CODEWITHSUNDEEP
python-3.xnumpymultidimensional-array
Grmrk
Grmrk

Reputation: 45

Join two python arrays via index

i have a problem in Python. I am creating two numpy arrays from dict entries. I want to join those two numpy arrays in a specific way like this:

# create array with classes
probVec = filePickle['classID']
a = np.empty([0, 1])

for x in np.nditer(probVec):
    a = np.append(a,x)

timeVec = filePickle['start']
timeVec = np.asarray(timeVec)
b = np.empty([0, 1])

for x in np.nditer(timeVec):
    b = np.append(b,x)

# create input-vectors for clustering
c = np.vstack((b,a)).transpose()

Now, if i want to join them in a more specific way, like taking only specific items of array "probVec" to join them with the corresponding entry of array "timeVec" like this:

for x in np.nditer(probVec):
    if x == 3.0:
        a = np.append(a,x)

for x in np.nditer(timeVec):
    b = append with x values that have the same indices as the ones appended in the first loop

Because both arrays contain values corresponding to each other they have the same length. So my goal is something like this:

probVec = [2.0, 1.0, 3.0, 3.0, 4.0, 3.0...]

timeVec = [t1, t2, t3, t4, t5, t6...]

c = [[3.0 t3]
     [3.0 t4]
     [3.0 t6]
         .
         .
         .
     ]

I just don't know what's the best way to realize that.

Upvotes: 1

Views: 5699

Answers (1)

Sammelsurium
Sammelsurium

Reputation: 516

Using a comparison operator on an array, like a == 3.0, you get a boolean array that can be used for indexing, selecting the rows where the condition is true.

In [87]: a = np.random.randint(low=1, high=4, size=10)   # example data

In [88]: a
Out[88]: array([3, 1, 3, 1, 1, 3, 2, 2, 2, 2])

In [89]: b = np.arange(10)

In [90]: c = np.column_stack((a, b))

In [91]: c[a == 3]
Out[91]: 
array([[3, 0],
       [3, 2],
       [3, 5]])

Upvotes: 1

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