# Basics¶

Here we discuss some of the most basic aspects of expression manipulation in Diofant.

## Assumptions¶

The assumptions system allows users to declare certain mathematical properties on symbols, such as being positive, imaginary or integer.

By default, all symbols are complex valued. This assumption makes it easier to treat mathematical problems in full generality.

```
>>> sqrt(x**2)
____
╱ 2
╲╱ x
```

Yet obviously we can simplify above expression if some additional
mathematical properties on `x`

are assumed. This is where
assumptions system come into play.

Assumptions are set on `Symbol`

objects
when they are created. For instance, we can create a symbol that is
assumed to be positive.

```
>>> p = symbols('p', positive=True)
```

And then, certain simplifications will be possible:

```
>>> sqrt(p**2)
p
```

The assumptions system additionally has deductive capabilities. You
might check assumptions on any expression with `is_assumption`

attributes, like `is_positive`

.

```
>>> p.is_positive
True
>>> (1 + p).is_positive
True
>>> (-p).is_positive
False
```

Note

`False`

is returned also if certain assumption doesn’t make sense
for given object.

In a three-valued logic, used by system, `None`

represents the
“unknown” case.

```
>>> (p - 1).is_positive is None
True
```

## Substitution¶

One of the most common things you might want to do with a mathematical
expression is substitution with `subs()`

method. It replaces all instances of something in an expression with
something else.

```
>>> expr = cos(x) + 1
>>> expr.subs({x: y})
cos(y) + 1
>>> expr
cos(x) + 1
```

We see that performing substitution leaves original expression
`expr`

unchanged.

Note

Almost all Diofant expressions are immutable. No function (or method) will change them in-place.

Use several method calls to perform a sequence of substitutions in same variable:

```
>>> x**y
y
x
>>> _.subs({y: x**y}).subs({y: x**x})
⎛ ⎛ x⎞⎞
⎜ ⎝x ⎠⎟
⎝x ⎠
x
```

Use flag `simultaneous`

to do all substitutions at once.

```
>>> (x - y).subs({x: y, y: x})
0
>>> (x - y).subs({x: y, y: x}, simultaneous=True)
-x + y
```

## Numerics¶

To evaluate a numerical expression into a floating point number with
arbitrary precision, use `evalf()`

.
By default, 15 digits of precision are used.

```
>>> expr = sqrt(8)
>>> expr.evalf()
2.82842712474619
```

But you can change that. Let’s compute the first 70 digits of \(\pi\).

```
>>> pi.evalf(70)
3.141592653589793238462643383279502884197169399375105820974944592307816
```

Sometimes there are roundoff errors smaller than the desired precision
that remain after an expression is evaluated. Such numbers can be
removed by setting the `chop`

flag.

```
>>> one = cos(1)**2 + sin(1)**2
>>> (one - 1).evalf(strict=False)
-0.e-146
>>> (one - 1).evalf(chop=True)
0
```

Discussed above method is not effective enough if you intend to evaluate an expression at many points, there are better ways, especially if you only care about machine precision.

The easiest way to convert a Diofant expression to an expression that
can be numerically evaluated with libraries like `numpy`

— use
the `lambdify()`

function. It acts
like a `lambda`

form, except it converts the Diofant names to
the names of the given numerical library.

```
>>> import numpy
>>> a = numpy.arange(5)
>>> expr = sin(x)
>>> f = lambdify(x, expr, "numpy")
>>> f(a)
[ 0. 0.84147098 0.90929743 0.14112001 -0.7568025 ]
```

You can use other libraries than NumPy. For example, the standard
library `math`

module.

```
>>> f = lambdify(x, expr, "math")
>>> f(0.1)
0.09983341664682815
```