My function doesn't append to it's list

%calculates polynomials of form Pk = ((2k-1)t/k)*Pk-1 - ((k-1)/k)*Pk-2
function [poly, filllist] = calc_poly (n, t, filllist)
    if n == 0
        filllist = [filllist; ones(size(t))];
        poly = ones(size(t));
        return
    end
    if n == 1
        filllist = [filllist; t];
        poly = t;
        return
    end
    poly = ((2*n-1)/n)*t.*calc_poly(n-1, t, filllist) - ((n-1)/n)*calc_poly(n-2, t, filllist);
    filllist = [filllist; poly];
end

This function is supposed to recursively calculate the polys and append them into filllist, but the array remians unalatered after running it. I know that as it stands there would be duplicates in the list, but I first want to get the function to actually do anything.

Answer 1

[Steven Lord]

Your recursive calls only call your function with one output argument, so the modified second output argument is not returned from that function. Therefore any changes you make to your third input argument are discarded when each recursive call returns.

[Kacper]

I tried it with only one output and it still doesn't work. I don't actually care about the filllist output since it's supposed to operate on the given array, not an array of it's own context.

[Kacper]

%calculates polynomials of form Pk = ((2k-1)t/k)*Pk-1 - ((k-1)/k)*Pk-2
function [poly, filllist] = calc_poly (n, t, filllist)
    if n == 0
        filllist = [filllist; ones(size(t))];
        poly = ones(size(t));
        return
    end
    if n == 1
        filllist = [filllist; t];
        poly = t;
        return
    end
    [poly1, filllist] = calc_poly(n-1, t, filllist);
    poly2 = calc_poly(n-2, t, []);
    poly = ((2*n-1)/n)*t.*poly1 - ((n-1)/n)*poly2;
    filllist = [filllist; poly];
end

This is a revised function where it returns the fillist with each call, but it still does nothing. I call it like this:

syms z;
list = [];
[polynomials, list] = calc_poly(1, z, list);

What do I do now? There's nothing on passing by reference or copy in matlab's variables and matricies docs.

[Walter Roberson]

It gives the result I would expect from that code ?

syms z;
list = [];
[polynomials, list] = calc_poly(1, z, list)

polynomials = 

z

list = 

z

%calculates polynomials of form Pk = ((2k-1)t/k)*Pk-1 - ((k-1)/k)*Pk-2
function [poly, filllist] = calc_poly (n, t, filllist)
    if n == 0
        filllist = [filllist; ones(size(t))];
        poly = ones(size(t));
        return
    end
    if n == 1
        filllist = [filllist; t];
        poly = t;
        return
    end
    [poly1, filllist] = calc_poly(n-1, t, filllist);
    poly2 = calc_poly(n-2, t, []);
    poly = ((2*n-1)/n)*t.*poly1 - ((n-1)/n)*poly2;
    filllist = [filllist; poly];
end

[Kacper]

huh. The list is empty when I run it.

[Walter Roberson]

Experiment with creating a script that has both parts together, and running the script. You can use the "Copy" box on the code I posted to get the code to be pasted into the script.

[Kacper]

I pasted your code to a new script and it worked, despite the previous script being the same. huh. Anyway, thx for your answers!