Python pandas: Add a column to my dataframe that counts a variable

Question

I have a dataframe 'gt' like this:

org     group
org1      1
org2      1
org3      2
org4      3
org5      3
org6      3

and I would like to add column 'count' to gt dataframe to counts number member of the groups, expected results like this:

org     group   count
org1      1       2
org2      1       2
org3      2       1
org4      3       3
org5      3       3
org6      3       3

I know how to do it per one item of the group, but do not know how to make the count repeated for all of the group items, here is the code I have used:

gtcounts = gt.groupby('group').count()

This question includes [this](https://stackoverflow.com/questions/17709270/create-column-of-value-counts-in-pandas-dataframe) one. The other link has more responses and therefore might be also helpful for those who landed here. — J Agustin Barrachina, Feb 20 '23 at 16:04

score 116 · Accepted Answer · edited Jun 08 '17 at 12:29

116

Call transform this will return a Series aligned with the original df:

In [223]:

df['count'] = df.groupby('group')['group'].transform('count')
df
Out[223]:
    org  group  count
0  org1      1      2
1  org2      1      2
2  org3      2      1
3  org4      3      3
4  org5      3      3
5  org6      3      3

edited Jun 08 '17 at 12:29

Linda

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4
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answered Apr 22 '15 at 08:35

EdChum

376,765
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813
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Would it be possible to get the same result with apply() or map()? – skan Jan 09 '23 at 17:33
Take a look [here](https://stackoverflow.com/questions/17709270/create-column-of-value-counts-in-pandas-dataframe) – J Agustin Barrachina Feb 20 '23 at 16:04

score 0 · Answer 2 · answered Mar 17 '23 at 22:29

It can also be done with a combination of value_counts() and map as well. Basically, the idea is to find the counts of each group; then map these counts back to the groups.

df['count'] = df['group'].map(df['group'].value_counts())

# or
df['count'] = df['group'].map(df.groupby('group')['group'].count())

Python pandas: Add a column to my dataframe that counts a variable

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