python 字典

# #有 3 个字典方法，它们将返回类似列表的值，分别对应于字典的键、值和键-值对
# keys()  values()  items()
spam = {‘color‘: ‘red‘, ‘age‘: 42}
for v in spam.values():  # spam.keys() # items() # items() 打印为包含键和值的元组
    print(v)
spam = {‘color‘: ‘red‘, ‘age‘: 42}
spam.keys()  # dict_keys([‘color‘, ‘age‘])
list(spam.keys())  # [‘color‘, ‘age‘]

#也可以利用多重赋值的技巧，在 for 循环中将键和值赋给不同的变量。在交互
#式环境中输入以下代码：
spam = {‘color‘: ‘red‘, ‘age‘: 42}
for k, v in spam.items():
    print(‘Key: ‘ + k + ‘ Value: ‘ + str(v))
# Key: age
# Value: 42
# Key: color
# Value: red

#在访问一个键的值之前，检查该键是否存在于字典中，这很麻烦。好在，字典有一
#个get()方法，它有两个参数：要取得其值的键，以及如果该键不存在时，返回的备用值。
picnicItems = {‘apples‘: 5, ‘cups‘: 2}
‘I am bringing ‘ + str(picnicItems.get(‘cups‘, 0)) + ‘ cups.‘
##‘I am bringing 2 cups.‘
‘I am bringing ‘ + str(picnicItems.get(‘eggs‘, 0)) + ‘ eggs.‘
##‘I am bringing 0 eggs.‘


#计算元素出现次数
message = ‘It was a bright cold day in April, and the clocks were striking thirteen.‘
count = {}
for character in message:
    # print(character)
    count.setdefault(character, 0)
    # 注意把一个不同的默认值赋给已经存在的键，不会改变原来的值，仍将返回初始值
    count[character] += 1
    print(count)
    # 漂亮打印
    import pprint
    pprint.pprint(count)