%%%------------------------------------------------------------------------------------------------------------
\begin{tikzpicture}
\begin{axis}[  
  name=relu,
  width=6cm, height=4.5cm, 
  xtick={-1.0,-0.5,...,1.0},
  ytick={0.5,1.0},
  xlabel={$x$},
  ylabel={$y$},
  xlabel style={xshift=2.4cm,yshift=0.7cm},
  axis y line=middle,%y轴居中
  ylabel style={xshift=0.1cm,yshift=0cm},
  x axis line style={->},%x轴箭头
  axis line style={very thick},
  axis x line*=middle,%将上面的x轴去掉，仅保留下面的x轴
  xmin=-1.2,
  xmax=1.2,
  ymin=0,
  ymax=1.2]
\addplot[domain=-1:0,draw=red,very thick]{0};
\addplot[domain=0:1,draw=red,very thick]{x};
\end{axis}
\node [anchor=north] (labelc) at (2.3,-0.5) {\footnotesize{(a)}};
\begin{axis}[  
  at={(relu.east)},
  anchor=east, 
  xshift=6cm,
  yshift=0cm, 
  width=6cm, height=4.5cm, 
  xtick={-4,-2,...,4},
  ytick={0,0.2,...,1.0},
  xlabel={$x$},
  ylabel={$y$},
  xlabel style={xshift=2.4cm,yshift=0.7cm},
  axis y line=middle,%y轴居中
  ylabel style={xshift=0.1cm,yshift=0cm},
  x axis line style={->},%x轴箭头
  axis line style={very thick},
  axis x line*=middle,%将上面的x轴去掉，仅保留下面的x轴
  xmin=-5,
  xmax=5,
  ymin=0,
  ymax=1.2]
\addplot[domain=-4:0,draw=ublue,very thick]{0};
\addplot[domain=0:4,draw=ublue,very thick]{1};
\end{axis}
\node [anchor=north] (labelc) at (8.2,-0.5) {\footnotesize{(b)}};
\end{tikzpicture}
%%%------------------------------------------------------------------------------------------------------------