Functions and Lists

≈ 30 min

Functions: naming a block of steps

When you find yourself writing the same steps again and again, wrap them in a function. A function has a name, can take inputs (parameters) inside the brackets, and can return a result.

def area(length, width):
    return length * width

print(area(5, 3))   # 15
print(area(10, 2))  # 20

def starts the definition. You call the function by writing its name with values in the brackets. return hands a value back to whoever called it.

python

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Lists: many values in one box

A single variable holds one value. A list holds many, in order, inside square brackets:

temps = [21, 24, 19, 28, 23]
print(temps[0])     # 21  (first item, index 0)
print(len(temps))   # 5   (how many items)
temps.append(30)    # add 30 on the end

Each item has an index (position) starting at 0. Useful tools: len(list) counts items, list.append(x) adds one, sum(list) totals numbers.

Looping over a list

A for loop can walk through every item in a list, which is where functions and lists become powerful together:

temps = [21, 24, 19, 28, 23]
hot_days = 0
for t in temps:
    if t >= 25:
        hot_days = hot_days + 1
print(hot_days)   # 1

The loop variable t becomes each temperature in turn, and the if counts the hot ones.

python

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Given this list:

rivers = ["Orange", "Vaal", "Limpopo", "Tugela"]
print(rivers[1])

What is printed?

$Orange$

$Vaal$

$Limpopo$

an error

A function returns the average of a list:

def average(values):
    return sum(values) / len(values)

marks = [60, 80, 70, 90]
print(average(marks))

What number is printed?

Which Python list method adds a new item to the end of a list, as in chores.____("sweep")? (One word, no brackets.)