python using multiple conditions with numpy where

Question

I have a giant array called AllDays storing datetime.

I generated an array which stores day of week information for each day.

I am trying to extract the weekends only from the original datetime array AllDays.

So, from the day of week I am trying the following:

DayOfWeek = np.asarray([x.weekday() for x in AllDays])
#AllDays stores datetime objects
ind = np.where(DayOfWeek == 0 or DayOfWeek == 6) #gives Error

I aim to use it as following to extract only the weekends:

weekends = AllDays[ind]

Error at line

ind = np.where(DayOfWeek == 0 or DayOfWeek == 6)
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

You could define a variable weekend_indexes = (0, 6) and check DayOfWeek in weekend_indexes. Then you'd have a single condition to put in your np.where() — Jeremy Weirich
– Jeremy Weirich, Commented Jun 3, 2016 at 13:48
The ValueError comes up frequently. It indicates that an array is being used in a context that expects a scalar True/False (e.g. the or). — hpaulj
– hpaulj, Commented Jun 3, 2016 at 16:11

Stephan W. · Accepted Answer · 2016-06-03 13:49:41Z

2

The problem here is the "or" which is not defined for numpy boolean arrays. You can just use a sum instead:

np.where((DayOfWeek == 0) + (DayOfWeek == 6))

edit: You can also use the bitwise or operator:

np.where((DayOfWeek == 0) | (DayOfWeek == 6))

which gives the same result but is somewhat nicer as we are working with booleans...

edited Jun 3, 2016 at 13:49

answered Jun 3, 2016 at 13:44

Stephan W.

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4 Comments

user2285236 Over a year ago

The element-wise OR operator is |. np.where((DayOfWeek == 0) | (DayOfWeek == 6))

Stephan W. Over a year ago

Yeah, that is even nicer. Thanks!

Stephan W. Over a year ago

because true + true = true does not really make sense as far as logics are concerned. In the implementation, it works just as good.

hpaulj Over a year ago

The () are equally important. You want to evaluate the == first.

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