Counting unique values in a column in pandas dataframe like in Qlik?

问题

If I have a table like this:

df = pd.DataFrame({
         'hID': [101, 102, 103, 101, 102, 104, 105, 101],
         'dID': [10, 11, 12, 10, 11, 10, 12, 10],
         'uID': ['James', 'Henry', 'Abe', 'James', 'Henry', 'Brian', 'Claude', 'James'],
         'mID': ['A', 'B', 'A', 'B', 'A', 'A', 'A', 'C']
})

I can do count(distinct hID) in Qlik to come up with count of 5 for unique hID. How do I do that in python using a pandas dataframe? Or maybe a numpy array? Similarly, if were to do count(hID) I will get 8 in Qlik. What is the equivalent way to do it in pandas?

回答1:

Count distict values, use nunique:

df['hID'].nunique()
5

Count only non-null values, use count:

df['hID'].count()
8

Count total values including null values, use size attribute:

df['hID'].size
8

Edit to add condition

Use boolean indexing:

df.loc[df['mID']=='A','hID'].agg(['nunique','count','size'])

OR using query:

df.query('mID == "A"')['hID'].agg(['nunique','count','size'])

Output:

nunique    5
count      5
size       5
Name: hID, dtype: int64

回答2:

If I assume data is the name of your dataframe, you can do :

data['race'].value_counts()

this will show you the distinct element and their number of occurence.

回答3:

Or get the number of unique values for each column:

df.nunique()

dID    3
hID    5
mID    3
uID    5
dtype: int64

---

New in pandas 0.20.0 pd.DataFrame.agg

df.agg(['count', 'size', 'nunique'])

         dID  hID  mID  uID
count      8    8    8    8
size       8    8    8    8
nunique    3    5    3    5

---

You've always been able to do an agg within a groupby. I used stack at the end because I like the presentation better.

df.groupby('mID').agg(['count', 'size', 'nunique']).stack()


             dID  hID  uID
mID                       
A   count      5    5    5
    size       5    5    5
    nunique    3    5    5
B   count      2    2    2
    size       2    2    2
    nunique    2    2    2
C   count      1    1    1
    size       1    1    1
    nunique    1    1    1

回答4:

You can use nunique in pandas:

df.hID.nunique()
# 5

回答5:

To count unique values in column, say hID of dataframe df, use:

len(df.hID.unique())

回答6:

you can use unique property by using len function

len(df['hID'].unique()) 5

来源：https://stackoverflow.com/questions/45759966/counting-unique-values-in-a-column-in-pandas-dataframe-like-in-qlik