# Functions#

We have used functions is past lessons. Functions have a name and parameters. Some of them return a result, others don’t. We typically call them using `result = name(parameters)`

.

See also

Let’s take a look at some functions, for example `print(text)`

and `pow(x, y)`

. The print function takes a parameter (or multiple parameters) and returns nothing:

```
result = print('Hello world')
```

```
Hello world
```

```
result
```

The pow function has two parameters and returns a result:

```
result = pow(2, 3)
```

```
result
```

```
8
```

## Custom functions#

You can DEFine your own functions using the `def`

statement. After the def statement, you should specify your functions’ name and in brackets its parameters. Afterwards follows a colon `:`

and all following lines of code which are indented are part of this function. A final `return`

statement sends the result back to from where the function was called.

```
def sum_numbers(a, b):
result = a + b
return result
```

You can then call your function as often as you like

```
sum_numbers(3, 4)
```

```
7
```

```
sum_numbers(5, 6)
```

```
11
```

Sometimes, you want to save the result of your function in a variable.

```
c = sum_numbers(4, 5)
print(c)
```

```
9
```

## Simplify code using functions#

Assume you have a complicated algorithm which can tell you if a number if odd or even. Let’s put this algorithm in a function and call it later on. For our algorithm, we will use the modulo operator %.

```
def print_odd_or_even(number):
if number % 2 == 0:
print(number, "is even")
else:
print(number, "is odd")
```

```
print_odd_or_even(3)
```

```
3 is odd
```

```
print_odd_or_even(4)
```

```
4 is even
```

```
print_odd_or_even(10)
```

```
10 is even
```

Thus, instead of writing the same `if-else`

block again and again, we can just call our custom `print_odd_or_even`

function.

## Documenting functions#

You can document what a function does in its so called doc string. The doc string follows right after the functions header and looks like this:

```
def square(number):
'''
Squares a number by multiplying it with itself and returns its result.
'''
return number * number
```

You can then later read the documentation of the function like this:

```
print(square.__doc__)
```

```
Squares a number by multiplying it with itself and returns its result.
```

Also try this if you want to have the docstring shown side-by-side in your notebook:

```
square?
```

```
Signature: square(number)
Docstring: Squares a number by multiplying it with itself and returns its result.
File: /var/folders/p1/6svzckgd1y5906pfgm71fvmr0000gn/T/ipykernel_11914/1507435947.py
Type: function
```

By the way, you can do this with any function:

```
import math
print(math.sqrt.__doc__)
```

```
Return the square root of x.
```

```
print(math.exp.__doc__)
```

```
Return e raised to the power of x.
```

## Exercise#

Write a function that takes two parameters: `number_of_points_in_exam`

and `number_of_total_points_in_exam`

and returns a grade from 1 to 5. Students with > 95% of the points get grade 1, above 80% they get grade 2, above 60% grade 3 and above 50% grade 4. Students with less than 50% get grade 5 and have to repeat the exam. Then, call the function for three students who had 15, 25 and 29 points in an exam with 30 total points.