Arrays have a fixed size. You can't append to them. If you know the upper bounds of the array, you can preallocate the array with

np.empty((10, 10, 10))

For a 10x10x10 matrix. You can then keep 3 indices x,y,z to track the actual size you have. Eg, add a new element with:

matrix[x,y,z] = newElement
x += 1

Then when you're done, you can extract the submatrix with

finalMatrix = matrix[:x,:y,:z]
Answer from Oren Bell on Stack Overflow
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TutorialsPoint
tutorialspoint.com โ€บ return-a-new-three-dimensional-array-without-initializing-entries-in-numpy
Return a new Three-Dimensional array without initializing entries in Numpy
import numpy as np # To return a new 3D array without initializing entries, use the numpy.empty() method in Python Numpy # The 1st parameter is the Shape of the empty array arr = np.empty([3, 3, 3]) # Display the array print("Array... ",arr) # Get the type of the array print(" Array type...
Discussions

Initializing a 3D Numpy array with random values in Python - Python - Data Science Dojo Discussions
In the realm of data science and computational tasks, 3D Numpy arrays are a vital tool for managing multi-dimensional data. This thread explores the different techniques of initializing these arrays with random values, along with example codes. 1. Using the np.empty function: 2. Using the np.zeros ... More on discuss.datasciencedojo.com
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January 30, 2023
python - Creating an empty multidimensional array - Stack Overflow
In Python when using np.empty(), ... an array that is really empty/have no values but have given dimensions/shape? ... I think OP is looking for a variable that has shape but doesn't really have values. ... Yes, what @Sraw said is what I mean by empty. ... A numpy array always ... More on stackoverflow.com
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May 29, 2018
How can I create a truly empty numpy array which can be merged onto (by a recursive function)?
I can't say I fully followed your problem statement, but you can create an array with a total size of zero if any of the dimensions has size zero: a = np.empty((0, 3)) # Doesn't really matter if you use `empty`, `zeros` or `ones` here Zero-size arrays are the neutral element wrt. concatenation along their zero-size dimension (if that's what you mean by "merging"): b = np.random.uniform(size=(20, 3)) c = np.concatenate([a, b], 0) (c == b).all() # True More on reddit.com
๐ŸŒ r/learnpython
2
2
September 21, 2023
Declaring a 3D array (list?) in Python
First of all, you wouldn't get confused with your bracketing if you wrote it a little differently: int map[2][3][5] = { { {0,0,0,0,0}, {0,0,0,0,0}, {0,0,0,0,0} }, { {0,0,0,0,0}, {0,0,0,0,0}, {0,0,0,0,0} } } Anyway, you basically have two options: Option 1: list of lists of lists x, y, z = 2, 3, 5 map = [] for _ in range(x): sublist = [] for _ in range(y): subsublist = [0] * z sublist.append(subsublist) map.append(sublist) This can be written more compactly using nested list comprehensions: x, y, z = 2, 3, 5 map = [[[0 for c in range(z)] for b in range(y)] for a in range(x)] Doing it this way, your dimensions are fixed size (you'll get IndexError if you try to access invalid indices), but it is non-ideal because it is really a list of lists of lists. You'll have to be careful not to accidentally make multiple references to the same sublists, because that would probably lead to confusing unexpected results. Option 2: dictionary with (x, y, z) tuples as the keys x, y, z = 2, 3, 5 map = {(a, b, c): 0 for a in range(x) for b in range(y) for c in range(z)} # Now access items like map[(2, 3, 5)] # or even map[2, 3, 5] This is nice because the set-up is simpler. The downside is that there's no safeguard against adding new items with out-of-bounds indices. EDIT: Probably a good idea to call this something other than map, because map() is a built-in function that you may find yourself needing to use. More on reddit.com
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11
9
November 8, 2013
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NumPy
numpy.org โ€บ doc โ€บ stable โ€บ reference โ€บ generated โ€บ numpy.empty.html
numpy.empty โ€” NumPy v2.5 Manual
Array of uninitialized (arbitrary) data of the given shape, dtype, and order. Object arrays will be initialized to None. ... Return an empty array with shape and type of input.
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NumPy
numpy.org โ€บ doc โ€บ 2.3 โ€บ reference โ€บ generated โ€บ numpy.empty.html
numpy.empty โ€” NumPy v2.3 Manual
Array of uninitialized (arbitrary) data of the given shape, dtype, and order. Object arrays will be initialized to None. ... Return an empty array with shape and type of input.
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Appdividend
appdividend.com โ€บ create-and-check-if-a-numpy-array-is-empty
How to Create and Check If a Numpy Array is Empty
July 10, 2025 - The easiest and most robust way to check if a numpy 1d, 2d, 3d, or multi-dimensional array is empty using np.ndarray.size attribute.
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NumPy
numpy.org โ€บ doc โ€บ 1.25 โ€บ reference โ€บ generated โ€บ numpy.empty.html
numpy.empty โ€” NumPy v1.25 Manual
Array of uninitialized (arbitrary) data of the given shape, dtype, and order. Object arrays will be initialized to None. ... Return an empty array with shape and type of input.
Find elsewhere
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w3resource
w3resource.com โ€บ numpy โ€บ array-creation โ€บ empty.php
NumPy: numpy.empty() function - w3resource
We should use numpy.empty() when we need to create an array of a specific shape and type but do not require the elements to be initialized to any particular value, which can save time when dealing with large arrays.
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NumPy
numpy.org โ€บ devdocs โ€บ reference โ€บ generated โ€บ numpy.empty.html
numpy.empty โ€” NumPy v2.6.dev0 Manual
Array of uninitialized (arbitrary) data of the given shape, dtype, and order. Object arrays will be initialized to None. ... Return an empty array with shape and type of input.
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SciPy
docs.scipy.org โ€บ doc โ€บ numpy-1.13.0 โ€บ reference โ€บ generated โ€บ numpy.empty.html
numpy.empty โ€” NumPy v1.13 Manual
Return a new array of given shape and type, without initializing entries. ... empty, unlike zeros, does not set the array values to zero, and may therefore be marginally faster.
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Data Science Dojo
discuss.datasciencedojo.com โ€บ python
Initializing a 3D Numpy array with random values in Python - Python - Data Science Dojo Discussions
January 30, 2023 - In the realm of data science and computational tasks, 3D Numpy arrays are a vital tool for managing multi-dimensional data. This thread explores the different techniques of initializing these arrays with random values, along with example codes. 1. Using the np.empty function: 2. Using the np.zeros function: 3. Using the np.random.random_sample function: All these methods will create a 3-dimensional NumPy array of shape (3, 4, 5) and fill it with random values in the range [0, 1).
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NumPy
numpy.org โ€บ doc โ€บ 2.2 โ€บ reference โ€บ generated โ€บ numpy.empty.html
numpy.empty โ€” NumPy v2.2 Manual
Array of uninitialized (arbitrary) data of the given shape, dtype, and order. Object arrays will be initialized to None. ... Return an empty array with shape and type of input.
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NumPy
numpy.org โ€บ doc โ€บ 2.1 โ€บ reference โ€บ generated โ€บ numpy.empty.html
numpy.empty โ€” NumPy v2.1 Manual
Array of uninitialized (arbitrary) data of the given shape, dtype, and order. Object arrays will be initialized to None. ... Return an empty array with shape and type of input.
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Reddit
reddit.com โ€บ r/learnpython โ€บ how can i create a truly empty numpy array which can be merged onto (by a recursive function)?
r/learnpython on Reddit: How can I create a truly empty numpy array which can be merged onto (by a recursive function)?
September 21, 2023 -

I'm kind of stuck conceptually on how to make this happen. I have a recursive method that builds a binary tree, and stores the tree as an instance variable. However, the function is not allowed to return anything, so each recursive call should (according to me) modify in-place the tree instance variable. However, I'm not sure how to set up my instance variable such that all said and done it holds a multidimensional array that represents the tree.

Say I set initialize it as a 1x1 array with element zero as a placeholder. Then as I go about recursing through my tree I can merge to it... but at the end I'm left with a spare [0] element that I don't need. In this case, I'd need some kind of final stop condition and function to remove that unnecessary placeholder stump. I don't think this is possible?

Otherwise, say I initialize the instance variable as None. Then when the first series of recursive calls, it would have to reassign the tree variable to change from None to an ndarray object, but all future calls would have to merge to the array. I don't think this is what the function should be asked to do?

Is there a way to make a truly empty array that I can merge onto? (e.g. np.empty doesn't reallly give an empty array, it gives an array with placeholder values so I'm still left with a useless stump at the end).

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Python Guides
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Create An Empty Array Using NumPy In Python
May 7, 2025 - It allocates an array of the requested shape without setting the values, so the contents are whatever happened to be in that memory. The signature is in the numpy.empty reference.
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NumPy
numpy.org โ€บ doc โ€บ 1.18 โ€บ reference โ€บ generated โ€บ numpy.empty.html
numpy.empty โ€” NumPy v1.18 Manual
May 24, 2020 - Array of uninitialized (arbitrary) data of the given shape, dtype, and order. Object arrays will be initialized to None. ... Return an empty array with shape and type of input.
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Educative
educative.io โ€บ answers โ€บ how-to-create-an-empty-numpy-array
How to create an empty NumPy array
numpy.zeros(shape, dtype=float, order='C') numpy.empty(shape, dtype=float, order='C') # Shape -> Shape of the new array, e.g., (2, 3) or 2. # dtype -> The desired data-type for the array,e.g., numpy.int8. Default is numpy.float64. This parameter is optional. # order -> Indicates whether multi-dimensional data should be stored in row-major (C-style) or column-major (Fortran-style) order in memory.
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NumPy
numpy.org โ€บ doc โ€บ 2.4 โ€บ reference โ€บ generated โ€บ numpy.empty.html
numpy.empty โ€” NumPy v2.4 Manual
Array of uninitialized (arbitrary) data of the given shape, dtype, and order. Object arrays will be initialized to None. ... Return an empty array with shape and type of input.
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Note.nkmk.me
note.nkmk.me โ€บ home โ€บ python โ€บ numpy
NumPy: Create an empty array (np.empty, np.empty_like) | note.nkmk.me
January 22, 2024 - The NumPy version used in this article is as follows. Note that functionality may vary between versions. ... To create an empty array specifying shape and data type (dtype), use np.empty().