Find peaks in a matrix containing zeros

Question

Askeladden2 2024-1-17

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https://ww2.mathworks.cn/matlabcentral/answers/2071026-find-peaks-in-a-matrix-containing-zeros

评论： Askeladden2 2024-1-17

Dear All Community Members,

I have two matrices, 'A' and 'B'. Matrix 'A' represents the time history of velocities, while matrix 'B' represents the corresponding times. I want to extract all the peak velocity values from matrix 'A' that are above a threshold value of 3.

Matrix A is as follows:

A=[ 1 3 -1 1 5 -1 0;1 6 3 -2 0 0 0;2 3 9 -2 1 11 -1;4 1 -2 8 5 0 0;2 -2 6 -1 0 0 0;-1 13 -2 0 0 0 0];

Matrix B is as follows:

B=[1 1.5 2.5 3 3.5 4 0;1.5 3 5 6 0 0 0;0.5 2 2.5 2.75 3 4 5;2 4 4.25 4.5 6 0 0;1 2 3 4 0 0 0;0.5 3 4 0 0 0 0];

Specifically, I am looking to extract the value 5 from the first row, the value 6 from the second row, the values 9 and 11 from the third row, and then values 8,6,13 from the from the fourth and to the last row in matrix A.

I have tried the findpeaks function, but I am getting errors because of the ending zeros in matrix B.

Can anyone help me out?

Thank you in advance.

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Dyuman Joshi 2024-1-17

编辑：Dyuman Joshi 2024-1-17

I am not sure if I get the logic used to extract the values.

In the 4th row, 4 is above 3, but they are not extracted.

How does the values from B come into play here?

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Answer 1

Stephen23 2024-1-17

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2071026-find-peaks-in-a-matrix-containing-zeros#answer_1391586

在 MATLAB Online 中打开

A = [1,3,-1,1,5,-1,0;1,6,3,-2,0,0,0;2,3,9,-2,1,11,-1;4,1,-2,8,5,0,0;2,-2,6,-1,0,0,0;-1,13,-2,0,0,0,0]
A = 6×7
     1     3    -1     1     5    -1     0
     1     6     3    -2     0     0     0
     2     3     9    -2     1    11    -1
     4     1    -2     8     5     0     0
     2    -2     6    -1     0     0     0
    -1    13    -2     0     0     0     0
F = @(a)findpeaks(a,'Threshold',3);
C = cellfun(F,num2cell(A,2),'uni',0)
C = 6×1 cell array
    {[   5]}
    {[   6]}
    {[9 11]}
    {[   8]}
    {[   6]}
    {[  13]}

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Stephen23 2024-1-17

在 MATLAB Online 中打开

V = [1,1,2,4,2,-1]
V = 1×6
     1     1     2     4     2    -1
X = findpeaks(V)
X = 4
Y = islocalmax(V)
Y = 1×6 logical array
   0   0   0   1   0   0

By specifying the threshold explicitly then of course I can change what peaks are detected:

X = findpeaks(V, 'Threshold',3)
X =

  1×0 empty double row vector

Clearly 4 is not >=3 compared to the 2's on either side, so it is (according to my requirement) not a maxima. In fact, there are no maxima in that vector according to the threshold that I specified.

"but then the labelling that it finds the local maxima is plainly wrong and 'maxima' should be removed from the documentation."

I do not follow your argument: the definition of what is a local maxima depends on the prominence, the window, etc... I do not see what is "plainly" wrong here.

Askeladden2 2024-1-17