Error converting function handle to double?

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Pizza Pie
Pizza Pie 2016-5-4
评论： Star Strider 2016-5-5
Hey guys, any idea why this won't run? I attached the code below as well as the original problem. Thanks!
% HW 9
% define the mesh in space
dx = 1e-2;
x = 0:dx:10;
x = x';
% define the mesh in time
dt = 1e-4;
t = 0:dt:2;
c = 1/2;
% display('this number should be between -1 and 1 for stability:')
mu = c*dt/dx;
% choose the wave number of the initial data and give its decay rate
u = zeros(N+1,M+1);
u(:,1) = @(t) exp(-20*((x-3)^2));
u_exact(:,1) = u(:,1);
% I want to do the Lax Wendroff scheme:
%
% u_new(j) = u_old(j) - (mu/2)*(u_old(j+1)-u_old(j-1))
% + (mu^2/2)*(u_old(j-1)-2*u_old(j)+u_old(j+1))
for j=1:M
for k=2:N
u(k,j+1) = u(k,j) - (mu/2)*(u(k+1,j)-u(k-1,j)) + (mu^2/2)*(u(k+1,j)-2*u(k,j)+u(k-1,j));
end
% I code in the exact values at the endpoints.
u(1,j+1)=1;
u(N+1,j+1)=0;
X = x-c*t(j+1);
u_exact(:,j+1) = @(t) 0.4*sin(2*t);
end

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回答（2 个）

Geoff Hayes
Geoff Hayes 2016-5-5
Pizza Pie - the full error message is
The following error occurred converting from function_handle to double:
Error using double
Conversion to double from function_handle is not possible.
Error in ***** (line 18)
u(:,1) = @(t) exp(-20*((x-3)^2));
You have defined u as a matrix of zeros (doubles) with the line of code
u = zeros(N+1,M+1);
and then you are trying to assign a anonymous function handle to all elements in the first column of u. Hence the error.
I suspect that you want to evaluate the expression instead though it is unclear how t should be used in it. If, for the moment, we ignore the t then something like the following will work
u(:,1) = exp(-20*((x-3).^2));
(The above assumes that N is 1000.)
What is the expression that you wish to evaluate? How should t be used?
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Geoff Hayes
Geoff Hayes 2016-5-5
There is no image attached.
Please copy and paste the full error message. Also, what are N and M?

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Star Strider
Star Strider 2016-5-5
The problem is with your anonymous functions. I created versions that work, and made what seem to me to be the appropriate substitutions in your ‘u’ and ‘u_exact’ assignments. (You didn’t supply ‘N’ and ‘M’, so I made up some values for them to test the code.) I will leave you to determine if your code with my changes produces the correct results:
N = 3;
M = 4;
% define the mesh in space
dx = 1e-2;
x = 0:dx:10;
x = x';
% define the mesh in time
dt = 1e-4;
t = 0:dt:2;
c = 1/2;
% display('this number should be between -1 and 1 for stability:')
mu = c*dt/dx;
% choose the wave number of the initial data and give its decay rate
u = zeros(N+1,M+1);
u_fcn = @(x) exp(-20*((x-3).^2)); % <— CONVERT TO ANONYMOUS FUNCTION
u_exact_fcn = @(t) 0.4*sin(2*t); % <— CONVERT TO ANONYMOUS FUNCTION
u_exact(:,1) = u_fcn(u(:,1));
% I want to do the Lax Wendroff scheme:
%
% u_new(j) = u_old(j) - (mu/2)*(u_old(j+1)-u_old(j-1))
% + (mu^2/2)*(u_old(j-1)-2*u_old(j)+u_old(j+1))
for j=1:M
for k=2:N
u(k,j+1) = u_fcn(u(k,j)) - (mu/2)*(u_fcn(u(k+1,j))-u_fcn(u(k-1,j))) + (mu^2/2)*(u_fcn(u(k+1,j))-2*u_fcn(u(k,j))+u_fcn(u(k-1,j)));
end
% I code in the exact values at the endpoints.
u(1,j+1)=1;
u(N+1,j+1)=0;
X = x-c*t(j+1);
u_exact(:,j+1) = u_exact_fcn(t(j));
end
  2 个评论
Pizza Pie
Pizza Pie 2016-5-5
This isn't giving me any error messages so far, however - how would i go about plotting this every 10% of the way? I can't use the normal plot function like this so I'm guessing it has to be in the for loop somewhere?
Star Strider
Star Strider 2016-5-5
What do you want to plot?
Also, does the notation in the screen shot image indicate that this is a homegeneous partial differntial equation?

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