Plotting Piecewise function produces white empty graph

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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!

Accepted Answer

Walter Roberson
Walter Roberson on 6 Apr 2021
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);
  1 Comment
Tarek Hajj Shehadi
Tarek Hajj Shehadi on 6 Apr 2021
Thank you very much! I will practice harder to ensure not to face the same problem

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