Numpy 中的索引

Question

In the following example, why does the first numpy index gives [1], but the second numpy index gives [ ]? Thanks in advance for the help!

a = np.array([1,2,3])
print(a[a<2])
>>> [1]
print(a[True, False, False])
>>> []

Answer 1

for the first one, you are selecting index: a[a<2] = a[[True, False, False]] which is different to your second. a[True, False, False]

for your second, you are selecting True, False, False index, which doesn't exists in the array. so nothing returns.

Answer 2

here you told the numpy array a to select the elements which are less than 2 and there is only one element it is the element "1" which has as index 0 in your array.

look at this illustrative example:

print(a[a<1])
>>> []
print(a[a<2])
>>> [1]
print(a[a<3])
>>> [1,2]
print(a[a<4])
>>> [1,2,3]

try

print(a[[True, False, False]])

instead of

print(a[True, False, False])