CODEWITHSUNDEEP
pythonnumpy
vini_i
vini_i

Reputation: 335

Python NumPy adding [] after operator such as where

I'm super new to python. I'm trying to understand some code. The code uses the NumPy library to analyze a data stream. 

in0 = input_items[0]
mask = np.where(in0 > 0.9)[0]
(start, stop) = (mask[0], mask[-1])
blank = int(50e-6*sample_rate) # Skip first 50 us.
start = start+blank
foo = in1[start:stop] > 0.5
preamble_location = np.where(foo)[0][0]

In the second line np.where returns where the elements of in0 are greater than 0.9. What I don't understand is what the [0] in that line does. Similar to the last line I'm not sure what [0][0] does as well.

Upvotes: 0

Views: 43

Answers (2)

Sheldore
Sheldore

Reputation: 39072

Suppose you have the following array in0. np.where(in0 > 0.9) will return you a tuple of indices.

in0 = np.array([0.1, 0.5, 0.95, 1.3, 0.5, 0.2])

This can be checked by printing the type 

print (type(np.where(in0 > 0.9)))
# <class 'tuple'>

The length of this tuple is 1

print (len(np.where(in0 > 0.9)))
# 1

Now you need the indices of in0 array which fulfill this condition. But np.where returns a tuple. 

print (np.where(in0 > 0.9))
# (array([2, 3]),)

To get the list of indices, you need to use the index [0] 

print (np.where(in0 > 0.9)[0])
# [2 3]

Now let's come to the second question about [0][0]. Consider the following example:

foo = in0[0:4] > 0.5
print (foo)

# array([False, False,  True,  True])

Now np.where again returns a tuple as shown above. To get the array of indices, you need to access it using index [0]. This will return 

preamble_location = np.where(foo)[0]
print (preamble_location)
# [2 3]

Now [0][0] will just return the first element of this array of indices, which is the value 2. If you use [0][1], you will get the second element i.e., 3.

preamble_location = np.where(foo)[0][0]
print (preamble_location)
# 2

Upvotes: 3

Hippolyte BRINGER
Hippolyte BRINGER

Reputation: 863

It's a function of Numpy : https://docs.scipy.org/doc/numpy/reference/generated/numpy.where.html

[0] is the first element of the return.

Like :

list = [3,2,5]
print(list[0])

The return will be :

3

Upvotes: 0

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