**TLDR**

I wirte a code to realize SDP problem. I used `subs`

to change syms into numerical value before cvx content and I did see the numerical value after I printed out the variables. But `whos()`

did show that the variable is still syms type which I think might be the key point where I went wrong. The error message shows like this:

Unable to convert ‘cvx’ to ‘sym’.

**Details**

I’m trying to write a SDP problem looks like follows:

```
function [res] = NH_SDP(rhot,p,value,t)
dim = size(rhot,1);
for i = 1:p
drhot(:,:,i) = diff(rhot,t(i));
end
drhot = subs(drhot,t,value);
rhot = subs(rhot,t,value);
rhott = kron(eye(p),rhot);
% cvx process
cvx_begin sdp
% cvx part
variable L_ele(dim,dim,(1+p)*p/2) hermitian;
variable X_ele(dim,dim,p) hermitian;
cnt = 1;
for i = 1:p
for j = i:p
if i == j
L((i-1)*dim+1:i*dim,(j-1)*dim+1:j*dim) = L_ele(:,:,cnt);
cnt = cnt + 1;
else
L((i-1)*dim+1:i*dim,(j-1)*dim+1:j*dim) = L_ele(:,:,cnt);
L((j-1)*dim+1:j*dim,(i-1)*dim+1:i*dim) = L_ele(:,:,cnt);
cnt = cnt + 1;
end
end
end
for i = 1:p
X((i-1)*dim+1:i*dim,1:dim) = X_ele(:,:,i);
end
for i = 1:p
X2(1:dim,(i-1)*dim+1:i*dim) = X_ele(:,:,i);
end
% rhott = [[0, 0, 0, 0, 0, 0, 0, 0]
% [0, 1/2, 7/20, 0, 0, 0, 0, 0]
% [0, 7/20, 1/2, 0, 0, 0, 0, 0]
% [0, 0, 0, 0, 0, 0, 0, 0]
% [0, 0, 0, 0, 0, 0, 0, 0]
% [0, 0, 0, 0, 0, 1/2, 7/20, 0]
% [0, 0, 0, 0, 0, 7/20, 1/2, 0]
% [0, 0, 0, 0, 0, 0, 0, 0]];
rhott = double(rhott);
rhot = double(rhot);
drhot = double(drhot);
minimize(trace(rhott*L));
subject to
% rhot = [0 0 0 0;0 1/2 7/20 0;0 7/20 1/2 0;0 0 0 0];
% drhot
for i =1:p
trace(rhot*X_ele(:,:,i)) == value(i);
end
for i = 1:p
for j = 1:p
if i == j
trace(drhot(:,:,i)*X_ele(:,:,j)) == 1;
else
trace(drhot(:,:,i)*X_ele(:,:,j)) == 0;
end
end
end
% semidefinit constraint.
[L X; X2 eye(dim)] >= 0;
cvx_end
res = cvx_optval;
X_ele
end
```

There are three line in the above code:

```
rhott = double(rhott);
rhot = double(rhot);
drhot = double(drhot);
```

If I delete those three lines, there will be error `Unable to convert 'cvx' to 'sym'.`

while my `rhot`

have already changed into numerical value by `subs`

function and I print it out to see numerical numbers even though `whos`

function show that the type of `rhot`

is still syms.

Any idea? Thanks in advance!