# Logical operators for boolean indexing in Pandas

I'm working with boolean index in Pandas. The question is why the statement:

```
a[(a['some_column']==some_number) & (a['some_other_column']==some_other_number)]
```

works fine whereas

```
a[(a['some_column']==some_number) and (a['some_other_column']==some_other_number)]
```

exists with error?

Example:

```
a=pd.DataFrame({'x':[1,1],'y':[10,20]})
In: a[(a['x']==1)&(a['y']==10)]
Out: x y
0 1 10
In: a[(a['x']==1) and (a['y']==10)]
Out: ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
```

When you say

```
(a['x']==1) and (a['y']==10)
```

You are implicitly asking Python to convert `(a['x']==1)`

and `(a['y']==10)`

to boolean values.

NumPy arrays (of length greater than 1) and Pandas objects such as Series do not have a boolean value -- in other words, they raise

```
ValueError: The truth value of an array is ambiguous. Use a.empty, a.any() or a.all().
```

when used as a boolean value. That's because its unclear when it should be True or False. Some users might assume they are True if they have non-zero length, like a Python list. Others might desire for it to be True only if **all** its elements are True. Others might want it to be True if **any** of its elements are True.

Because there are so many conflicting expectations, the designers of NumPy and Pandas refuse to guess, and instead raise a ValueError.

Instead, you must be explicit, by calling the `empty()`

, `all()`

or `any()`

method to indicate which behavior you desire.

In this case, however, it looks like you do not want boolean evaluation, you want **element-wise** logical-and. That is what the `&`

binary operator performs:

```
(a['x']==1) & (a['y']==10)
```

returns a boolean array.

By the way, as alexpmil notes, the parentheses are mandatory since `&`

has a higher operator precedence than `==`

. Without the parentheses, `a['x']==1 & a['y']==10`

would be evaluated as `a['x'] == (1 & a['y']) == 10`

which would in turn be equivalent to the chained comparison `(a['x'] == (1 & a['y'])) and ((1 & a['y']) == 10)`

. That is an expression of the form `Series and Series`

. The use of `and`

with two Series would again trigger the same `ValueError`

as above. That's why the parentheses are mandatory.

From: stackoverflow.com/q/21415661

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