# Python: Functions

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In the previous session, we tested the code below for different values of `n`.

if n % 2 == 0:
print(n,'is even')
elif n % 2 == 1:
print(n,'is odd')
else:
print(n, 'is not an integer')

This was quite tedious as we replaced by hand the name `n` by different numbers.

Everything that is boring and tedious in programming should be automatized.

Do it once by hand. Do it a second time by copy-paste. The third time write a program doing it for you.

One great way of not having to copy-paste code are functions.

So instead of replacing the name `n` by hand we now treat it as a variable and use it to define a function that we can call and reuse instead of changing the code by hand each time.

def parity(n):
if n % 2 == 0:
print(n,'is even')
elif n % 2 == 1:
print(n,'is odd')
else:
print(n, 'is not an integer')

The function `parity` checks whether a number is even or odd.

Paste the code above into the Python REPL. Then call the function `parity` with various arguments such as

parity(2)
parity(3)
parity(2.5)
...

Instead of working with the REPL, you can also download the code [`parity.pl`](https://github.com/alexhkurz/introduction-to-numbers/blob/master/src/parity.pl) and run with different values of `n`. Notice that thanks to having a function, you need to change the value of `n` in one place only.

**Homework/Activity:**
- Write a function `hello()` that prints `Hello, here is Python`.
- Implement the mathematical function $f(x)=x^2+2x+1$ in Python. Use the Python function to compute $f(-2)$, $f(-1)$ and $f(0)$.
- Invent some of your own functions.