[python] Unsuccessful append to an empty NumPy array

I might understand the question incorrectly, but if you want to declare an array of a certain shape but with nothing inside, the following might be helpful:

Initialise empty array:

>>> a = np.zeros((0,3)) #or np.empty((0,3)) or np.array([]).reshape(0,3)
>>> a
array([], shape=(0, 3), dtype=float64)

Now you can use this array to append rows of similar shape to it. Remember that a numpy array is immutable, so a new array is created for each iteration:

>>> for i in range(3):
...     a = np.vstack([a, [i,i,i]])
...
>>> a
array([[ 0.,  0.,  0.],
       [ 1.,  1.,  1.],
       [ 2.,  2.,  2.]])

np.vstack and np.hstack is the most common method for combining numpy arrays, but coming from Matlab I prefer np.r_ and np.c_:

Concatenate 1d:

>>> a = np.zeros(0)
>>> for i in range(3):
...     a = np.r_[a, [i, i, i]]
...
>>> a
array([ 0.,  0.,  0.,  1.,  1.,  1.,  2.,  2.,  2.])

Concatenate rows:

>>> a = np.zeros((0,3))
>>> for i in range(3):
...     a = np.r_[a, [[i,i,i]]]
...
>>> a
array([[ 0.,  0.,  0.],
       [ 1.,  1.,  1.],
       [ 2.,  2.,  2.]])

Concatenate columns:

>>> a = np.zeros((3,0))
>>> for i in range(3):
...     a = np.c_[a, [[i],[i],[i]]]
...
>>> a
array([[ 0.,  1.,  2.],
       [ 0.,  1.,  2.],
       [ 0.,  1.,  2.]])

Examples related to python

programming a servo thru a barometer Is there a way to view two blocks of code from the same file simultaneously in Sublime Text? python variable NameError Why my regexp for hyphenated words doesn't work? Comparing a variable with a string python not working when redirecting from bash script is it possible to add colors to python output? Get Public URL for File - Google Cloud Storage - App Engine (Python) Real time face detection OpenCV, Python xlrd.biffh.XLRDError: Excel xlsx file; not supported Could not load dynamic library 'cudart64_101.dll' on tensorflow CPU-only installation

Examples related to arrays

PHP array value passes to next row Use NSInteger as array index How do I show a message in the foreach loop? Objects are not valid as a React child. If you meant to render a collection of children, use an array instead Iterating over arrays in Python 3 Best way to "push" into C# array Sort Array of object by object field in Angular 6 Checking for duplicate strings in JavaScript array what does numpy ndarray shape do? How to round a numpy array?

Examples related to numpy

Unable to allocate array with shape and data type How to fix 'Object arrays cannot be loaded when allow_pickle=False' for imdb.load_data() function? Numpy, multiply array with scalar TypeError: only integer scalar arrays can be converted to a scalar index with 1D numpy indices array Could not install packages due to a "Environment error :[error 13]: permission denied : 'usr/local/bin/f2py'" Pytorch tensor to numpy array Numpy Resize/Rescale Image what does numpy ndarray shape do? How to round a numpy array? numpy array TypeError: only integer scalar arrays can be converted to a scalar index

Examples related to append

List append() in for loop ValueError: all the input arrays must have same number of dimensions Append a tuple to a list - what's the difference between two ways? How merge two objects array in angularjs? How to add an element at the end of an array? Appending a list or series to a pandas DataFrame as a row? Can someone explain how to append an element to an array in C programming? How to append elements at the end of ArrayList in Java? Append value to empty vector in R? How to append new data onto a new line