MATLAB symbolic to double error

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Andrew
Andrew 2013-11-7
评论： Walter Roberson 2013-11-7
Hi, I am trying to run this code but there are errors. I don't know how to remove them. Here is code: This error comes when converting symbolic to double. Any help much appreciated.
clear all;
clc;
nel=4; % number of elements
totlength=10;
nne=3;
nnp=2*nel+1;
le=totlength/nel;
x=0:le:totlength;
element_nodes=[1:2:(nnp-2);2:2:(nnp-1);3:2:nnp]';
%element_nodes=[1:nel;2:nnp]';
B=[];
syms xlc
%Finding out B matrix
for i=1:1:nel
j=element_nodes(i,1)
B(i)=[(2*xlc-x(i+2)-x(j+1))/(2*le^2),(2*xlc-x(j+2)-x(i))/(-1*le^2),(2*xlc-x(j+1)-x(i))/(2*le^2)];
end
Any help or suggestions? Thanks.

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回答（1 个）

Walter Roberson
Walter Roberson 2013-11-7
Experiment with starting with
B = sym([]);
I think you are going to have trouble with your assignment to B(i) anyhow, as you appear to be attempting to assign a list of 3 elements into a single storage location.
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Andrew
Andrew 2013-11-7
I modified my code. Instead of writing 3 values in 1 row, I created two nested loops to write values at each location. But still error is present. Here is code:
clear all;
clc;
nel=4; % number of elements
totlength=10;
nne=3;
nnp=2*nel+1;
le=totlength/nel;
x=0:le:totlength;
element_nodes=[1:2:(nnp-2);2:2:(nnp-1);3:2:nnp]';
%element_nodes=[1:nel;2:nnp]';
B=[];
syms xlc
%Finding out B matrix
for i=1:1:nel
k=1:1:nne
j=element_nodes(i,1);
if(k==1)
B(i,k)=vpa((2*xlc-x(i+2)-x(j+1))/(2*le^2));
elseif (k==2)
B(i,k)=vpa((2*xlc-x(j+2)-x(i))/(-1*le^2));
else
B(i,k)=vpa((2*xlc-x(j+1)-x(i))/(2*le^2));
end
%B(i)=[(2*xlc-x(i+2)-x(j+1))/(2*le^2),(2*xlc-x(j+2)-x(i))/(-1*le^2),(2*xlc-x(j+1)-x(i))/(2*le^2)];
end
Walter Roberson
Walter Roberson 2013-11-7
B = sym([]);
And in the loop,
B(i,:) = [(2*xlc-x(i+2)-x(j+1))/(2*le^2),(2*xlc-x(j+2)-x(i))/(-1*le^2),(2*xlc-x(j+1)-x(i))/(2*le^2)];

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