fouriertransform with piecewise function

I am trying to apply a fft (fourier transform) into a piecewise function, but i receive the following message: "Error using fft

Invalid data type. First argument must be double, single, int8, uint8, int16, uint16, int32, uint32, or logical."

Code:

figure(1);
syms t
x = piecewise(-1/2<=t<=1/2, 1-abs(t), t<-1/2, 0 , t>1/2, 0);
fplot(x,[-2,2])
grid
xlabel("Time")
ylabel("Amplitude")


fa = 15000; 
t = 0:1/fa:2; 

y=fft(x);
f = -fa:1:fa;

figure(2);
subplot(2,1,1);

plot(f,abs(y_ajustado)/fa);
xlabel("Frequency (Hz)")
ylabel("Amplitude")
xlim([0 400]) 

subplot(2,1,2)
plot(f,angle(y_ajustado));
xlabel("Frequency (Hz)")
ylabel("Fase (Graus)")
xlim([0 400])

Answer 1

[VBBV]

use double and subs() the result of x

figure(1);
syms t
x = piecewise(-1/2<=t<=1/2, 1-abs(t), t<-1/2, 0 , t>1/2, 0)

x = 

fplot(x,[-2,2])
grid
xlabel("Time")
ylabel("Amplitude")

fa = 15000; 
t = 0:1/fa:2; 
y=fft(double(subs(x,t)));
plot(t,real(y),t,imag(y))

[Elisa]

thank you!!!!

[Paul]

Why is y plotted agains t? Shouldn't y be plotted against frequency? And the fft is only taken for the portion of the signal for t>=0, but it's non-zero for t < 0.