Pandas count unique values for list of values

Question

I am trying to figure out, how to count unique values in some column depending on another value. My DataFrame looks like this:

   id_user  id_track  
       1        1           
       1        2 
       1        4
       3        1
       3        1    
       3        4
       1        1
       2        5

Basically I have a table with id's of users and songs they listened to. I want to count for each user, how many unique songs did he listen and to sort it by this value. The output should be like this:

id_user uniqueTracks
   1         3
   3         2
   2         1

I tried to do this in this way (hearings is my DataFrame):

uniqueTracks=[]  #list of numbers of unique tracks
for i in range(len(hearings['id_user'].unique())):
     uniqueTracks.append(len(hearings[hearings['id_user']==i['titles'].unique()))           

but it works terribly slow for table of 27 million rows and 70k unique users. Does anyone have a clue how to do this in Pandas? Thank you in advance:)

Answer 1

Here's a NumPy approach meant for performance -

def nunique_groupby_col0_in_col1(a):
    b = a[np.lexsort(a[:,::-1].T)]

    m = np.r_[True, b[1:,1] != b[:-1,1]]
    split_idx = np.r_[0, np.flatnonzero(b[1:,0] != b[:-1,0])+1]
    m[split_idx] = 1
    count = np.add.reduceat(m,split_idx)
    userIDs = b[split_idx,0]

    sidx = count.argsort()[::-1]
    out_data = np.column_stack(( userIDs, count ))[sidx]
    return out_data

Sample run -

In [69]: df
Out[69]: 
   id_user  id_track
0        1         1
1        1         2
2        1         4
3        3         1
4        3         1
5        3         4
6        1         1
7        2         5

In [70]: out_data = nunique_groupby_col0_in_col1(df.values)
    ...: cnames = list(['id_user','uniqueTracks'])
    ...: dfout = pd.DataFrame(out_data,columns=cnames)
    ...: 

In [71]: dfout
Out[71]: 
   id_user  uniqueTracks
0        1             3
1        3             2
2        2             1