如何将几个列表中的特定对象附加到新文件中？

Question

我有数百个包含不同压力的数字列表，我需要将每个列表中的第5个对象附加到一个新文件中。该程序所做的是遍历数百个文本文件，每个文件中包含数千行数据。程序遍历路径中的每个文件，并将数据分为两个列表：质量电荷和压力。我的代码是：

import os

os.chdir('file_path')
path = os.listdir('file_path')
for file in path:
    MtoC = []
    Pressure = []
    readfile = open(file,'r')
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()
    line = readfile.readline()

    datalines = readfile.read().split("\n")
    useful_data = []

    for line in datalines:
        line = ''.join([x for x in line if x not in [",", "\r"]])
        data = [float(item) for item in line.split()]
        useful_data.append(data)
    combined_data = [MtoC.extend(sub_list) for sub_list in useful_data]
    Mass_to_charge = MtoC[::2]
    Pressure = MtoC[1::2]

如果我在压力下的下一行是print Pressure，那么它将打印出数百个压力列表，每个列表包含大约1000个对象。我需要将每个列表的压力[4]附加到一个新文件中。我在想：

for p in Pressure:
    file.append(p[4])

但这不起作用。我也只是试着打印每个压力表的第五个对象，但我也不知道

Answer 1

首先，变量名使用小写字母。现在，要从列表列表中获取每个列表的第5个元素，可以使用map对每个项目应用函数并返回结果列表：

list_of_fifths = map(lambda x: x[4], Pressure)

运行此命令的示例如下：

# create many lists...
>>> list_of_lists = [range(i) for i in range(5, 15, 2)]
>>> list_of_lists
[[0, 1, 2, 3, 4], [0, 1, 2, 3, 4, 5, 6], [0, 1, 2, 3, 4, 5, 6, 7, 8], [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10], [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]]
# take fifth element of each
>>> list_of_fifths = map(lambda x: x[4], list_of_lists)
>>> list_of_fifths
[4, 4, 4, 4, 4]