MATLAB: How to get the y value of xx of spline function

spline

Official example：

       x = -4:4; y = [0 .15 1.12 2.36 2.36 1.46 .49 .06 0];
       cs = spline(x,[0 y 0]);
       xx = linspace(-4,4,101);
       plot(x,y,'o',xx,ppval(cs,xx),'-');

I want to get the 101 y values corresponding to xx.

Thanks

Best Answer

 y = ppval(cs,xx);

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MATLAB: Determining function for smoothing spline

The equations of the cubics between each pair of knots is given in the coefs field. See how I took the demo script from the help and printed them out:

x = -4:4;
y = [0 .15 1.12 2.36 2.36 1.46 .49 .06 0];
cs = spline(x,[0 y 0]);
xx = linspace(-4,4,101);
plot(x,y,'o',xx,ppval(cs,xx),'-');
% Now print out the coefficients
coefficients = cs.coefs
fprintf('The equation for the different segments are:\n');
for k = 1 : size(coefficients, 1)
  fprintf('y = %7.4f * x^3 + %7.4f * x^2 + %7.4f * x + %7.4f\n', coefficients(k,:));
end

In the command window, you will see:

coefficients =
          0.20344440353461       -0.0534444035346097                         0                         0
       -0.0903332106038293          0.55688880706922          0.50344440353461                      0.15
        -0.392111561119293         0.285889175257731          1.34622238586156                      1.12
         0.148779455081001        -0.890445508100147         0.741666053019146                      2.36
         0.136993740795287        -0.444107142857143        -0.592886597938144                      2.36
          0.13324558173785       -0.0331259204712812         -1.07011966126657                      1.46
       -0.0599760677466861         0.366610824742268        -0.736634756995581                      0.49
       -0.0633413107511046         0.186682621502209        -0.183341310751105                      0.06
The equation for the different segments are:
y =  0.2034 * x^3 + -0.0534 * x^2 +  0.0000 * x +  0.0000
y = -0.0903 * x^3 +  0.5569 * x^2 +  0.5034 * x +  0.1500
y = -0.3921 * x^3 +  0.2859 * x^2 +  1.3462 * x +  1.1200
y =  0.1488 * x^3 + -0.8904 * x^2 +  0.7417 * x +  2.3600
y =  0.1370 * x^3 + -0.4441 * x^2 + -0.5929 * x +  2.3600
y =  0.1332 * x^3 + -0.0331 * x^2 + -1.0701 * x +  1.4600
y = -0.0600 * x^3 +  0.3666 * x^2 + -0.7366 * x +  0.4900
y = -0.0633 * x^3 +  0.1867 * x^2 + -0.1833 * x +  0.0600

MATLAB: How to find value on a curve for different x

% 

y=[0.13 0.31 0.45 0.59 0.70 0.79 0.85 0.92 0.97 1];
x=0.1:0.1:1;
plot(x,y,'-')
% xlim([0 1]);
% 
% ylim([0 1]);
xi =0.01 ;
yi = interp1(x,y,xi,'spline') ;
hold on
plot(xi,yi,'*r')

Read about interp1

Best Answer

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MATLAB: Determining function for smoothing spline

MATLAB: How to find value on a curve for different x

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