Define function as interpolation of x,y data

I have 2 columns of x y data in data.txt like this:

0  0
1  1
2  4
3  9
4  16
5  25

Now I want to define a function f(x) where x is the first column and f(x) is the second column, and then be able to print values of this function like so:

f(2)

Which should give me 4.

How do I achieve this?

Assuming that you want some return value for numbers between the ones you have as reference, you can use linear interpolation:

    function y= linearLut(x)
         xl = [0 1 2 3 4 5];
         yl = [0 1 4 9 16 25];
         y = interp1(xl,yl,x);
    end

A more generic version of the function might be:

    function y= linearLut(xl,yl,x)
         y = interp1(xl,yl,x);
    end

And then you can create specific instances by using anonymous functions:

    f = @(x)(linearLut([0 1 2 3 4],[0 1 4 9 16],x));
    f(4);