Warm-up 3

Price difference

Write difference(a, b) that returns how far apart two prices are, always as a positive number (or zero).

difference(40, 25)  ->  15
difference(25, 40)  ->  15

def difference(a, b):
    ...

Even number?

Write is_even(n) that returns True if n is even.

is_even(4)  ->  True
is_even(7)  ->  False

def is_even(n):
    ...

Comma list

Write listing(items) that returns the items joined into one string separated by a comma and a space.

listing(["apple", "bread", "milk"])  ->  "apple, bread, milk"
listing([])                          ->  ""

def listing(items):
    ...

How many of this item?

Write how_many(basket, item) that returns how many times item appears in the list basket.

how_many(["apple", "bread", "apple"], "apple")  ->  2
how_many(["apple"], "milk")                     ->  0

def how_many(basket, item):
    ...

Cheapest price

prices maps item names to prices and is not empty. Write cheapest_price(prices) that returns the lowest price.

cheapest_price({"apple": 40, "milk": 2.5, "bread": 25})  ->  2.5

def cheapest_price(prices):
    ...

Every other item

Write every_other(items) that returns a new list with the first item, the third, the fifth, and so on.

every_other(["a", "b", "c", "d", "e"])  ->  ["a", "c", "e"]
every_other([])                         ->  []

def every_other(items):
    ...

Swap a pair

Write swap(pair) that takes a tuple of two values and returns a tuple with them the other way round.

swap(("bread", 25))  ->  (25, "bread")

def swap(pair):
    ...

Smallest with a heap

Write smallest(nums) that returns the smallest number in a non-empty list, without changing the list. min would do it; the point here is practice with heapq. Remember that heapify rearranges the list it is given, so if you use it, work on a copy.

smallest([40, 25, 2.5])  ->  2.5

import heapq

def smallest(nums):
    ...

Reverse the text

Write backwards(text) that returns the text reversed.

backwards("bread")  ->  "daerb"

def backwards(text):
    ...

Contains milk?

Write has_milk(order) that returns True if the letters milk appear anywhere in the order text, in any mix of capitals. (So "milkshake" counts too.)

has_milk("2 x Milk, 1 x bread")  ->  True
has_milk("bread")                ->  False

def has_milk(order):
    ...

Next: Standard 1.