CODEWITHSUNDEEP
pythonnumpy
Samuel
Samuel

Reputation: 6237

merge two numpy.array without a loop

I have a two numpy.arrays, I want to get following result efficiently

1.add the element's of b to a's sub-array

    a=numpy.array([(1,2,3),(1,2,3)])
    b=numpy.array([0,0])
->
    c=[(0,1,2,3),(0,1,2,3)]

code in a loop 

a=numpy.array([(1,2,3),(1,2,3)])
b=numpy.array([(0,0)])
c=numpy.zeros(2 , 4)
idx=0
for x in a:
   c[idx]=(a[idx][0],a[idx][1],a[idx][2], b[idx])
   idx = idx+1

and
2. Get an 2-D array with dimension(a.dim*b.dim, 2) from two 1-D arrays

    a=numpy.array([(1,2)])
    b=numpy.array([(3,4)])
->
    c=[(1,3),(1,4),(2,3),(2,4)]

code in a loop 

a=numpy.array([(1,2)])
b=numpy.array([(3,4)])
c=numpy.zeros(a.size*b.size , 2)
idx=0
for x in a:
    for y in b:
        c[idx]=(x,y)
        idx = idx+1

Upvotes: 0

Views: 202

Answers (2)

zs2020
zs2020

Reputation: 54514

For the first one, you can do

>>> a=numpy.array([(1,2,3),(1,2,3)])
>>> b=numpy.array([0,0])
>>> [tuple(numpy.insert(x, 0, y)) for (x,y) in zip(a,b)]
[(0, 1, 2, 3), (0, 1, 2, 3)]

For the 2nd one, you can get the 2-D array like this

>>> a=numpy.array([(1,2)])
>>> b=numpy.array([(3,4)])
>>> import itertools
>>> c = list(itertools.product(a.tolist()[0], b.tolist()[0]))
[(1, 3), (1, 4), (2, 3), (2, 4)]

Upvotes: 2

Paulo Almeida
Paulo Almeida

Reputation: 8061

For the first problem, you can define b differently and use numpy.hstack:

a = numpy.array([(1,2,3),(1,2,3)])
b = numpy.array([[0],[0]])
numpy.hstack((b,a))

Regarding the second problem, I would probably use sza's answer and create the numpy array from that result, if necessary. That technique was suggested in an old Stack Overflow question.

Upvotes: 3

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