Question

我有一个如下所示的数据集：

id,created_at,username
1,2006-10-09T18:21:51Z,hey
2,2007-10-09T18:30:28Z,bob
3,2008-10-09T18:40:33Z,bob
4,2009-10-09T18:47:42Z,john
5,2010-10-09T18:51:04Z,brad
...

我包含1M +行。我想使用python从中提取用户名列表而不重复。到目前为止，我的代码看起来像这样：

import csv

file1 = file("sample.csv", 'r')
file2 = file("users.csv", 'w')

reader = csv.reader(file1)
writer = csv.writer(file2)

rownum = 0
L = []
for row in reader:
    if not rownum == 0:
        if not row[2] in L:
            L.append(row[2])
            writer.writerow(row[2])

    rownum += 1

我有几个问题： 1 - users.csv中的输出如下所示：

h,e,y
b,o,b
j,o,h,n
b,r,a,d

如何删除每个字母之间的逗号？

2 - 我的代码不是很优雅，有没有办法将csv文件作为矩阵导入以选择最后一行，然后在javascript中使用像underscore.js这样的优雅库来删除重复项？

非常感谢

Answer 1

您可以在此处使用set，与O(1)列表相比，它提供O(N)项目查找。

seen = set()
add_  = seen.add
next(reader) #skip header
writer.writerows([row[-1]] for row in reader if row[-1] not in seen
                                                        and not add_(row[-1]))

并始终使用with语句处理文件，它会自动为您关闭文件：

with file("sample.csv", 'r') as file1, file("users.csv", 'w') as file2:
    #Do stuff with file1 and file2 here

Answer 2

更改

writer.writerow(row[2])

到

writer.writerow([row[2]])

另外，检查列表中的成员资格是计算上昂贵的[O（n）]。如果您要检查大量项目的成员资格并经常这样做，请使用set [O（1）]：

L = set()
reader.next() # Skip the header
for row in reader:
    if row[2] not in L:
        L.add(row[2])
        writer.writerow([row[2]])

替代地

如果您可以使用几兆内存，请执行以下操作：

with open("sample.csv", "rb") as infile:
    reader = csv.reader(infile)
    reader.next()
    no_duplicates = set(tuple(row) for row in reader)

    with open("users.csv", "wb") as outfile:
        csv.writer(outfile).writerows(no_duplicates)

如果订单很重要，请使用OrderedDict代替集合：

from collections import OrderedDict
with open("sample.csv", "rb") as infile:
    reader = csv.reader(infile)
    reader.next()
    no_duplicates = OrderedDict.fromkeys(tuple(row) for row in reader)

    with open("users.csv", "wb") as outfile:
        csv.writer(outfile).writerows(no_duplicates.keys())

Answer 3

轻松简短！

for line in reader:
    string = str(line)
    split = string.split("," , 2)
    username = split[2][2:-2]

从CSV文件中提取没有重复的列表

3 个答案:

替代地