Concatenating two one-dimensional NumPy arrays

Question

I have two simple one-dimensional arrays in NumPy  I should be able to concatenate them using numpy concatenate  But I get this error for the code below   TypeError  only length-1 arrays can be converted to Python scalars  Code import numpy a   numpy array  1  2  3   b   numpy array  5  6   numpy concatenate a  b   Why

User · Answer

There are several possibilities for concatenating 1D arrays, e.g.,

numpy.r_[a, a],
numpy.stack([a, a]).reshape(-1),
numpy.hstack([a, a]),
numpy.concatenate([a, a])

All those options are equally fast for large arrays; for small ones, concatenate has a slight edge:

The plot was created with perfplot:

import numpy
import perfplot

perfplot.show(
    setup=lambda n: numpy.random.rand(n),
    kernels=[
        lambda a: numpy.r_[a, a],
        lambda a: numpy.stack([a, a]).reshape(-1),
        lambda a: numpy.hstack([a, a]),
        lambda a: numpy.concatenate([a, a]),
    ],
    labels=["r_", "stack+reshape", "hstack", "concatenate"],
    n_range=[2 ** k for k in range(19)],
    xlabel="len(a)",
)

User · Answer

The line should be  numpy concatenate  a b    The arrays you want to concatenate need to be passed in as a sequence  not as separate arguments  From the NumPy documentation   numpy concatenate  a1  a2        axis 0  Join a sequence of arrays together   It was trying to interpret your b as the axis parameter  which is why it complained it couldn t convert it into a scalar

User · Answer

Here are more approaches for doing this by using numpy ravel    numpy array    utilizing the fact that 1D arrays can be unpacked into plain elements     we ll utilize the concept of unpacking In  15     a   b  Out 15    1  2  3  5  6     using  numpy ravel    In  14   np ravel   a   b   Out 14   array  1  2  3  5  6      wrap the unpacked elements in  numpy array    In  16   np array   a   b   Out 16   array  1  2  3  5  6

User · Answer

The first parameter to concatenate should itself be a sequence of arrays to concatenate    numpy concatenate  a b     Note the extra parentheses

User · Answer

An alternative ist to use the short form of  concatenate  which is either  r        or  c        as shown in the example code beneath  see http   wiki scipy org NumPy for Matlab Users for additional information     pylab vector a   r  0  10    short form of  arange  vector b   array  1 1 1 1   vector c   r  vector a vector b  print vector a print vector b print vector c    n n   a   ones  3 4   4 print a    n  c   array  1 1 1   b   c  a c  print b    n n   a   ones  4 3   4 print a    n  c   array   1 1 1    b   r  a c  print b  print type vector b    Which results in     0   1   2   3   4   5   6   7   8   9    1 1 1 1    0   1   2   3   4   5   6   7   8   9   1   1   1   1         4   4   4   4      4   4   4   4      4   4   4   4         4   4   4   4   1      4   4   4   4   1      4   4   4   4   1          4   4   4      4   4   4      4   4   4      4   4   4         4   4   4      4   4   4      4   4   4      4   4   4      1   1   1

User · Answer

Some more facts from the numpy docs    With syntax as numpy concatenate  a1  a2        axis 0  out None   axis   0 for row-wise concatenation axis   1 for column-wise concatenation    gt  gt  gt  a   np array   1  2    3  4     gt  gt  gt  b   np array   5  6       Appending below last row  gt  gt  gt  np concatenate  a  b   axis 0  array   1  2           3  4           5  6       Appending after last column  gt  gt  gt  np concatenate  a  b T   axis 1       Notice the transpose array   1  2  5           3  4  6       Flattening the final array  gt  gt  gt  np concatenate  a  b   axis None  array  1  2  3  4  5  6     I hope it helps

[python] Concatenating two one-dimensional NumPy arrays

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