# Linear interpolation using inter1 function in matlab

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Hassan on 5 Aug 2022
Commented: Steven Lord on 5 Aug 2022
I have data in this format. I am reading the data using readtable command.
T G
2 6
3 8
4 9
5 0
I am using this code to perform linear interpolation in matlab. Data is read using following command X_t = readtable()
for i = 1:height(X_t)-1
x1(i) = X_t.T(i);
x2(i) = X_t.T(i+1);
y1(i) = X_t.G(i);
y2(i) = X_t.G(i+1);
for j = X_t.T(i) : X_t.T(i+1) -1
y_inter = interp1([x1,x2],[y1,y2]);
end
end
I am not sure if I am doing this correctly or not. Please help me.

Bjorn Gustavsson on 5 Aug 2022
Edited: Bjorn Gustavsson on 5 Aug 2022
First you should learn to read the help to the function you struggle with. Do that carefully and you will see that the typical use of interp1 is:
Yi = interp1(x,y,xi);
Where you send in a third input argument for the points along the x-direction that you want the interpolated values Yi at. You have to change your call to something like:
x = X_t.T;
y = X_t.G;
Xi = linspace(min(x),max(x),10*numel(x)); % this you might want to modify to get values at your points-of-interest
y_inter = interp1(x,y,Xi);
HTH
##### 2 CommentsShowHide 1 older comment
Steven Lord on 5 Aug 2022
That means two or more of your X values are the same. MATLAB can't interpolate the data in that case. Consider a simple example:
X = [0 0.5 0.5 1];
Y = [0 0 1 1];
plot(X, Y, 'o-') If I were to interpolate this data to try to find the value of Y at X = 0.5, what should that value be? Should it be 0, 1, or a value somewhere inbetween? Since that question is ambiguous, MATLAB throws an error.
interp1(X, Y, 0.5) % Error
Error using matlab.internal.math.interp1
Sample points must be unique.

Error in interp1 (line 188)
VqLite = matlab.internal.math.interp1(X,V,method,method,Xqcol);

R2021a

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