Print the Length of DataFrames at a Specified Location

Let's find out how length and slicing operations work in pandas DataFrames.

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Try it yourself

Try executing the code below to see the result.

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import pandas as pd
df = pd.DataFrame([
  [1, 1, 1],
  [2, 2, 2],
  [3, 3, 3],
  [4, 4, 4],
  [5, 5, 5],
])
print(len(df.loc[1:3]))

Explanation

Slices in Python are half-open rangesA half-open range includes the first element but excludes the last one.. We get values from the first index up to, but not including, the last index:

In [1]: chars = ['a', 'b', 'c', 'd', 'e']
In [2]: chars[1:3]
Out[2]: ['b', 'c']

Most of the time, pandas works in the same way, which we can see below:

In [3]: s = pd.Series(chars)
In [4]: s[1:3]
Out[4]:
1 b
2 c
dtype: object

Here are three different ways to slice a pandas.Series or a pandas.DataFrame:

  • Use loc, which works by label.
  • Use iloc, which works by offset.
  • Use a slice notation such as s[1:3], which works like iloc.

The loc function works by label, and it slices on a closed range that includes the last index, like this:

In [5]: df[1:3]
Out[5]:
  0 1 2
1 2 2 2
2 3 3 3
In [6]: df.iloc[1:3]
Out[6]:
  0 1 2
1 2 2 2
2 3 3 3
In [7]: df.loc[1:3]
Out[7]:
  0 1 2
1 2 2 2
2 3 3 3
3 4 4 4

We should remember to watch out for this off-by-one error when using .loc.

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