MATLAB: Finding slope of a curve at some specific points

at different pointsslope

I have a a column data set which is the transverse strain of a composite material. I want to calculate slope of it at some specific points

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Your data appear to be smooth enough that you could fit them with a low-order (about 3 or 4) polynomial with polyfit, dividing ‘Index’ by 1E+5 first. Then use polyder to take the derivative of the polyfit coefficients, and use polyval to evaluate the derivative at the values of ‘Index’ you want.

Related Solutions

MATLAB: Differentiation question using diff and polyder

Problems like this are in a sense a bit artful because you have options. I suggest:

% % Part A:

Don't you actually want to calculate:

df1 = diff(y);

instead?

In that situation, I suggest perhaps plotting df1 as:

plot(x(1:length(df1)), df1)

% % Part B:

If you want to calculate a 3rd order polynomial fit, I suggest:

df2 = polyfit(x,y,3);

You might also find the polyval function helpful.

MATLAB: How to get the slope of the peak

Try this:

% Your existing code:
data=xlsread('data.xlsx',1,'A:B'); %import data,column A means the frame number; column B means the intensity%
frame=data(:,1); 
time=data(:,1)./25;
avgI=data(:,2);
B=transpose(avgI); %to make column into row%
[TF,S1,S2]=ischange(B,'linear','Threshold',200);
segline = S1.*(1:3012) + S2; % to get a linear line that fit the peak%
plot(time,B,time,segline)
legend('Data','Linear Regime')
xlabel('Time / s')
ylabel('Mean gray value')
%% New Code
x = time;
y = B;
% Get index for section from time = 54 to 63
index1 = find(x > 54, 1, 'first');
index2 = find(x > 63, 1, 'first');
% Compute slope in that section:
coefficients = polyfit(x(index1:index2), y(index1:index2), 1)
yFit = polyval(coefficients, x(index1:index2));
hold on;
plot(x(index1:index2), yFit, 'r-', 'LineWidth', 2);
caption = sprintf('Slope from time = %f to time = %f is %f', x(index1), x(index2), slope)
title(caption, 'FontSize', 20)
slope = coefficients(1)

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Related Solutions

MATLAB: Differentiation question using diff and polyder

MATLAB: How to get the slope of the peak

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