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 $f$ that takes one parameter $x$, and returns the value of $x^2$. For example, $f(3)$ is 9, and if $a=5$, then $f(a)$ 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
>>>