# Python Pandas For Your Grandpa | Section 2.7 | Series Boolean Indexing

# Course Contents

- Introduction
- Series

2.1 Series Creation

2.2 Series Basic Operations

2.3 Series Basic Indexing

2.4 Series Overwriting Data

2.5 Series Apply

2.6 Series Concatenation

**2.7 Series Boolean Indexing**

2.8 Series View Vs Copy

2.9 Series Missing Values

2.10 Series Challenges

```
import numpy as np
import pandas as pd
```

Just like NumPy arrays, you can subset a pandas Series using a boolean index. For example, if you have the Series

```
foo = pd.Series([20, 50, 11, 45, 17, 31])
```

and you check `foo < 20`

, you’ll get back a corresponding series of boolean values.

```
lt20 = foo < 20
lt20
## 0 False
## 1 False
## 2 True
## 3 False
## 4 True
## 5 False
## dtype: bool
```

Then you can use this Series of boolean values to subset `foo`

via `foo.loc[lt20]`

, the result of which is the subset of `foo`

with elements less than 20.

```
foo.loc[lt20]
## 2 11
## 4 17
## dtype: int64
```

Or if you wanted to avoid the intermediate step, you can do a one-liner like

```
foo.loc[foo < 20]
## 2 11
## 4 17
## dtype: int64
```

Now, you might be thinking that this operation works something like this:

For each element in

`lt20`

, if the value is True, keep the corresponding element of`foo`

otherwise exclude it.

And you’d kind of be right. But watch what happens if we swap index labels 4 and 5 in `foo`

and then we do the same exact boolean subset using `lt20`

.

```
foo.index = [0, 1, 2, 3, 5, 4]
foo.loc[lt20]
## 2 11
## 4 31
## dtype: int64
```

This time, the result includes 31 and excludes 17. That’s because `foo.loc`

is looking for the elements of `foo`

whose index label matches those of `lt20`

where `lt20`

has a True value.

Usually this behavior is fine, but in some cases it might not be what you want and if you’d rather just include or exclude the corresponding values of `foo`

by the position of True and False elements of `lt20`

, just use the underlying NumPy array to subset `foo`

since the underlying NumPy array doesn’t have index labels.

```
foo.loc[lt20.to_numpy()]
## 2 11
## 5 17
## dtype: int64
```

## Combining Boolean Series

If you want to combine boolean Series together, you can do that too using an ampersand for ‘and’ and a pipe for ‘or’. for example, you can build complex logical subsets like

```
foo.loc[(foo < 40) & (foo > 20)]
## 4 31
## dtype: int64
```

and

```
foo.loc[(foo > 40) | (foo < 20)]
## 1 50
## 2 11
## 3 45
## 5 17
## dtype: int64
```

You can also negate a boolean series with a tilde like

```
foo.loc[~(foo % 10 == 0)]
## 2 11
## 3 45
## 5 17
## 4 31
## dtype: int64
```

Just make sure you wrap each condition in parentheses, otherwise the interpreter will read things in the wrong order and you’ll probably get an error.

```
foo.loc[foo > 40 | foo < 20] # error
```