logical

Determine if symbolic equation, inequality, or condition is true

Syntax

``tf = logical(cond)``

Description

example

````tf = logical(cond)` checks if the conditions in `cond` are true and returns an array of logical values. To test conditions that require assumptions or mathematical transformations, use `isAlways` instead.```

Examples

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Test if `3/5` is less than `2/3`.

`tf = logical(sym(3)/5 < sym(2)/3)`
```tf = logical 1 ```

To check if several conditions are true at the same time, combine them by using logical operators. For example, check if `1` is less than `2` and if `exp(log(x)) == x`. Note that when you define a condition that uses other functions, such as `exp` and `log`, these functions are evaluated when defining the condition.

```syms x cond1 = 1 < 2 & exp(log(x)) == x```
`cond1 = $x=x$`
`tf = logical(cond1)`
```tf = logical 1 ```

For multiple conditions, you can represent the conditions as a symbolic array.

`cond2 = [1 < 2; exp(log(x)) == x]`
```cond2 =  $\left(\begin{array}{c}1\\ x=x\end{array}\right)$```
`tf = logical(cond2)`
```tf = 2x1 logical array 1 1 ```

Test an inequality that has a symbolic type on the left side and a numeric type on the right side. The expressions on both sides have compatible data types, and the inequality is true.

`tf = logical(sym(11)/4 - sym(1)/2 > 2)`
```tf = logical 1 ```

`logical` also checks the validity of equations and inequalities involving other functions, such as `int`. For example, the left side of this equation that contains an integral evaluates to `sym(1)`. Both sides of the equation are equal, and `logical` returns `1` (`true`).

```syms x tf = logical(int(x,x,0,2) - 1 == 1)```
```tf = logical 1 ```

`logical` does not simplify or apply mathematical transformations when checking if a condition is true. For example, check the equality of ${\left(\mathit{x}+1\right)}^{2}$ and ${\mathit{x}}^{2}+2\mathit{x}+1$ using `logical`.

```syms x tf = logical((x+1)^2 == x^2+2*x+1)```
```tf = logical 0 ```

Simplify the condition represented by the symbolic equation using `simplify`. The `simplify` function returns the symbolic logical constant `symtrue` because the equation is always true for all values of `x`.

`cond = simplify((x+1)^2 == x^2+2*x+1)`
`cond = $\mathrm{symtrue}$`

Using logical on `symtrue` converts the symbolic logical constant to logical `1` (`true`).

`tf = logical(cond)`
```tf = logical 1 ```

To check an equation that requires simplifications, you can use `isAlways` instead.

`tf = isAlways((x+1)^2 == x^2+2*x+1)`
```tf = logical 1 ```

Check if an equation that involves a function, such as `sqrt`, is true. Without an additional assumption that `x` is nonnegative, `sqrt(x^2)` does not evaluate to `x`.

```syms x tf = logical(x == sqrt(x^2))```
```tf = logical 0 ```

Use `assume` to set an assumption that `x` is nonnegative. Now the expression `sqrt(x^2)` evaluates to `x`, and `logical` returns `1` because `x == x` is true.

```assume(x >= 0) tf = logical(x == sqrt(x^2))```
```tf = logical 1 ```

Note that the `logical` function itself ignores assumptions on symbolic variables.

```syms x assume(x == 5) tf = logical(x == 5)```
```tf = logical 0 ```

To compare expressions taking into account assumptions on their variables, use `isAlways`.

`tf = isAlways(x == 5)`
```tf = logical 1 ```

For further computations, clear the assumption on `x` by recreating it using `syms`.

`syms x`

The `logical` function does not simplify or apply mathematical transformations when checking if a condition is true. For example, `logical` does not recognize the mathematical equivalence of this equation.

```syms x tf = logical(sin(x)/cos(x) == tan(x))```
```tf = logical 0 ```

`logical` also considers this inequality to be true.

`tf = logical(sin(x)/cos(x) ~= tan(x))`
```tf = logical 1 ```

To test the validity of equations and inequalities that require simplification or mathematical transformations, use `isAlways` instead. `isAlways` issues a warning when returning `false` for undecidable inputs.

`tf = isAlways(sin(x)/cos(x) == tan(x))`
```tf = logical 1 ```
`tf = isAlways(sin(x)/cos(x) ~= tan(x))`
```Warning: Unable to prove 'sin(x)/cos(x) ~= tan(x)'. ```
```tf = logical 0 ```

Input Arguments

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Input, specified as a symbolic equation, inequality, or a symbolic array of equations or inequalities. You also can combine several conditions by using the logical operators `and`, `or`, `xor`, `not`, or their shortcuts.

Tips

• For symbolic equations, `logical` returns logical `1` (`true`) only if the left and right sides are equal. Otherwise, it returns logical `0` (`false`).

• For symbolic inequalities constructed with `~=`, `logical` returns logical `0` (`false`) only if the left and right sides are equal. Otherwise, it returns logical `1` (`true`).

• For all other inequalities (constructed with `<`, `<=`, `>`, or `>=`), `logical` returns logical `1` if it can prove that the inequality is true and logical `0` if it can prove that the inequality is false. If `logical` cannot determine whether an inequality is true or false, it returns an error.

• `logical` does not simplify or mathematically transform a conditional statement. To compare a conditional statement applying mathematical transformations and simplifications, use `isAlways`.

• If you use `logical` to check a conditional statement that involves a symbolic type, then the data types of the compared expressions must be compatible. For example, `logical(1==sym(1))` returns `1` (`true`). If the expressions do not have compatible data types, then `logical` returns an error. For example, ```syms f(x) g(y); tf = logical(f~=g)``` returns an error.

• `logical` ignores assumptions on symbolic variables.

Version History

Introduced in R2012a