np.ones() – Create 1D / 2D Numpy Array filled with ones (1’s)

In this article, we will discuss how to create 1D or 2D numpy arrays filled with ones (1s).

numpy.ones()

Python’s Numpy module provides a function to create a numpy array of given shape & type and filled with 1’s i.e,

numpy.ones(shape, dtype=float, order='C')

Arguments:

• shape: Shape of the numpy array. Single integer or sequence of integers.
• dtype: (Optional) Data type of elements. Default is float64.
• order: (Optional) Order in which data is stored in multi-dimension array i.e. in row major(‘F’) or column major (‘C’). Default is ‘C’.

Returns:

• It returns a numpy array of given shape but filled with ones.

Let’s understand with some examples but first we need to import the numpy module,

import numpy as np

Create 1D Numpy Array of given length and filled with ones

Suppose we want to create a numpy array of five ones (1s). For that we need to call the numpy.ones() function with argument 5 i.e.

np.ones(5)

It returns a 1D numpy array with five 1s,

array([1., 1., 1., 1., 1.])

We can assign the array returned by ones() to a variable and print its type to confirm if it is a numpy array or not,

arr = np.ones(5)
print(arr)
print(type(arr))

Output:

[1. 1. 1. 1. 1.]
<class 'numpy.ndarray'>

Create Numpy array with ones of integer data type

By default numpy.ones() returns a numpy array of float ones. But if we want to create a numpy array of ones as integers, then we can pass the data type too in the ones() function. For example,

arr = np.ones(5, dtype=np.int64)
print(arr)

Output:

[1 1 1 1 1]

It returned a numpy array of ones as integers because we pass the datatype as np.int64.

Create two dimensional (2D) Numpy Array of ones

To create a multidimensional numpy array filled with ones, we can pass a sequence of integers as the argument in ones() function. For example, to create a 2D numpy array or matrix of 4 rows and 5 columns filled with ones, pass (4, 5) as argument in the ones() function.

arr_2d = np.ones( (4, 5) , dtype=np.int64)
print(arr_2d)

Output:

[[1 1 1 1 1]
 [1 1 1 1 1]
 [1 1 1 1 1]
 [1 1 1 1 1]]

It returned a matrix or 2D Numpy Array of 4 rows and 5 columns filled with 1s.

Create 3D Numpy Array filled with ones

To create a 3D Numpy array filled with ones, pass the dimensions as the argument in ones() function. For example,

arr_3d = np.ones( (2, 4, 5) , dtype=np.int64)
print(arr_3d)

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 1]
  [1 1 1 1 1]
  [1 1 1 1 1]
  [1 1 1 1 1]]]

It created a 3D Numpy array of shape (2, 4, 5) filled with 1s.

Summary:

In this article we learned how to create 1D or 2D numpy array of given shape and filled with ones.