Curve fitting in matlab

Question

For example i have 5 point like this,

(1,1) (2,-1) (3,2) (4,-2) (5,2)

Now,

• 1) I want a function to interpolation these points in Matlab.
• 2) I want to Plot this function.
• 3) Read a number from input and write F(x) to output.

How can I do this??

Answer 1

To fit a polynom to given datapoints you can use polyfit(x,y,n) where x is a vector with points for x, y is a vector with points for y and n is the degree of the polynom. See example at Mathworks polyfit documentation

In your case:

x=[1,2,3,4,5];
y=[1,-1,-2,-2,2];
n=3;
p = polyfit(x,y,n)

And then to plot, taken from example

f = polyval(p,x);
plot(x,y,'o',x,f,'-')

Or, to make a prettier plot of the polynomial (instead of above plot)

xx=0:0.1:5;
yy = erf(xx);
f = polyval(p,xx);
plot(x,y,'o',xx,f,'-')

Answer 2

If you are not sure what a good fit would be and want to try out different fit, use the curve fitting toolbox, cftool. You will need to create two vectors with x and y coordinates and then you can play around with cftool.

Another option would be to use interp1 function for interpolation. See matlab documentation for more details.