You question is not very clear, so I will try to guess what you are trying to do.
If you want to define a purely symbolic function f(x), use the following syntax.
If you want to create a function from data points x and y, use the griddedInterpolant function to create a interpolation of x and y.
x=[1 5 0 32 8];
y=[10 1 654 32 100];
[~, index] = sort(x);
F = griddedInterpolant(x(index), y(index));