在从CSV文件（Python）读取的多个列表中查找重复

Question

Title seems confusing, but let's say I'm working with the following CSV file ('names.csv').

    name1,name2,name3
    Bob,Jane,Joe
    Megan,Tom,Jane
    Jane,Joe,Rob

My question is, how would I go about making code that returns the string that occurs at least 3 times. So the output should be 'Jane', because that occurs at least 3 times. Really confused here.. perhaps some sample code would help me better understand?

So far I have:

    import csv
    reader = csv.DictReader(open("names.csv"))

    for row in reader:
        names = [row['name1'], row['name2'], row['name3']]
        print names

This returns:

    ['Bob', 'Jane', 'Joe']
    ['Megan', 'Tom', 'Jane']
    ['Jane', 'Joe', 'Rob']

Where do I go from here? Or am I going about this wrong? I'm really new to Python (well, programming altogether), so I have close to no clue what I'm doing..

Cheers

Answer 1

I'd do it like this:

>>> from collections import defaultdict
>>> d = defaultdict(int)
>>> rows = [['Bob', 'Jane', 'Joe'],
... ['Megan', 'Tom', 'Jane'],
... ['Jane', 'Joe', 'Rob']]
...
>>> for row in rows:
...     for name in row:
...         d[name] += 1
... 
>>> filter(lambda x: x[1] >= 3, d.iteritems())
[('Jane', 3)]

It uses dict with default value of 0 to count how many times each name happens in the file, and then it filters the dict with according condition (count >= 3).

Answer 2

Putting it altogether (and showing proper csv.reader usage):

import csv
import collections
d = collections.defaultdict(int)
with open("names.csv", "rb") as f: # Python 3.x: use newline="" instead of "rb"
    reader = csv.reader(f):
    reader.next() # ignore useless heading row
    for row in reader:
        for name in row:
            name = name.strip()
            if name:
                d[name] += 1
 morethan3 = [(name, count) for name, count in d.iteritems() if count >= 3]
 morethan3.sort(key=lambda x: x[1], reverse=True)
 for name, count in morethan3:
    print name, count

Update in response to comment:

You need to read through the whole CSV file whether you use the DictReader approach or not. If you want to eg ignore the 'name2' column ( not row ), then ignore it. You don't need to save all the data as your use of the variable name "rows" suggests. Here is code for a more general approach that doesn't rely on the column headings being in a particular order and allows selection/rejection of particular columns.

    reader = csv.DictReader(f):
    required_columns = ['name1', 'name3'] #### adjust this line as needed ####
    for row in reader:
        for col in required_columns:
            name = row[col].strip()
            if name:
                d[name] += 1