# Condition for Specific Data collection

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Adil Sardar on 17 Nov 2020
Commented: Adil Sardar on 19 Nov 2020
Hi everyone
I am facing a problem i have different Excel files in which there is a lot of data. i want to find some common condition for all these file to get the specific data (e.g i circle some example in the graph). I also attach 3 excel file as an example. I want to find some condition according to current or volatge column in the excel file. there is 3 column time, voltage and current. you can use any columnn for finding the condition to get some specific data and discard the remaining data. waiting for your kind help.
Thank you.
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Adil Sardar on 17 Nov 2020
This data is from the running condition of a system output. I want to select some data as i circle in the graph for this i want to have a general code which select the data like i circle. i don't know what kind of condition or creteria i used to get random data for some time. or we can say that it the direction of voltage or current is changing for some time then save that data or any other condition if you suggest.
thank you

Mathieu NOE on 17 Nov 2020
hello
see example below
this could be interesting for data 1 example
but the two other data it's not feasible unless someone come's with a clever condtion equation to select such fraction of data
time = num(:,1); % s
current = num(:,2); % A
voltage = num(:,3); % V
samples = length(time);
dt = (time(end)-time(1))/(samples-1);
Fs = 1/dt;
% main loop : extract min max vales for every 5 min data %
[dvoltage, ddvoltage] = firstsecondderivatives(time,voltage);
figure(1);
subplot(2,1,1),plot(time,voltage,'-b');grid
subplot(2,1,2),plot(time,dvoltage,'-b');grid
% selected buffer is defined between the two positive peaks of dvoltage
dvoltage_max = max(dvoltage(dvoltage>0));
threshold = dvoltage_max/2;
ind = find(dvoltage > threshold);
ind_start = ind(1)-100;
ind_stop = ind(end)-100;
% plot selected portion of data
figure(2);
plot(time,voltage,'-b',time(ind_start:ind_stop),voltage(ind_start:ind_stop),'or');grid
function [dy, ddy] = firstsecondderivatives(x,y)
% The function calculates the first & second derivative of a function that is given by a set
% of points. The first derivatives at the first and last points are calculated by
% the 3 point forward and 3 point backward finite difference scheme respectively.
% The first derivatives at all the other points are calculated by the 2 point
% central approach.
% The second derivatives at the first and last points are calculated by
% the 4 point forward and 4 point backward finite difference scheme respectively.
% The second derivatives at all the other points are calculated by the 3 point
% central approach.
n = length (x);
dy = zeros;
ddy = zeros;
% Input variables:
% x: vector with the x the data points.
% y: vector with the f(x) data points.
% Output variable:
% dy: Vector with first derivative at each point.
% ddy: Vector with second derivative at each point.
dy(1) = (-3*y(1) + 4*y(2) - y(3)) / 2*(x(2) - x(1)); % First derivative
ddy(1) = (2*y(1) - 5*y(2) + 4*y(3) - y(4)) / (x(2) - x(1))^2; % Second derivative
for i = 2:n-1
dy(i) = (y(i+1) - y(i-1)) / 2*(x(i+1) - x(i-1));
ddy(i) = (y(i-1) - 2*y(i) + y(i+1)) / (x(i-1) - x(i))^2;
end
dy(n) = (y(n-2) - 4*y(n-1) + 3*y(n)) / 2*(x(n) - x(n-1));
ddy(n) = (-y(n-3) + 4*y(n-2) - 5*y(n-1) + 2*y(n)) / (x(n) - x(n-1))^2;
end
Adil Sardar on 18 Nov 2020
Thank you soo much for your time and help.

Peter Perkins on 19 Nov 2020
Unless you are using a fairly old version of MATLAB, I strognly recommend that you stay away from xlsread, and use readtable or readtimetable instead.
Adil Sardar on 19 Nov 2020
Thank you for your suggestion but my problem is not reading the data my problem is to find a condition for selecting some specific data.