求助：如何画出分段函数曲面？

[SFWQ1120]

题目图片的截图地址如下：
http://www.yupoo.com/photos/view?id=ff808081132e9d9701133e237f841c4a

我编的M文件是：
[x,y]=meshgrid(-5:0.5:5,-5:0.5:5);
if (x+y)>1;
z=0.54*exp(-0.75*x*x-3.75*y*y-1.5*y);
mesh(z);
end
if (x+y)>-1&（x+y)<=1;
z=0.7575*exp(-1*x*x-6*y*y);
mesh(z);
end
if (x+y)<=-1;
z=0.5457*exp(-0.75*x*x-3.75*y*y+1.5*y);
mesh(z);
end
运行后没有反应，为什么呢？

[freedomlh]

[x,y]=meshgrid(-5:0.5:5,-5:0.5:5);
for i=1:length(x)
if (x(i)+y(i))>1
z=0.54*exp(-0.75*x*x-3.75*y*y-1.5*y);
end
if (x(i)+y(i))>-1 and （x(i)+y(i))<=1;
z=0.7575*exp(-1*x*x-6*y*y);
mesh(z);
end
if (x(i)+y(i))<=-1;
z=0.5457*exp(-0.75*x*x-3.75*y*y+1.5*y);
mesh(z);
end
end