creating list from dataframe

Question

I am a newbie to python. I am trying iterate over rows of individual columns of a dataframe in python. I am trying to create an adjacency list using the first two columns of the dataframe taken from csv data (which has 3 columns).

The following is the code to iterate over the dataframe and create a dictionary for adjacency list:

df1 = pd.read_csv('person_knows_person_0_0_sample.csv', sep=',', index_col=False, skiprows=1) 

src_list = list(df1.iloc[:, 0:1])
tgt_list = list(df1.iloc[:, 1:2])
    adj_list = {}

    for src in src_list:
        for tgt in tgt_list:
            adj_list[src] = tgt


    print(src_list) 
    print(tgt_list)
    print(adj_list)

and the following is the output I am getting:

['933']
['4139']
{'933': '4139'}

I see that I am not getting the entire list when I use the list() constructor. Hence I am not able to loop over the entire data.

Could anyone tell me where I am going wrong?

To summarize, Here is the input data:

A,B,C
933,4139,20100313073721718
933,6597069777240,20100920094243187
933,10995116284808,20110102064341955
933,32985348833579,20120907011130195
933,32985348838375,20120717080449463
1129,1242,20100202163844119
1129,2199023262543,20100331220757321
1129,6597069771886,20100724111548162
1129,6597069776731,20100804033836982

the output that I am expecting:

933: [4139,6597069777240, 10995116284808, 32985348833579, 32985348838375]
1129: [1242, 2199023262543, 6597069771886, 6597069776731]

Could you please add the input data and expected output to the question as well? There should be some better solution that iterating over all values in a for-loop. — Shaido
– Shaido, Commented Aug 16, 2018 at 7:56

jezrael · Accepted Answer · 2018-08-16 08:08:32Z

2

Use groupby and create Series of lists and then to_dict:

#selecting by columns names
d = df1.groupby('A')['B'].apply(list).to_dict()

#seelcting columns by positions
d = df1.iloc[:, 1].groupby(df1.iloc[:, 0]).apply(list).to_dict()

print (d)
{933: [4139, 6597069777240, 10995116284808, 32985348833579, 32985348838375],
 1129: [1242, 2199023262543, 6597069771886, 6597069776731]}

answered Aug 16, 2018 at 8:08

jezrael

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2 Comments

sindhuja Over a year ago

Yes sure, I have tried this method, it works. But in general, I just want to know if there is a way to loop through columns of dataframes. Why does the list() constructor return only the first value? Is there a way to do the same task by iterating through the rows?

jezrael Over a year ago

@sindhuja - First rule in pandas is avoid loops, because slow. And for selcet values need df1.iloc[:, 1] instaed df1.iloc[:, 0:1]

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