp)];
end
toc
tic
n=2^13;
idxB=cell(2*n-1,1);
for i=1:2*n-1
temp=zeros(i,2);
for j=1:i
if j>n
idx1=0;
else
idx1=j;
end
if i-j+1>n
idx2=0;
else
idx2=i-j+1;
end
temp(j,:)=[idx1,idx2];
end
idxB{i}=temp;
end
toc
isequal(idxA, idxB)
Result:
Elapsed time is 1.842785 seconds.
Elapsed time is 15.006841 seconds.
ans =
logical
1

6 个评论
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I want to make a code that solve convolution equations, I want to solve for x such as:
G(f)*x(f)=F(f)
Where * is the convolution operator. So I need to set up the lhs of the equation based on the convolution formula (you can find it in the more about here: https://www.mathworks.com/help/matlab/ref/conv.html
Ameer Hamza
Ameer Hamza 2020-4-3
编辑：Ameer Hamza 2020-4-3
Does the code given in my answer work for your problem?
I have a second part that I want to make faster(it has a somewhat similar structure). After getting the indices from the first part I setup the equations as follows. It's also taking me so much time. G is a vector of complex numbers having a length n.
counter=0;
for i=n/2+1:n/2+n
temp=idx{i};
indices = find(temp(:,2)==0);
temp(indices,:) = [];
for j=1:length(temp)
if temp(j,1)==0
rhs=0;
else
rhs=G(temp(j,1));
end
mat(counter,temp(j,2))=rhs;
end
counter=counter+1;
end
Ameer Hamza
Ameer Hamza 2020-4-3
Are you trying to implement the convolution function?
Yes. I can't use the ifft(fft) trick since I need to obtain a system of equations to solve.
Ameer Hamza
Ameer Hamza 2020-4-3
Sorry, I don't understand this code, so it is difficult for me to suggest an optimization by vectorizing. This code runs quite slow, and maybe you need to rethink about your problem from basics. There might be some other efficient way to get the system of equations you want.

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