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In a pandas dataframe I have a field 'amp' that should be populated by a list of length 495. Is there a panda-ic way to quickly filter on this length, such that all rows with field 'amp' are not equal to 495 are dropped?
I tried
df[len(df['amp']) == 495]
and this returned
KeyError: False
478
To filter strings of an Array based on length in JavaScript, call Array. filter() method on this String Array, and pass a function as argument that returns true for the specific condition on string length or false otherwise.
Get the number of rows: len(df) The number of rows of pandas. DataFrame can be obtained with the Python built-in function len() . In the example, it is displayed using print() , but len() returns an integer value, so it can be assigned to another variable or used for calculation.
Using query() to Filter by Column Value in pandas DataFrame. query() function is used to filter rows based on column value in pandas. After applying the expression, it returns a new DataFrame.
If you specifically need len, then @MaxU's answer is best.
For a more general solution, you can use the map method of a Series.
df[df['amp'].map(len) == 495]
This will apply len to each element, which is what you want. With this method, you can use any arbitrary function, not just len.
159
Try this:
df[df['amp'].str.len() == 495]
Demo:
In [77]: df
Out[77]:
a
0 [1, 2, 3, 4, 5]
1 [1, 2, 3]
2 [-1]
In [78]: df.a.str.len()
Out[78]:
0 5
1 3
2 1
Name: a, dtype: int64
In [79]: df[df.a.str.len() == 3]
Out[79]:
a
1 [1, 2, 3]
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