2.4 Introduction to functions

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This is a very short introduction to functions.

We will soon go into more detail on functions, in the page on call expressions.

For now, we start with something simple, the function cos.

Remember functions from mathematics? We might write something like this, in mathematics:

\[y = cos(0)\]

You can read $cos(0)$ as:

Call the function “cos” with the value 0, and return the result.

Then $y = cos(0)$ means:

Call the function “cos” with the value 0, and return the result, storing the result in “y”.

We pass a value to the function. In our case we pass the value $0$.

The function returns a value. In our case it returns the calculation of the cosine of $0$.

You may remember that the cosine of 0 is 1. So, after $y = cos(0)$, we expect $y$ to equal 1.

Functions in Python work in a similar way.

First we load up the cos function from a library called numpy.

Don’t worry about the command to load the function, for now. We will come back to that later. Here is the command you need. Just run it in the notebook.

# Get the cos function from the numpy library.
from numpy import cos

Now we call the cos function with the value 0, and see what we get:

cos(0)

1.0
  • cos is the name of the function;
  • 0 is the value we as passing to the function.
  • The value we pass to the function goes between the parentheses ( and ).
  • 1.0 is the value that the function returns.

The value we pass to the function is also called the function argument.

In our case we pass 0 as the argument to the cos function.

Let’s try the same thing with the sin function:

# Get the sin function from the numpy library.
from numpy import sin

sin(0)

0.0
  • sin - the function name;
  • 0 - the value we pass to the function;
  • The value goes after the function name, and between parentheses;
  • 0.0 - the value that the function returns.

We pass 0 as the argument to the sin function.

Now your turn.

Try importing the sqrt function.

# Get the sqrt function from the numpy library.
# Your code here.

Call the sqrt function with the value 9; you should see this returns the value 3.0.

# Your code here.

We will see more about functions in the section on call expressions.