find indicies of maximum and minmum elements of an array in its each segmented smaller arrays, plus mean
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I have a large array A (1x3,000,000) and it was segmented by an indexA (1x5,000). Part of A (first 100 element) and indexA are as below:
A=[0 0 0 0 0 0 ...
0 0 0 0 0 0 ...
0 0 0 0 0 0 ...
0 0 0 0 0 0 ...
0 0 0 0 0 0 ...
0.4665982 1.0120536 1.0120536 1.0120536 1.0120536 1.0120536 ...
2.2720058 5.6612258 6.2438807 6.2438807 6.2438807 0.1706306 ...
3.4740953 7.5749402 8.3634338 8.3634338 8.3634338 0.9215111 ...
5.1563759 9.1346588 9.5029898 9.5029898 0.3987859 2.7512887 ...
7.7671609 11.3574429 11.4436398 0.3256813 1.5965981 5.1405444 ...
10.3805666 13.0157471 0 0.6612921 2.3093321 6.6952753 ...
11.4918060 12.5332108 0.1177574 1.0389878 3.9089594 8.5320177 ...
10.2545567 10.2545567 0.0723169 2.8528883 7.3057065 8.5047731 ...
0.0136646 0.8193274 4.6227546 8.9367990 9.6189337 0.2354149 ...
1.6503071 5.8358650 10.0257235 10.5574694 0.4393677 2.0512471 ...
6.2374287 10.5120144 11.1818972 0.4886205 2.0159214 5.8866639 ...
10.4216042 11.6498079 0.3699284 1.7069057];
indexA=[8 16 30 37 43 48 66 71 76 81 86 91 97];
The problem is to find index of max/min of A in each small segemented array based of the indexA, such as the segmented array are A(1:8-1), A(8:16-1),...A(91:97-1), A(97, end).
my solution using for loop is as below:
if indexA(1)>1
indexA=[1 indexA];
elseif indexA(end)<length(A)
indexA=[indexA length(A)];
end
Amax=[];idxAmax=[];Amin=[];idxAmin=[];Amean=[];
for i=1:length(indexA)-1
[val, idx] = max(A(indexA(i):indexA(i+1)-1);
idxAmax=[idxAmax idx+indexA(i)-1];
Amax =[Amax val];
[val, idx] = min(A(indexA(i):indexA(i+1)-1);
Amin =[Amin val];
idxAmin=[idxAmin idx+indexA(i)-1];
Amean=[Amean mean(A(indexA(i):indexA(i+1)-1)];
end
Is there other solution simpler and faster than using for loop? (The problem is updated. Elements of IndexA is the begining index of each segment in A now. And you can copy A directly to Matlab now).
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채택된 답변
Bruno Luong
2020년 8월 11일
편집: Bruno Luong
2020년 8월 11일
Test example
A=1:100;
indexA=[8 16 30 37 43 48 66 71 76 81 86 91 97];
Code
I = zeros(size(A));
I([0 indexA]+1) = 1;
I(length(A)+1:end) = []; % incase indexA(end) contains the last index of A
I = cumsum(I);
[minAI,maxAI] = splitapply(@bounds, A, I);
meanAI = splitapply(@mean, A, I);
댓글 수: 6
Bruno Luong
2020년 8월 12일
편집: Bruno Luong
2020년 8월 12일
Yeah as most of the recent MATHWORKS functions (SplitApply) it's slow. If speed is important and you are willing to go with histotical function, they usually faster. In this case the old ACCUMARRAY will do nicely the job and fast (which SPLITAPPLY tries to replace).
%%
A=1:1e6+1;
indexA=8:8:1e6;
tic
NewIndex=diff([0,indexA,numel(A)]);
cellA=mat2cell(A,1,NewIndex);
[~,idxMaxA]=cellfun(@max,cellA);
idxMaxA=idxMaxA+[0,indexA];
toc % Elapsed time is 0.346554 seconds.
tic
I = zeros(size(A));
I([0 indexA]+1) = 1;
I(length(A)+1:end) = []; % incase indexA(end) contains the last index of A
I = cumsum(I);
maxAI = splitapply(@max, A, I);
toc % Elapsed time is 2.071576 seconds.
tic
I = zeros(size(A));
I([0 indexA]+1) = 1;
I(length(A)+1:end) = []; % incase indexA(end) contains the last index of A
I = cumsum(I);
idxmaxAI = accumarray(I(:),A(:),[],@max)';
toc % Elapsed time is 0.120087 seconds.
추가 답변 (1개)
Fangjun Jiang
2020년 8월 11일
편집: Fangjun Jiang
2020년 8월 11일
Here is one way to find the max index without for-loop. Not sure if it is faster for larger data
%%
A=1:100;
indexA=[8 16 30 37 43 48 66 71 76 81 86 91 97];
NewIndex=diff([0,indexA,numel(A)]);
cellA=mat2cell(A,1,NewIndex);
[~,idxMaxA]=cellfun(@max,cellA);
idxMaxA=idxMaxA+[0,indexA]
idxMaxA =
8 16 30 37 43 48 66 71 76 81 86 91 97 100
댓글 수: 5
Fangjun Jiang
2020년 8월 12일
Answer for the modified question.
NewIndex=diff([1,indexA,numel(A)+1]);
cellA=mat2cell(A,1,NewIndex);
[~,idxMaxA]=cellfun(@max,cellA);
idxMaxA=idxMaxA+[0,indexA-1]
idxMaxA =
7 15 29 36 42 47 65 70 75 80 85 90 96 100
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