(2) Given a function $f(x)$ defined on $\mathbf{R}$ that satisfies $f(x+1)=f(-x)$, and $f(x)=\left\{\begin{array}{ll}1, & -1<x \leqslant 0, \\ -1, & 0<x \leqslant 1,\end{array}\right.$ then $f(f(3.5))=$ $\qquad$ .
2. -1 Hint: Since $f(x+1)=-f(x)$, then $f(x+2)=f(x)$, $f(3.5)=f(-0.5)=1$. Therefore,
$$
f(f(3.5))=f(1)=-1 .
$$