# Substitute Elements in Symbolic Matrices

Create a 2-by-2 matrix `A` with automatically generated elements using `sym`. The generated elements ${A}_{1,1}$, ${A}_{1,2}$, ${A}_{2,1}$, and ${A}_{2,2}$ do not appear in the MATLAB® workspace.

`A = sym('A',[2 2])`
```A =  $\left(\begin{array}{cc}{A}_{1,1}& {A}_{1,2}\\ {A}_{2,1}& {A}_{2,2}\end{array}\right)$```

Substitute the element ${A}_{1,2}$ with a value 5. Assign the value directly by indexing into the matrix element.

`A(1,2) = 5`
```A =  $\left(\begin{array}{cc}{A}_{1,1}& 5\\ {A}_{2,1}& {A}_{2,2}\end{array}\right)$```

Alternatively, you can create a 2-by-2 matrix using `syms`. Create a matrix `B` using `syms`.

```syms B [2 2] B```
```B =  $\left(\begin{array}{cc}{B}_{1,1}& {B}_{1,2}\\ {B}_{2,1}& {B}_{2,2}\end{array}\right)$```

The generated elements ${B}_{1,1}$, ${B}_{1,2}$, ${B}_{2,1}$, and ${B}_{2,2}$ appear as symbolic variables `B1_1`, `B1_2`, `B2_1`, and `B2_2` in the MATLAB workspace. Use `subs` to substitute the element of `B` by specifying the variable name. For example, substitute `B2_2` with 4.

`B = subs(B,B2_2,4)`
```B =  $\left(\begin{array}{cc}{B}_{1,1}& {B}_{1,2}\\ {B}_{2,1}& 4\end{array}\right)$```

You can also create a matrix by specifying the elements individually. Create a 3-by-3 circulant matrix `M`.

```syms a b c M = [a b c; b c a; c a b]```
```M =  $\left(\begin{array}{ccc}a& b& c\\ b& c& a\\ c& a& b\end{array}\right)$```

Replace variable `b` in the matrix `M` by the expression `a + 1`. The subs function replaces all `b` elements in matrix `M` with the expression `a + 1`.

`M = subs(M,b,a+1)`
```M =  $\left(\begin{array}{ccc}a& a+1& c\\ a+1& c& a\\ c& a& a+1\end{array}\right)$```

Next, replace all elements whose value is `c` with `a + 2`. You can specify the value to replace as `c`, `M(1,3)` or `M(3,1)`.

`M = subs(M,M(1,3),a+2)`
```M =  $\left(\begin{array}{ccc}a& a+1& a+2\\ a+1& a+2& a\\ a+2& a& a+1\end{array}\right)$```

To replace a particular element of a matrix with a new value while keeping all other elements unchanged, use the assignment operation. For example, `M(1,1) = 2` replaces only the first element of the matrix `M` with the value 2.

Find eigenvalues and eigenvectors of the matrix `M`.

`[V,E] = eig(M)`
```V =  $\left(\begin{array}{ccc}1& \frac{\sqrt{3}}{2}-\frac{1}{2}& -\frac{\sqrt{3}}{2}-\frac{1}{2}\\ 1& -\frac{\sqrt{3}}{2}-\frac{1}{2}& \frac{\sqrt{3}}{2}-\frac{1}{2}\\ 1& 1& 1\end{array}\right)$```
```E =  $\left(\begin{array}{ccc}3 a+3& 0& 0\\ 0& \sqrt{3}& 0\\ 0& 0& -\sqrt{3}\end{array}\right)$```

Replace the symbolic parameter `a` with the value 1.

`subs(E,a,1)`
```ans =  $\left(\begin{array}{ccc}6& 0& 0\\ 0& \sqrt{3}& 0\\ 0& 0& -\sqrt{3}\end{array}\right)$```