Why asarray and list react diffently?

Question

I'm trying to understand why :

 w=[0.1,0.2,0.3,0.5,0]
 print(w[w!=0])

outputs : 0.2,

while

 w=[0.1,0.2,0.3,0.5,0]
 w=np.asarray(w)
 print(w[w!=0])

outputs : [0.1 0.2 0.3 0.5], which seems more logical

So : why lists do return the second element ?

Ultimately, they react differently because they are different types. Much like a cat and a tiger react differently to a poke with a stick — donkopotamus
– donkopotamus, Commented Oct 9, 2019 at 9:35
It wasn't a surprise to know that there are two different data type. I was looking for a comprehension of WHY does list reacts by throwing the second element ? — NanBlanc
– NanBlanc, Commented Oct 9, 2019 at 9:45

donkopotamus · Accepted Answer · 2019-10-09 09:52:53Z

1

A list and an ndarray implement comparison differently. In particular:

a list returns a single bool value of True or False when compared to something else. Clearly a list w is not the value 0.2 so w != 0.2 returns True
an ndarray implements comparison by returning an ndarray of booleans, representing each array element’s comparison. Thus, w != 0.2 returns [True False True True]

Thus

for a list, w[w!=0.2] is w[True] and this is treated as meaning w[1]
for an ndarray it is w[ ndarray([True False True True]) ] which then leverages numpy’s array indexing to return only those elements where the Boolean is True

answered Oct 9, 2019 at 9:47

donkopotamus

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1 Comment

This is clarifying why i can poke a cat with a stick and not a tiger ! Thanks