# syms

Create symbolic variables and functions

## Syntax

``syms var1 ... varN``
``syms var1 ... varN [n1 ... nM]``
``syms var1 ... varN n``
``syms ___ set``
``syms f(var1,...,varN)``
``syms f(var1,...,varN) [n1 ... nM]``
``syms f(var1,...,varN) n``
``syms(symArray)``
``syms``
``S = syms``

## Description

example

````syms var1 ... varN` creates symbolic variables `var1 ... varN`. Separate different variables by spaces. `syms` clears all assumptions from the variables.```

example

````syms var1 ... varN [n1 ... nM]` creates symbolic arrays `var1 ... varN`, where each array has the size `n1`-by-`...`-by-`nM` and contains automatically generated symbolic variables as its elements. For example, ```syms a [1 3]``` creates the symbolic array `a = [a1 a2 a3]` and the symbolic variables `a1`, `a2`, and `a3` in the MATLAB® workspace. For multidimensional arrays, these elements have the prefix `a` followed by the element’s index using `_` as a delimiter, such as `a1_3_2`.```

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````syms var1 ... varN n` creates `n`-by-`n` symbolic matrices filled with automatically generated elements.```

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````syms ___ set` sets the assumption that the created symbolic variables belong to `set`, and clears other assumptions. Here, `set` can be `real`, `positive`, `integer`, or `rational`. You also can combine multiple assumptions using spaces. For example, ```syms x positive rational``` creates a variable `x` with a positive rational value.```

example

````syms f(var1,...,varN)` creates the symbolic function `f` and the symbolic variables `var1,...,varN`, which represent the input arguments of `f`. You can create multiple symbolic functions in one call. For example, `syms f(x) g(t)` creates two symbolic functions (`f` and `g`) and two symbolic variables (`x` and `t`).```

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````syms f(var1,...,varN) [n1 ... nM]` creates an `n1`-by-`...`-by-`nM` symbolic array with automatically generated symbolic functions as its elements. This syntax also generates the symbolic variables `var1,...,varN` that represent the input arguments of `f`. For example, ```syms f(x) [1 2]``` creates the symbolic array `f(x) = [f1(x) f2(x)]`, the symbolic functions `f1(x)` and `f2(x)`, and the symbolic variable `x` in the MATLAB workspace. For multidimensional arrays, these elements have the prefix `f` followed by the element’s index using `_` as a delimiter, such as `f1_3_2`.```

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````syms f(var1,...,varN) n` creates an `n`-by-`n` symbolic matrix filled with automatically generated elements.```

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````syms(symArray)` creates the symbolic variables and functions contained in `symArray`, where `symArray` is either a vector of symbolic variables or a cell array of symbolic variables and functions. Use this syntax only when such an array is returned by another function, such as `solve` or `symReadSSCVariables`.```

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````syms` lists the names of all symbolic variables, functions, and arrays in the MATLAB workspace.```

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````S = syms` returns a cell array of the names of all symbolic variables, functions, and arrays.```

## Examples

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Create symbolic variables `x` and `y`.

```syms x y x```
`x = $x$`
`y`
`y = $y$`

Create a 1-by-4 symbolic vector `a` with the automatically generated elements ${a}_{1},\dots ,{a}_{4}$. This command also creates the symbolic variables `a1`, ..., `a4` in the MATLAB workspace.

```syms a [1 4] a```
`a = $\left(\begin{array}{cccc}{a}_{1}& {a}_{2}& {a}_{3}& {a}_{4}\end{array}\right)$`
`whos`
``` Name Size Bytes Class Attributes a 1x4 8 sym a1 1x1 8 sym a2 1x1 8 sym a3 1x1 8 sym a4 1x1 8 sym ```

You can change the naming format of the generated elements by using a format character vector. Declare the symbolic variables by enclosing each variable name in single quotes. `syms` replaces `%d` in the format character vector with the index of the element to generate the element names.

```syms 'p_a%d' 'p_b%d' [1 4] p_a```
`p_a = $\left(\begin{array}{cccc}{p}_{\mathrm{a1}}& {p}_{\mathrm{a2}}& {p}_{\mathrm{a3}}& {p}_{\mathrm{a4}}\end{array}\right)$`
`p_b`
`p_b = $\left(\begin{array}{cccc}{p}_{\mathrm{b1}}& {p}_{\mathrm{b2}}& {p}_{\mathrm{b3}}& {p}_{\mathrm{b4}}\end{array}\right)$`

Create a 3-by-4 symbolic matrix with automatically generated elements. The elements are of the form ${A}_{i,j}$, which generates the elements ${A}_{1,1},\dots ,{A}_{3,4}$.

```syms A [3 4] A```
```A =  $\left(\begin{array}{cccc}{A}_{1,1}& {A}_{1,2}& {A}_{1,3}& {A}_{1,4}\\ {A}_{2,1}& {A}_{2,2}& {A}_{2,3}& {A}_{2,4}\\ {A}_{3,1}& {A}_{3,2}& {A}_{3,3}& {A}_{3,4}\end{array}\right)$```

Create symbolic variables `x` and `y`, and assume that they are integers.

`syms x y integer`

Create another variable `z`, and assume that it has a positive rational value.

`syms z positive rational`

Check assumptions.

`assumptions`
`ans = $\left(\begin{array}{cccc}x\in \mathbb{Z}& y\in \mathbb{Z}& z\in \mathbb{Q}& 0`

Alternatively, check assumptions on each variable. For example, check assumptions set on the variable `x`.

`assumptions(x)`
`ans = $x\in \mathbb{Z}$`

Clear assumptions on `x`, `y`, and `z`.

```assume([x y z],'clear') assumptions```
``` ans = Empty sym: 1-by-0 ```

Create a 1-by-3 symbolic array `a` and assume that the array elements have real values.

```syms a [1 3] real assumptions```
`ans = $\left(\begin{array}{ccc}{a}_{1}\in \mathbb{R}& {a}_{2}\in \mathbb{R}& {a}_{3}\in \mathbb{R}\end{array}\right)$`

Create symbolic functions with one and two arguments.

`syms s(t) f(x,y)`

Both `s` and `f` are abstract symbolic functions. They do not have symbolic expressions assigned to them, so the bodies of these functions are `s(t)` and `f(x,y)`, respectively.

Specify the following formula for `f`.

`f(x,y) = x + 2*y`
`f(x, y) = $x+2 y$`

Compute the function value at the point `x = 1` and `y = 2`.

`f(1,2)`
`ans = $5$`

Create a symbolic function and specify its formula by using a symbolic matrix.

```syms x M = [x x^3; x^2 x^4]; f(x) = M```
```f(x) =  $\left(\begin{array}{cc}x& {x}^{3}\\ {x}^{2}& {x}^{4}\end{array}\right)$```

Compute the function value at the point `x = 2`:

`f(2)`
```ans =  $\left(\begin{array}{cc}2& 8\\ 4& 16\end{array}\right)$```

Compute the value of this function for `x = [1 2 3; 4 5 6]`. The result is a cell array of symbolic matrices.

```xVal = [1 2 3; 4 5 6]; y = f(xVal)```
```y=2×2 cell array {2x3 sym} {2x3 sym} {2x3 sym} {2x3 sym} ```

Access the contents of a cell in the cell array by using braces.

`y{1}`
```ans =  $\left(\begin{array}{ccc}1& 2& 3\\ 4& 5& 6\end{array}\right)$```

Create a 2-by-2 symbolic matrix with automatically generated symbolic functions as its elements.

```syms f(x,y) 2 f```
```f(x, y) =  $\left(\begin{array}{cc}{f}_{1,1}\left(x,y\right)& {f}_{1,2}\left(x,y\right)\\ {f}_{2,1}\left(x,y\right)& {f}_{2,2}\left(x,y\right)\end{array}\right)$```

Assign symbolic expressions to the symbolic functions `f1_1(x,y)` and `f2_2(x,y)`. These functions are displayed as ${f}_{1,1}\left(x,y\right)$ and ${f}_{2,2}\left(x,y\right)$ in the Live Editor. When you assign these expressions, the symbolic matrix `f` still contains the initial symbolic functions in its elements.

```f1_1(x,y) = 2*x; f2_2(x,y) = x - y; f```
```f(x, y) =  $\left(\begin{array}{cc}{f}_{1,1}\left(x,y\right)& {f}_{1,2}\left(x,y\right)\\ {f}_{2,1}\left(x,y\right)& {f}_{2,2}\left(x,y\right)\end{array}\right)$```

Substitute the expressions assigned to `f1_1(x,y)` and `f2_2(x,y)` by using the `subs` function.

`A = subs(f)`
```A(x, y) =  $\left(\begin{array}{cc}2 x& {f}_{1,2}\left(x,y\right)\\ {f}_{2,1}\left(x,y\right)& x-y\end{array}\right)$```

Evaluate the value of the symbolic matrix `A`, which contains the substituted expressions at `x = 2` and `y = 3`.

`A(2,3)`
```ans =  $\left(\begin{array}{cc}4& {f}_{1,2}\left(2,3\right)\\ {f}_{2,1}\left(2,3\right)& -1\end{array}\right)$```

Certain functions, such as `solve` and `symReadSSCVariables`, can return a vector of symbolic variables or a cell array of symbolic variables and functions. These variables or functions do not automatically appear in the MATLAB workspace. Create these variables or functions from the vector or cell array by using `syms`.

Solve the equation `sin(x) == 1` by using `solve`. The parameter `k` in the solution does not appear in the MATLAB workspace.

```syms x eqn = sin(x) == 1; [sol, parameter, condition] = solve(eqn, x, 'ReturnConditions', true); parameter```
`parameter = $k$`

Create the parameter `k` by using `syms`. The parameter `k` now appears in the MATLAB workspace.

`syms(parameter)`

Similarly, use `syms` to create the symbolic objects contained in a vector or cell array. Examples of functions that return a cell array of symbolic objects are `symReadSSCVariables` and `symReadSSCParameters`.

Create some symbolic variables, functions, and arrays.

```syms a f(x) syms A [2 2]```

Display a list of all symbolic objects that currently exist in the MATLAB workspace by using `syms`.

`syms`
```Your symbolic variables are: A A1_1 A1_2 A2_1 A2_2 a f x ```

Instead of displaying a list, return a cell array of all symbolic objects by providing an output to `syms`.

`S = syms`
```S = 8x1 cell {'A' } {'A1_1'} {'A1_2'} {'A2_1'} {'A2_2'} {'a' } {'f' } {'x' } ```

Create several symbolic objects.

`syms a b c f(x)`

Return all symbolic objects as a cell array by using the `syms` function. Use the `cellfun` function to delete all symbolic objects in the cell array `symObj`.

```symObj = syms; cellfun(@clear,symObj)```

Check that you deleted all symbolic objects by calling `syms`. The output is empty, meaning no symbolic objects exist in the MATLAB workspace.

`syms`

## Input Arguments

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Symbolic variables, matrices, or arrays, specified as valid variable names separated by spaces. Each variable name must begin with a letter and can contain only alphanumeric characters and underscores. To verify that the name is a valid variable name, use `isvarname`.

Example: `x y123 z_1`

Vector, matrix, or array dimensions, specified as a vector of integers. As a shortcut, you can create a square matrix by specifying only one integer. For example, `syms x 3` creates a square `3`-by-`3` matrix.

Example: `[2 3]`, `[2,3]`, `[2;3]`

Assumptions on a symbolic variable or matrix, specified as `real`, `positive`, `integer`, or `rational`.

You can combine multiple assumptions using spaces. For example, ```syms x positive rational``` creates a variable `x` with a positive rational value.

Example: `rational`

Symbolic function with its input arguments, specified as an expression with parentheses. The function name `f` and the variable names `var1...varN` must be valid variable names. That is, they must begin with a letter and can contain only alphanumeric characters and underscores. To verify that the name is a valid variable name, use `isvarname`.

Example: `s(t)`, `f(x,y)`

Symbolic variables or functions, specified as a vector of symbolic variables or a cell array of symbolic variables and functions. Such a vector or array is typically the output of another function, such as `solve` or `symReadSSCVariables`.

## Output Arguments

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Names of all symbolic variables, functions, and arrays in the MATLAB workspace, returned as a cell array of character vectors.

## Tips

• `syms` is a shortcut for `sym`. This shortcut lets you create several symbolic variables in one function call. Alternatively, you can use `sym` and create each variable separately. However, when you create variables using `sym`, any existing assumptions on the created variables are retained. You can also use `symfun` to create symbolic functions.

• In functions and scripts, do not use `syms` to create symbolic variables with the same names as MATLAB functions. For these names, MATLAB does not create symbolic variables, but keeps the names assigned to the functions. If you want to create a symbolic variable with the same name as a MATLAB function inside a function or a script, use `sym` instead. For example, use ```alpha = sym('alpha')```.

• The following variable names are invalid with `syms`: `integer`, `real`, `rational`, `positive`, and `clear`. To create variables with these names, use `sym`. For example, `real = sym('real')`.

• `clear x` does not clear the symbolic object of its assumptions, such as real, positive, or any assumptions set by `assume`, `sym`, or `syms`. To remove assumptions, use one of these options:

• `syms x` clears all assumptions from `x`.

• `assume(x,'clear')` clears all assumptions from `x`.

• `clear all` clears all objects in the MATLAB workspace and resets the symbolic engine.

• `assume` and `assumeAlso` provide more flexibility for setting assumptions on variables.

• When you replace one or more elements of a numeric vector or matrix with a symbolic number, MATLAB converts that number to a double-precision number.

```A = eye(3); A(1,1) = sym(pi)```
```A = 3.1416 0 0 0 1.0000 0 0 0 1.0000```

You cannot replace elements of a numeric vector or matrix with a symbolic variable, expression, or function because these elements cannot be converted to double-precision numbers. For example, ```syms a; A(1,1) = a``` throws an error.

## Compatibility Considerations

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Behavior changed in R2018b

Warns starting in R2018b

#### Mathematical Modeling with Symbolic Math Toolbox

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