python中reduce函数详解 |
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1、python2 中,reduce函数是内置函数 2、python3 中,函数被移到了functools模块中 3、官方文档: reduce(...) reduce(function, sequence[, initial]) -> value Apply a function of two arguments cumulatively to the items of a sequence, from left to right, so as to reduce the sequence to a single value. For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates ((((1+2)+3)+4)+5). If initial is present, it is placed before the items of the sequence in the calculation, and serves as a default when the sequence is empty. 从左到右对一个序列的项累计地应用有两个参数的函数,以此合并序列到一个单一值。 例如,reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) 计算的就是((((1+2)+3)+4)+5)。 如果提供了 initial 参数,计算时它将被放在序列的所有项前面,如果序列是空的,它也就是计算的默认结果值了
4、解释: reduce有三个参数: function:有两个参数的额函数, 必需参数sequence: 元组、列表等可迭代对象, 必需参数inital: 初始值, 可选参数
5、reduce的工作过程是: 在迭代sequence(tuple ,list ,dictionary, string等可迭代物)的过程中, 首先把 前两个元素传给 函数参数,函数加工后,然后把得到的结果和第三个元素作为两个参数传给函数参数,函数加工后得到的结果又和第四个元素作为两个参数传给函数参数,依次类推如果传入了 initial 值, 那么首先传的就不是 sequence 的第一个和第二个元素,而是 initial值和 第一个元素。 经过这样的累计计算之后合并序列到一个单一返回值
7、初阶1 from functools import reduce def add(x,y): return x+y reduce(add,[1,2,3,4])其实就相当于 1 + 2 + 3 + 4 = 10, 如果把加号改成乘号, 就成了阶乘了 当然 仅仅是求和的话还有更简单的方法, sum([1,2,3,4]) 10
8、初阶2 把一个整数列表拼成整数: from functools import reduce reduce(lambda x, y: x * 10 + y, [1 , 2, 3, 4, 5]) 12345
9、进阶1 from functools import reduce scientists =({'name':'Alan Turing', 'age':105, 'gender':'male'}, {'name':'Dennis Ritchie', 'age':76, 'gender':'male'}, {'name':'Ada Lovelace', 'age':202, 'gender':'female'}, {'name':'Frances E. Allen', 'age':84, 'gender':'female'}) def reducer(accumulator , value): sum = accumulator['age'] + value['age'] return sum total_age = reduce(reducer, scientists) print(total_age) 'int' object is not subscriptable错误分析: 第一轮循环时:accumulator = {'name':'Alan Turing', 'age':105} value = {'name':'Dennis Ritchie', 'age':76} 第二轮循环时:accumulator = sum = 105 + 76 = 181 value = {'name':'John von Neumann', 'age':114} 所以报错,如下图:
修改: from functools import reduce scientists =({'name':'Alan Turing', 'age':105, 'gender':'male'}, {'name':'Dennis Ritchie', 'age':76, 'gender':'male'}, {'name':'Ada Lovelace', 'age':202, 'gender':'female'}, {'name':'Frances E. Allen', 'age':84, 'gender':'female'}) def reducer(accumulator , value): sum = accumulator + value['age'] return sum total_age = reduce(reducer, scientists, 0) print(total_age)结果: 467分析: reduce 有三个参数, 第三个参数是初始值的意思,是可有可无的参数。
修改之后就不出错了,流程如下: 这个仍然也可以用 sum 来更简单的完成 sum([x['age'] for x in scientists ])
10、进阶2 from functools import reduce scientists =({'name':'Alan Turing', 'age':105, 'gender':'male'}, {'name':'Dennis Ritchie', 'age':76, 'gender':'male'}, {'name':'Ada Lovelace', 'age':202, 'gender':'female'}, {'name':'Frances E. Allen', 'age':84, 'gender':'female'}) def group_by_gender(accumulator , value): accumulator[value['gender']].append(value['name']) return accumulator grouped = reduce(group_by_gender, scientists, {'male':[], 'female':[]}) print(grouped) {'male': ['Alan Turing', 'Dennis Ritchie'], 'female': ['Ada Lovelace', 'Frances E. Allen']}
参考: https://www.cnblogs.com/lonkiss/p/understanding-python-reduce-function.html |
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