You've already seen some of Python's built-in functions:

In [1]:

```
max(1, 3)
```

Out[1]:

In [2]:

```
pow(2, 4)
```

Out[2]:

You've also seen functions in math. For example:

$f(x) = 2x + 7$

This is the **definition** of $f(x)$. By itself it doesn't have a value. It just defines what it would mean to
call the function $f$ on some value.

Given a **value** for $x$, we can calculate the resulting value of $f(x)$.
$f(2)$ evaluates to 11, $f(-1$) gives 5.

In Python, in addition to using the built-in functions, we can define our own. For example, we can define a function equivalent to $f(x) = 2x + 7$:

In [3]:

```
def f(x):
return 2 * x + 7
```

In [4]:

```
f(2)
```

Out[4]:

In [5]:

```
f(-1)
```

Out[5]:

Here is the general form of a function definition:

`def`

:

- a keyword indicating a function definition

`<<function_name>>`

:

- the name of the function (use
`pothole_case`

name format)

`<<parameters>>`

:

- the parameter(s) of the function, 0 or more.
- Parameters are a comma-separated list of variables whose values will be supplied when the function is called.

`<<body>>`

:

- One or more statements, often ending with a
`return`

statement. - All body statements must be indented the same amount (usually 4 spaces).

The `return`

statement has this form:
`return «expression»`

The `return`

statement is executed as follows:

- Evaluate the expresssion. The produces a value (which has a memory address).
- Exit the function and produce that value to the caller.

Once a function has been defined, we can call on the function. A *function call* has the form:
`«function_name»(«arguments»)`

A function call is executed as follows:

- Evaluate the arguments. These produce values. Each value has a memory address.
- Assign those values to the parameters. This stores the memory addresses of the values in the parameters.
- Pause the current statement and execute the body of the function

Revisiting our earlier example, the function definition was:

def f(x): return 2 * x + 7

Answer the following questions:

- What is the name of the function?
- How many parameter(s) does the function have?
- What are the parameter(s) names?
- For function call
`f(8)`

, what is the argument? - When it is executed, what value does
`f(8)`

produce? - Consider this code:
`result = f(4)`

. After that statement is executed, what value does`result`

refer to?

When defining functions, we will follow this **Function Design Recipe**:

- Write an
**example**call (or two) of how we will use the function we plan to write. - Write the
**header**for the function. That's the`def`

line. - Write the
**type**contract -- the type for each parameter and the type of value that will be returned. - Write a
**description**of what the function does. - Finally, write the
**code**for the body of the function. - Then
**test**our function.

Let's write a function that squares a number.

Step 1: Give examples of how we will call the function:

>>> square(5) 25 >>> square(-6) 36

Of course `square`

isn't defined yet, so we can't *actually* call it. Instead just work out, what the function calls should produce.

Step 2: Write the header, which is the line that starts with `def`

. The function will have one parameter, which we named `num`

, but we could have named it `x`

, `value`

, `n`

, or even `banana`

. Of course, it is best to pick a meaningful name.

While you are at it, make the examples that you wrote be part of a triple-quoted string
inside the function (make sure to indent them 4 spaces). This triple-quoted string is called a *docstring*, which stands for documentation string.

In [6]:

```
def square(num):
"""
>>> square(5)
25
>>> square(-6)
36
"""
```

In [7]:

```
def square(num):
""" (number) -> number
>>> square(5)
25
>>> square(-6)
36
"""
```

Step 4: Write the description and it to the doctstring.

In [8]:

```
def square(num):
"""(number) -> number
Return num squared.
>>> square(5)
25
>>> my_abs(-6)
36
"""
```

`return`

statement to produce a value.

In [9]:

```
def square(num):
"""(number) -> number
Return num squared.
>>> square(5)
25
>>> my_abs(-6)
36
"""
return num * num
```

In [10]:

```
square(5)
```

Out[10]:

In [11]:

```
square(-6)
```

Out[11]:

Generally in Canada, people know their weight in pounds. Most of the calculations for determining dosages and other medical information, ask for a patient's weight in kilograms. Following the Function Design Recipe, write a function that will convert from pounds to kilograms. (1kg == 2.2 lbs)

**Phase I: [Your level] Session 1 Practice Exercise**. Our instance of PCRS is available here: https://pcrs.teach.cs.toronto.edu/C4M17