Counting values in pandas dataframe using lambda

Question

I have a list of years in a pandas data frame. I want to filter them using a lambda function, which I am trying to pass using count method. For me, using lambda is the most convenient way, I would prefer a solution that involves lambda.

print df['year_built'][:5]
print df['year_built'].count(lambda x: len(x) == 4)

0    1981
1    1980
2    1935
3    2007
4    1994
Name: year_built, dtype: object

AttributeError: 'RangeIndex' object has no attribute 'levels'

What is the optimal way to do this using lambda and without it?

What exactly are you trying to do? count doesn't even take a callable object. Are you trying to count the number of elements of length 4? — gmds
– gmds, Commented Apr 13, 2019 at 9:24

shammery · Accepted Answer · 2019-04-13 09:33:54Z

1

Why not just use list comprehension.

[x for x in df['year_built'] if len(x) == 4]

answered Apr 13, 2019 at 9:33

shammery

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2 Comments

user4718221 Over a year ago

this looks like the most elegant way! Could you let me know why the lambda wasn't working?

Parsa Noori Over a year ago

Iterating over large datasets by hand is forbidden in data science.

gmds · Accepted Answer · 2019-04-13 09:28:04Z

0

The right way to do this, I think, is (assuming year_built is a column of object type and contains strings:

df.loc[df['year_built'].str.len() == 4, 'year_built']

If it doesn't:

df.loc[(1000 <= df['year_built']) & (df['year_built'] < 9999), 'year_built']

answered Apr 13, 2019 at 9:28

gmds

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Comments

Andy L. · Accepted Answer · 2019-04-13 10:33:51Z

0

Assume year_build is column of string. Below will give you count of rows where len = 4

  year_build
0       1981
1       1980
2       1935
3       2007
4       1994
5         67
6         89

In [149]: (df['year_build'].str.len() == 4).sum()
Out[149]: 5

answered Apr 13, 2019 at 10:33

Andy L.

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