Evaluating argument list element number 1

Question

I have the following function:

% function file: lagrange.m
function Yint = lagrange( x, y, Xint )
    n = length( x );
    for i = 1 : n
        L(i) = 1;
        for j = 1 : n
            if j ~= i
                L(i) = L(i) * (Xint - x(j)) / (x(i) - x(j));
            end
        end
    end
    Yint = sum( y .* L );
end

Then I try to use it from this script:

% script file: roteiro_lagrange.m
clear all
x  = [1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28];
y  = [2.682942 3.818595 3.28224 2.486395 3.082151 5.441169 8.313973 9.978716 9.824237 8.911958 9.00002 10.92685 13.84033 15.98121 16.30058 15.42419 15.7721 16.49803$
xi = [16.8 18.2 2.8 15.1 19.2 5.6 13.1 19.8 8.2 10.9 20.0 10.7 8.4 19.8 13.0 5.8 19.2 15.0 3.0 18.3 16.7 0.2 16.9 18.1 2.6 15.3 19.1 5.4];

n  = length(x)
ny = length(y)
YI = [];
for i = 1 : n
    xp = xi(i);
    for k = 2 : n
        if x(k) > xp
            xx = [x(k-1) x(k)];
            yy = [y(k-1) y(k)];
            yp = lagrange( xx, yy, xp );
            YI = [YI yp];
            break
        endif
    endfor
endfor
y_interp = YI'
plot( x, y, 'r', xi, YI, 'k*' )

but I get the following error when trying to run it:

 lagrange
error: 'x' undefined near line 3 column 10
error: called from
    lagrange at line 3 column 2
error: evaluating argument list element number 1
error: called from
    lagrange at line 3 column 2

(see screenshots of the problem here: https://imgur.com/a/1b5WwDC )

Answer 1

You are calling the function directly, with no arguments (note, in matlab/octave, lagrange is exactly the same as doing lagrange() ).

You should be calling your script instead.