Zeros between Sign Change of Values in column

Hello,

i have a matrix A (1080x2). In the 2. Column i have values which contains sign changes

1.Column 2. Column

1                    0
2                    0
3                    0.00047
4                    -0.0177
5                    -0.0328
6                    0.0075
7                    0
8                    -0.0170
9                    0
10                  0 

The values between sign changes in 2. column should be zero and keep the first value of sign change

1.Column 2. Column

1                    0
2                    0
3                    0
4                    -0.0177
5                    0
6                    0.0075
7                    0
8                    -0.0170
9                    0
10                  0

Maybe helpful is this Question/Answer:

How to determine sign change in a matrix for each coulmn separatly - MATLAB Answers - MATLAB Central (mathworks.com)

Thank You

Answer 1

[Jan]

data = [ ...
    1                    0; ...
    2                    0; ...
    3                    0.00047; ...
    4                    -0.0177; ...
    5                    -0.0328; ...
    6                    0.0075; ...
    7                    0; ...
    8                    -0.0170; ...
    9                    0; ...
    10                   0];

idx          = [true; diff(data(:, 2) >= 0) == 0];
data(idx, 2) = 0

data = 10×2

1.0000 0 2.0000 0 3.0000 0 4.0000 -0.0177 5.0000 0 6.0000 0.0075 7.0000 0 8.0000 -0.0170 9.0000 0 10.0000 0

[Tommy Schumacher]

whith this solution zeros are sign change too.

For Example -0.01, 0, -0.01 will make zero a sign change too!

[Jan]

Does this comment mean, that you want to treat the zero in another way?

Then please post an exhaustive set of input data to define uniquely, what you want to achieve. What is the wanted output for:

[-2, -1, 0, -1, -2]
[-2, -1, 0,  1, -2]
[1,   1, 0,  1,  1]
[1,   1, 0, -1, -1]

It is not automatically clear, how you want to treat the zeros or where the "sign change" occurs in [-1, 0, 1].

Answer 2

[Mathieu NOE]

hello

this is my suggestion below + see attachements

data = importdata('data.txt');
x = data(:,1);
y = data(:,2);


threshold = 0; % 

[t0_pos,s0_pos,t0_neg,s0_neg]= crossing_V7(y,x,threshold,'linear'); % positive (pos) and negative (neg) slope crossing points 
% t0 => corresponding time (x) values 
% s0 => corresponding function (y) values , obviously they must be equal to "threshold"

figure(1)
plot(x,y,t0_pos,s0_pos,'+r',t0_neg,s0_neg,'+g','linewidth',2,'markersize',12);grid on
legend('signal','positive slope crossing points','negative slope crossing points');

% now let's  force  y values to be zero when x values are one step before positive and negative slope zero crossing points
dx = mean(diff(x)); % x axis spacing
indp_before= floor(t0_pos/dx); % index of values before positive slope  zero crossing points 
indn_before= floor(t0_neg/dx); % index of values before negative slope  zero crossing points 
yy = y;
yy(indp_before)= 0;
yy(indn_before)= 0;

figure(2)
plot(x,y,x,yy,t0_pos,s0_pos,'+r',t0_neg,s0_neg,'+g','linewidth',2,'markersize',12);grid on
legend('signal','signal modified','positive slope crossing points','negative slope crossing points');

[Mathieu NOE]

Forgot to mention my result :

yy =

0

0

0

-0.0177

0

0.0075

0

-0.0170

0

0

Answer 3

[Tommy Schumacher]

thanks. For the test values above its working, but for my real working variable (see data.mat attached) is not working (it zeros the found sign change value). You know why?

Thank You!

[Mathieu NOE]

hmm , I am not sure to understand the issue

I exported the original (first column) and modified signal (second column) in the attached excel file

can you add a 3rd column with the expected results ?

thanks