Difference between 'and' (boolean) vs. '&' (bitwise) in python. Why difference in behavior with lists vs numpy arrays?
What explains the difference in behavior of boolean and bitwise operations on lists vs numpy.arrays?
I'm getting confused about the appropriate use of the '
&' vs '
and' in python, illustrated in the following simple examples.
mylist1 = [True, True, True, False, True] mylist2 = [False, True, False, True, False] >>> len(mylist1) == len(mylist2) True # ---- Example 1 ---- >>>mylist1 and mylist2 [False, True, False, True, False] #I am confused: I would have expected [False, True, False, False, False] # ---- Example 2 ---- >>>mylist1 & mylist2 *** TypeError: unsupported operand type(s) for &: 'list' and 'list' #I am confused: Why not just like example 1? # ---- Example 3 ---- >>>import numpy as np >>> np.array(mylist1) and np.array(mylist2) *** ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all() #I am confused: Why not just like Example 4? # ---- Example 4 ---- >>> np.array(mylist1) & np.array(mylist2) array([False, True, False, False, False], dtype=bool) #This is the output I was expecting!
Note, in my particular situation, my desired output is a newlist where:
len(newlist) == len(mylist1) newlist[i] == (mylist1[i] and mylist2[i]) #for every element of newlist
Example 4, above, led me to my desired output, so that is fine.
But I am left feeling confused about when/how/why I should use 'and' vs '&'. Why do lists and numpy arrays behave differently with these operators?
Can anyone help me understand the difference between boolean and bitwise operations to explain why they handle lists and numpy.arrays differently?
I just want to make sure I continue to use these operations correctly going forward. Thanks a lot for the help!
Numpy version 1.7.1 python 2.7 References all inline with text.
1) Thanks @delnan for pointing out that in my original examples I had am ambiguity that was masking my deeper confusion. I have updated my examples to clarify my question.
and tests whether both expressions are logically
& (when used with
False values) tests if both are
In Python, empty built-in objects are typically treated as logically
False while non-empty built-ins are logically
True. This facilitates the common use case where you want to do something if a list is empty and something else if the list is not. Note that this means that the list [False] is logically
>>> if [False]: ... print 'True' ... True
So in Example 1, the first list is non-empty and therefore logically
True, so the truth value of the
and is the same as that of the second list. (In our case, the second list is non-empty and therefore logically
True, but identifying that would require an unnecessary step of calculation.)
For example 2, lists cannot meaningfully be combined in a bitwise fashion because they can contain arbitrary unlike elements. Things that can be combined bitwise include: Trues and Falses, integers.
NumPy objects, by contrast, support vectorized calculations. That is, they let you perform the same operations on multiple pieces of data.
Example 3 fails because NumPy arrays (of length > 1) have no truth value as this prevents vector-based logic confusion.
Example 4 is simply a vectorized bit
If you are not dealing with arrays and are not performing math manipulations of integers, you probably want
If you have vectors of truth values that you wish to combine, use