Plotting Piecewise function produces white empty graph

Consider the following code :

function [X] = FourierSinc(a,b,K)
w = linspace(-10,10,600);
syms w
U = piecewise(abs(w)<a*pi,K, abs(w)>a*pi, 0);
X=exp(-1i*b*w)*U;
figure, fplot(w,X); 

Taking simple values like a=2; b=4; c=3; produces a (correct) answer :

ans =
 
piecewise(abs(w) < 2*pi, 3*exp(-w*4i), 2*pi < abs(w), 0)

However the graph displays nothing but white and empty grid. I hope someone helps me and thank you very much!

Answer 1

[Walter Roberson]

function [X] = FourierSinc(a,b,K)
w = linspace(-10,10,600);

You assign a list of definite numeric values to w

syms w

You immediately throw away those values and do the equivalent of

w = sym('w')

U = piecewise(abs(w)<a*pi,K, abs(w)>a*pi, 0);
X=exp(-1i*b*w)*U;

If you ever switch back to numeric w, be careful because the left and right side of the * would be row vectors, which would be a problem for the * operator. With your current setting of syms w this is not a problem, but you could make it easier to understand by using .* instead of *

figure, fplot(w,X); 

fplot() accepts a symbolic expression or symbolic function (or array of those) in the first parameter; the second parameter is the bounds for plotting.

I would suggest that you want something like

function [X] = FourierSinc(a,b,K)
W = linspace(-10,10,600);
syms w
U = piecewise(abs(w)<a*pi,K, abs(w)>a*pi, 0);
X=exp(-1i*b*w).*U;
figure;
xw = double(subs(X, w, W));
plot(W, xw);

[Tarek Hajj Shehadi]

Thank you very much! I will practice harder to ensure not to face the same problem