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NumPy - Create array filled with all ones

Last Updated : 19 Sep, 2025
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To create an array filled with all ones in Python, NumPy provides the numpy.ones() function. You can specify the shape and data type of the array.

Example: This example creates a simple 1D array of ones with 5 elements.

Python
import numpy as np
arr = np.ones(5)
print(arr) 

Output
[1. 1. 1. 1. 1.]

Syntax

numpy.ones(shape, dtype=None, order='C')

Parameters:

  • shape (int or tuple): Defines size of array.
  • dtype (optional): Sets data type of elements.
  • order (optional): Memory layout, 'C' -> Row-wise (default, C-style) and 'F' -> Column-wise (Fortran-style).

2D Array of Ones

We can also create a 2D array (matrix) filled with ones by passing a tuple to the shape parameter.

Example: This example creates a 2D matrix of ones with 3 rows and 4 columns.

Python
import numpy as np
arr = np.ones((3, 4))
print(arr)

Output
[[1. 1. 1. 1.]
 [1. 1. 1. 1.]
 [1. 1. 1. 1.]]

Explanation:

  • The argument (3, 4) defines the shape of the array.
  • The resulting array has 3 rows and 4 columns, filled entirely with 1.0 (float).

Array with a Specific Data Type

We can specify the data type of the array using the dtype parameter.

Example: This example creates a 1D integer array of ones with 4 elements.

Python
import numpy as np
arr = np.ones(4, dtype=int)
print(arr)

Output
[1 1 1 1]

Explanation: By specifying dtype=int, we ensure that the array is of integer type instead of the default float64.

Multi-Dimensional Array of Ones

We can also create a higher-dimensional array (3D or more) by passing a tuple representing the shape.

Example: This example creates a 3D array of ones with shape (2, 3, 4).

Python
import numpy as np
arr = np.ones((2, 3, 4))
print(arr)

Output
[[[1. 1. 1. 1.]
  [1. 1. 1. 1.]
  [1. 1. 1. 1.]]

 [[1. 1. 1. 1.]
  [1. 1. 1. 1.]
  [1. 1. 1. 1.]]]

Explanation:

  • The argument (2, 3, 4) creates a 3D array.
  • It has 2 blocks, each containing a 3x4 matrix, filled with 1.0.

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