求助，matlab second derivative approximation,导数逼近

splelvis

clear all;
$ w4 W7 F. I& n8 M9 c) E% H& B3 pnx=50;
$ j, R' _1 {' B3 K- o3 Gny=30;
) o, F- ]9 Q6 d# @, k- \' E0 Bhx=pi/nx;
* ]* [) X9 h7 F; ^) j1 _x=linspace(0,pi,nx+1);
y=linspace(0,pi,ny+1);
x_plus_h=x+hx.*ones(1,nx+1);
" k) u  D/ u! \2 |1 ex_minus_h=x-hx.*ones(1,nx+1);
for i=1:nx+1
# j3 d* `" Z3 v0 c! D9 k, Q1 I1 Q6 c    for j=1:ny+1
        f_xx(j,i)=(f9(x_plus_h(i),y(j))-2*f9+f9(x_minus_h(i),y(j)))./(hx.^2);   %原式f(x+hx,y)-2f(x,y)+f(x-hx,y)./(hx^2)
    end;
end;
[xx,yy]=meshgrid(x,y);
surf(xx,yy,f_xx);
5 E  s5 L0 K1 p* xtitle('partial derivative w.r.t.xx')
，

另开一个f9文件，
  e7 K" b9 \: v  yfunction l=f9(x,y)
, e% ?% H( X1 @1 M% N# J; v9 q6 |l=exp(sin(x-y));


然后出来说错误了，
/ m) w( j# u! n9 P9 w5 Q& z??? Input argument "x" is undefined.

Error in ==> f9 at 2
l=exp(sin(x-y));
Error in ==> partial_derivxx at 11
8 F$ W$ \: n2 T# m1 ?        f_xx(j,i)=(f9(x_plus_h(i),y(j))-2*f9+f9(x_minus_h(i),y(j)))./(hx.^2);
7 t# i8 A* `( A
$ I8 X: p! G$ a各位要怎么改才对呢

splelvis

我自己找到了问题，f9应该是 f9(x(i),y(j))

douglas8j

abc~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

splelvis

不好意思，我也觉得自己太粗心了

douglas8j

henhaodeyipianwenzhang~~~~~~~~~~~~~~~`