Many scientific fields have a basic unit of study:

- Biology has the
*cell* - Chemistry has
*atoms*and*molecules*

In Computer Science, our basic unit of study is the *function*.

No matter what programming language you work in—whether Python, Java, C, C++, C#, JavaScript, Haskell, Scala, PHP, etc.—
you will work with functions. (In some contexts, functions are called *methods*, but they are still *functions*).

You are already familiar with functions from your high school math classes. You probably first encountered them in Algebra I.

For example, in math class, we write:

This defines a function that takes one parameter , and returns the value of . For example, is 9, and if , then is 25.

In Python, we write the same function this way:

```
def f(x):
return x**2
```

Then we can evaluate `f`

for various values of `x`

at the Python prompt:

```
>>> def f(x):
... return x**2
...
>>> f(3)
9
>>> a=5
>>> f(a)
25
>>>
```