Return 以菜品为键，菜品被点的次数为值的字典

Question

def get_quantities(table_to_foods):
    """ (dict of {str: list of str}) -> dict of {str: int}

    The table_to_foods dict has table names as keys (e.g., 't1', 't2', and so on) and each value
    is a list of foods ordered for that table.

    Return a dictionary where each key is a food from table_to_foods and each
    value is the quantity of that food that was ordered.

    >>> get_quantities({'t1': ['Vegetarian stew', 'Poutine', 'Vegetarian stew'], 't3': ['Steak pie', 'Poutine', 'Vegetarian stew'], 't4': ['Steak pie', 'Steak pie']})
    {'Vegetarian stew': 3, 'Poutine': 2, 'Steak pie': 3}    
    """

    food_to_quantity = {}

    # Accumulate the food information here.
    # I have no idea what it should be at here.

    return food_to_quantity

如何正确编写这段代码？当我用元组的时候，我试了一下，但我不知道如何计算次数。

Answer 1

from collections import counter
from itertools import chain
Counter(chain(*table_to_foods.values()))

但我不确定你的老师会接受这个...

Answer 2

您需要遍历提供给函数的字典值中的项目，并将它们添加到计数字典中。

def get_quantities(table_to_foods):
    food_to_quantity = {}
    for table_order in table_to_foods.itervalues():
        for menu_item in table_order:
            if menu_item in food_to_quantity:
                food_to_quantity[menu_item] += 1
            else:
                food_to_quantity[menu_item] = 1

    return food_to_quantity

如果您可以使用除基本知识以外的其他方法，我会使用 Joran 提供的方法以及 collections.Counter 和 itertools.chain.from_iterable。

Answer 3

collections 模块 Counter class 的简单用法：

>>> def get_quantities(table_to_foods):
    c = Counter()
    for li in table_to_foods.values():
        c.update(li)
    return dict(c)
>>> get_quantities(l1)
{'Steak pie': 3, 'Poutine': 2, 'Vegetarian stew': 3}

Answer 4

import collections

orders = {'t1': ['Vegetarian stew', 'Poutine', 'Vegetarian stew'], 
          't3': ['Steak pie', 'Poutine', 'Vegetarian stew'], 
          't4': ['Steak pie', 'Steak pie']}

# We're not interested in the table numbers so can initially just produce
# a flat list.

flatorders = []
for o in orders:
    flatorders += orders[o]

quantities = collections.Counter(flatorders)

# The "sorted" keyword prints the list in alphabeitcal order. If it is
# omitted the list is printed in order of quantities.

for k in sorted(quantities):
    print("{:15} {:3d}".format(k, quantities[k]))

Return 以菜品为键，菜品被点的次数为值的字典

Return a dictionary with the dish as the key, and the number of times that dish was ordered as the value

python

parallels

tuples