Functions I¶

Use comments to add documentation to programs.¶

# This sentence isn't executed by Python.
adjustment = 0.5   # Neither is this - anything after '#' is ignored.

A function may take zero or more arguments.¶

We have seen some functions already — now let’s take a closer look.
An argument is a value passed into a function.
len takes exactly one.
int, str, and float create a new value from an existing one.
print takes zero or more.
print with no arguments prints a blank line.
- Must always use parentheses, even if they’re empty, so that Python knows a function is being called.

print('before')
print()
print('after')

Every function returns something.¶

Every function call produces some result.
If the function doesn’t have a useful result to return, it usually returns the special value None. None is a Python object that stands in anytime there is no value.

result = print('example')
print('result of print is', result)

Commonly-used built-in functions include `max`, `min`, and `round`.¶

Use max to find the largest value of one or more values.
Use min to find the smallest.
Both work on character strings as well as numbers.
- “Larger” and “smaller” use (0-9, A-Z, a-z) to compare letters.

print(max(1, 2, 3))
print(min('a', 'A', '0'))

Use the built-in function help to get help for a function.¶

Every built-in function has online documentation.

help(round)

The Jupyter Notebook has two ways to get help.¶

Option 1: Place the cursor near where the function is invoked in a cell (i.e., the function name or its parameters),
- Hold down Shift, and press Tab.
- Do this several times to expand the information returned.
Option 2: Type the function name in a cell with a question mark after it. Then run the cell.

Write your own functions in python¶

Exercise: Write the code in your notebook and replace the underscored lines to get the desired result

def max_of_two( __, __ ):
    if __ > __:
        return __
    return __

def max_of_three( __, __, z ):
    return max_of_two( x, max_of_two( __, __ ) )

print(max_of_three(3, 6, -5))

See Solution

def max_of_two( x, y ):
    if x > y:
        return x
    return y
def max_of_three( x, y, z ):
    return max_of_two( x, max_of_two( y, z ) )

print(max_of_three(3, 6, -5))

Exercise: Write the code in your notebook and replace the underscored lines to get the desired result

def sum(______):
    total = 0
    for number in numbers:
        total += number
    return total
print(sum((8, 2, 3, 0, 7)))

See Solution

def sum(numbers):
    total = 0
    for number in numbers:
        total += number
    return total
print(sum((8, 2, 3, 0, 7)))

Python essentials for GIS learners