A function is a block of code with a name that we can call multiple times. It saves us repeating lots of code and can help us organize code and and hide complexity.

A function generally has

1. A name
2. Input data
3. Output data

Functions use the following pattern when they are executed:

output = name(input)

Like this:

result = add(1, 2)
print(result)

If there are multiple inputs, they are seperated by commas.

In this example the function name is add and the input data is the numbers 1 and 2.

The add function adds the input numbers and returns the result. Returning a result means to substitute the function call with the value. In our example, add(1, 2) gets substituted by the result, 3. This is is then assigned to the result.

To make this clearer, we can see multiple function calls on the same line. Once they are all called and the results are substituted, then the final value is assigned.

Let's pretend there's a function called mul which multiplies and one called add which adds.

result = mul( add(2, 2), add(34, 66) )

To execute this code like a computer, we first call add(2,2) and perform the substitution.

result = mul( 4, add(34, 66) )

Next we call and replace add(34,66).

result = mul( 4, 100 )

finally we get this, which we call mul(4, 100) to get 400

Which is assigned to result