Getting index of an array value

Question

I have two arrays as follows:

e = np.array([True, False, True, True])    
f = np.array([True, False, False, True])

I want to get the index i where e[i] and f[i] == True.

The expected output for the above will be:

[0,3] since e[0] == True and f[0] == True; e[3] and f[3] ==True

There may be more of such matches, so I need a list of all of the indices which satisfy the above condition.

I tried to find the list of matching values by doing this:

list(e&f)
Out[474]:
[True, False, False, True]

To get the index, I thought I could use .index(True) of list. But it doesn't work or just gives output as 0. Maybe only giving the first index and not all.

Finally, I need the 1 added to each element of output so instead of [0,3] output should be [1,4] but this is easy I can do that if I get the indices,

You've asked several questions about searching numpy arrays. Aren't you learning general principles from the answers? It seems like you're just giving us your entire homework assignment and asking us to do it for you. — Barmar
– Barmar, Commented Jun 23, 2016 at 0:39
No there is no homework. I am working on some work. I do know general principle of numpy array. But my understanding says to get an index we can use .index function of list. but it gives only one value — Baktaawar
– Baktaawar, Commented Jun 23, 2016 at 0:40

michael_j_ward · Accepted Answer · 2016-06-23 00:42:41Z

3

Take a look at numpy.where in the docs

np.where(e&f)[0]

Outputs:

 array([0, 3])

answered Jun 23, 2016 at 0:42

michael_j_ward

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1 Comment

mhawke Over a year ago

np.where(e&f)[0]+1 returns the array with 1-based indices.

mhawke · Accepted Answer · 2016-06-23 00:40:55Z

1

You can just use a list comprehension to pick them out:

>>> e = np.array([True, False, True, True])    
>>> f = np.array([True, False, False, True])
>>> [i for i,v in enumerate(e&f, 1) if v]
[1, 4]

Using enumerate() you can specify the initial index, in this case 1.

answered Jun 23, 2016 at 0:40

mhawke

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mhawke Over a year ago

@Baktaawar: you can use enumerate(), although there is probably a better numpy specific way to do it.

user3404344 · Accepted Answer · 2016-06-23 00:43:11Z

1

or simply this

np.where(e&f)

answered Jun 23, 2016 at 0:43

user3404344

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abarisone Over a year ago

Please edit with more information. Code-only and "try this" answers are discouraged, because they contain no searchable content, and don't explain why someone should "try this". We make an effort here to be a resource for knowledge.

Andy · Accepted Answer · 2016-06-23 00:54:48Z

1

Something else if you don't want to be dependent on numpy:

e=[True, False, True, True]
f=[True, False, False, True]

for idx, val in enumerate(e):
  if cmp(e[idx], f[idx]) is 0:
    print idx+1, val

edited Jun 23, 2016 at 0:54

Andy♦

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answered Jun 23, 2016 at 0:51

GreatSUN

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1 Comment

GreatSUN Over a year ago

Thanks Andy for fixing my copy&paste issue :D

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