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data_small.txt中内容如下：
343 0
5258 1
3973 2
data_big.txt中内容如下：
343 2009-05-30T17:01:58Z 39.04183745 -94.5914053833 9191
343 2009-05-28T23:40:31Z 39.0523183095 -94.6074986458 8904
23 2009-05-28T23:40:31Z 39.0523183095 -94.6074986458 8904
56 2009-05-27T18:59:50Z 39.0424168 -94.59061145 9188
5258 2009-05-15T00:09:42Z 38.9920234667 -94.5920920333 10927
5258 2009-05-27T18:59:50Z 39.0424168 -94.59061145 9188
545 2009-05-15T00:09:42Z 38.9920234667 -94.5920920333 10927
3973 2009-05-14T20:47:20Z 39.0142536 -94.5928215833 12305
3973 2009-05-14T20:43:05Z 39.0146281324 -94.5907831192 9627
需求：将data_big中有data_small第一列所对应的那一行重新写入新的text文件。
即得到new_data.txt如下：
343 2009-05-30T17:01:58Z 39.04183745 -94.5914053833 9191
343 2009-05-28T23:40:31Z 39.0523183095 -94.6074986458 8904
5258 2009-05-15T00:09:42Z 38.9920234667 -94.5920920333 10927
5258 2009-05-27T18:59:50Z 39.0424168 -94.59061145 9188
3973 2009-05-14T20:47:20Z 39.0142536 -94.5928215833 12305
3973 2009-05-14T20:43:05Z 39.0146281324 -94.5907831192 9627
代码：
''' 根据data_small筛选数据集，得到新的小数据集。 '''
fid = open('data_new', 'w')
with open(r'./data_small.txt', mode='r', encoding='utf8') as rf1, open(r'./data_big.txt', mode='r', encoding='utf8') as rf2:
content1 = rf1.readlines(-1) # 读取所有行
content2 = rf2.readlines(-1)
for i in content1:
x_1 = i.split()
for j in content2:
x_2 = j.split()
if x_1[0] == x_2[0]: # 如果相同写入新的文件
fid.write(j)
else:
pass
fid.close()
以上代码时间复杂度太高，可用以下代码：
fid = open('data_new_1', 'w')
with open(r'./data_small.txt', mode='r', encoding='utf8') as rf1, open(r'./data_big.txt', mode='r', encoding='utf8') as rf2:
content1 = rf1.readlines(-1) # 读取所有行
user_id = [line.split()[0] for line in content1]
content2 = rf2.readlines(-1)
for j in content2:
x_2 = j.split()
if x_2[0] in user_id:
fid.write(j)
fid.close()
tips：
r只读，r+读写，文件不存在报错
w只写，w+读写，若文件不存在可创建，新写入内容会覆盖之前内容
a附加写，不可读，a+附加读写，若文件不存在可创建，可追加写，不覆盖
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