Linear interpolation using inter1 function in matlab

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.

Answer 1

[Bjorn Gustavsson]

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

[Hassan]

Thank you very much for your reply. I tried doing this, but I am getting the error "sample points must be unique)

Xi =0;

Xi(i) = Xi + (X_t.T(i) + X_turb.T(i+1))/2;

y_inter = interp1([x1,x2],[y1,y2],Xi);

[Steven Lord]

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);