In an assignment A(:) = B, the number of elements in A and B must be the same.

My code is:

pvals = 10:1:15;
nump = length(pvals);
for pidx = 1 : nump
    p=pvals(pidx);
    [x(pidx),y(pidx)]=solve('9*x(pidx)+8*y(pidx)=p','13*x(pidx)+14*y(pidx)=p+2','x(pidx)','y(pidx)');
end
plot(pvals,x)

The error is:

In an assignment  A(:) = B, the number of elements in A and B must be the same.

Any help is appreciated! Thanks!

[Kevin Phung]

it sounds like the solve() function is returning more than 2 elements-- can you check that through the debugger?

[James_111]

Thank you so much. Could you help me modify the code? I am not sure how to fix it.

[Kevin Phung]

unfortunately I don't have the toolbox, so I cant actually run your code :( sorry

[James_111]

Thanks, anyway. No worry.

Answer 1

[Star Strider]

My version of the Symbolic Math Toolbox doesn’t like quoted strings, so I had to change your code a bit.

Try this:

syms p x y 
pvals = 10:1:15;
nump = length(pvals);
for pidx = 1 : nump
    p=pvals(pidx);
    [xs(pidx),ys(pidx)]=solve([9*x+8*y==p, 13*x+14*y==p+2], [x y]);
end
plot(pvals,xs)

[James_111]

Thank you so much for your help. Just a quick question. After run your code, how I check the values of xs and ys? As I clicked the xs and ys in the workspace, but it just shows the "1*1 sym". As my real code is very long vector, I cant show all the values in command window.

[Star Strider]

My pleasure.

If ‘xs’ and ‘ys’ do not contain symbolic variables (they do not in the example you posted), use the double function to convert them into double vectors:

xv = double(xs);
yv = double(ys);

The problem with your posted code is that you were assigning ‘x’ and ‘y’ in the output of the solve call, so this was creating the error you saw. My solution was to assign the outputs to slightly different names, eliminating the problem.

[James_111]

It is very interesting. Thanks. Btw, I don't quite understand "My version of the Symbolic Math Toolbox doesn’t like quoted strings". Do you mean that is there any simplier way to modify the mode if we have quoted strings?

[Star Strider]

As always, my pleasure.

Quoted strings were depricated in newer versions (although I do not remember when). My version (R2018b) throws errors with your posted code, even when I change the outputs to ‘xs’ and ‘ys’. The code I posted works in my version.

‘Do you mean that is there any simplier way to modify the mode if we have quoted strings?’

I cannot answer that. I cannot use quoted strings, so there is no way I can test that.

[James_111]

Got it. Thanks.

[Star Strider]

As always, my pleasure.