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預設的值。
collections.defaultdict在宣告時會給預設值。
背景知識
dict需要key, value
傳統dict
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| res = dict()
res['key']
print(res)
> KeyError: 'key'
res = dict()
res['key'] = 3
print(res)
> {'key': 3}
dic1 = {}
dic1["k1"].append("kkkkk") #error
print(dic1)
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改defaultDict
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| from collections import defaultdict
res = defaultdict(list) # ()內一定要指定型別
res['key']
print(res)
>>defaultdict(<class 'list'>, {'key': []})
import collections
dic = collections.defaultdict(list) #list
dic["k1"].append("kkkkk")
print(dic)
> defaultdict(<class 'list'>, {'k1': ['kkkkk']})
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理解問題
DefaultDict統計字元次數，並與counter比較。
思路視覺化
遇到相同字元+1
程式化
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| import collections
# from collections import defaultdict, Counter
C = collections.defaultdict(int) # 0開始
# collections.Counter()
# list(C) # 列出key值
s="AABBABC"
for v in s:
C[v]+=1
print(C)
> defaultdict(<class 'int'>, {'A': 3, 'B': 3, 'C': 1})
# Counter({'A': 3, 'B': 3, 'C': 1})
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| s = [('red', 1), ('blue', 2), ('red', 3), ('blue', 4), ('red', 1), ('blue', 4)]
d = defaultdict(set)
for k, v in s:
d[k].add(v)
print(d)
>defaultdict(<class 'set'>,'red': , {1, 3}, 'blue': {2, 4})
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| words = ['apple', 'pear', 'peach', 'banana', 'plum']
by_letter = defaultdict(list)
for word in words:
by_letter[word[0]].append(word)
print(by_letter)
> {'a': ['apple'], 'p': ['pear', 'peach', 'plum'], 'b': ['banana']})
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| members = [
# Age, name
['male', 'John'],
['male', 'Jack'],
['female', 'Lily'],
['male', 'Pony'],
['female', 'Lucy'],
]
result = defaultdict(list)
for sex, name in members:
result[sex].append(name)
print(result)
> defaultdict(<class 'list'>, {'male': ['John', 'Jack', 'Pony'], 'female': ['Lily', 'Lucy']})
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複雜度分析
TC: O(n)
SC: O(n)