Let `x`

be a NumPy array. The following:

```
(x > 1) and (x < 3)
```

Gives the error message:

ValueError: The truth value of an array with more than one element is

ambiguous. Use a.any() or a.all()

How do I fix this?

## Solution

If `a`

and `b`

are Boolean NumPy arrays, the `&`

operation returns the elementwise-and of them:

```
a & b
```

That returns a Boolean *array*. To reduce this to a single Boolean *value*, use either

```
(a & b).any()
```

or

```
(a & b).all()
```

Note: if `a`

and `b`

are *non-Boolean* arrays, consider `(a - b).any()`

or `(a - b).all()`

instead.

#### Rationale

The NumPy developers felt there was no one commonly understood way to evaluate an array in Boolean context: it could mean `True`

if *any* element is `True`

, or it could mean `True`

if *all* elements are `True`

, or `True`

if the array has non-zero length, just to name three possibilities.

Since different users might have different needs and different assumptions, the

NumPy developers refused to guess and instead decided to raise a `ValueError`

whenever one tries to evaluate an array in Boolean context. Applying `and`

to two numpy arrays causes the two arrays to be evaluated in Boolean context (by calling `__bool__`

in Python3 or `__nonzero__`

in Python2).

Source: StackOverflow.com