The truth value of an array with more than one element is ambiguous.

- numpy
- pandas

```
>>> x = np.arange(10)
>>> if x<5: print('Small!')
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
```

That `x<5`

is not actually a boolean value, but an array of 10 bools, indicating which values are under 5.

If what the user wants is `(x<5).any()`

or `(x<5).all()`

and they just failed to read the error, close as a dup (separate questions for `numpy`

and `pandas`

).

If what the user wanted was to use `x<5`

as a mask array to do further array processing instead of whatever loop with an `if`

or `while`

they were attempting, that could be a good question, or a “I don’t want to read the numpy tutorial, I want you to write me a new numpy tutorial instead” question, but probably not a dup.