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求和

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def pi(N):    ' 计算pi的值 '    # step 1: 创建一个奇数序列: 1, 3, 5, 7, 9, ...    # step 2: 取该序列的前N项: 1, 3, 5, 7, 9, ..., 2*N-1.    # step 3: 添加正负符号并用4除: 4/1, -4/3, 4/5, -4/7, 4/9, ...    # step 4: 求和:    odd_list = itertools.count(1, 2)  # 创建一个奇数序列    limit_odd_list = itertools.takewhile(lambda x: 2 * N - 1 >= x, odd_list)  # 截取N项的    def map_f(x):        if not (x // 2) % 2:            return 4 / x        else:            return -4 / x    lst = map(map_f, limit_odd_list)    result = sum(list(lst))    return result

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