Instance Counting in Data Frame

Question

I would like to count how many time a person shows up in a data frame.

Say our data set is-

dic = {'firstname':['John','John','John','John','John','Susan','Mike',
                    'Mike','Jacob','David','Jacob','David','Jacob','David','Mike'],
       'lastname':['Smith','Smith','Adams','Adams','Adams','Wilson',
                   'Jones','Jones','White','Miller','Peterson','Miller','White',
                   'Miller','Jones']}
df = pd.DataFrame(dic)
print(df)

with output-

   firstname  lastname
0       John     Smith
1       John     Smith
2       John     Adams
3       John     Adams
4       John     Adams
5      Susan    Wilson
6       Mike     Jones
7       Mike     Jones
8      Jacob     White
9      David    Miller
10     Jacob  Peterson
11     David    Miller
12     Jacob     White
13     David    Miller
14      Mike     Jones

I want to count how many times a person is in this data set by first and last name.

Desired output-

   firstname  lastname count
0       John     Smith     2
1       John     Adams     3
2      Susan    Wilson     1
3       Mike     Jones     3
4      Jacob     White     2
5      David    Miller     3
6      Jacob  Peterson     1

Does [this](https://stackoverflow.com/questions/20076195/what-is-the-most-efficient-way-of-counting-occurrences-in-pandas) help? I found it by pasting the title of your question into a web search. — David Wierichs, Jun 12 '20 at 17:22

score 1 · Accepted Answer · answered Jun 12 '20 at 17:24

Try this,

In [22]: df.groupby(['firstname', 'lastname']).size().reset_index(name='count')

Out[22]:
  firstname  lastname  count
0     David    Miller      3
1     Jacob  Peterson      1
2     Jacob     White      2
3      John     Adams      3
4      John     Smith      2
5      Mike     Jones      3
6     Susan    Wilson      1

Instance Counting in Data Frame

1 Answers1