# 10 Ways to Initialize a Numpy Array (How to create numpy arrays)

In Python, matrix-like data structures are most commonly represented with `numpy`

arrays. The `numpy`

Python package has been well-developed for efficient computation of matrices. The first step to using `numpy`

arrays is to initialize, or create, an array. In Python, there are many ways to create a `numpy`

array. In this article, I’ll demonstrate how to create `numpy`

arrays in ten different ways.

Ten common ways to initialize (or create) `numpy`

arrays are:

- From values (
`numpy.array([value, value, value])`

) - From a Python list or tuple (
`numpy.asarray(list)`

) - Empty array (
`numpy.empty(shape)`

) - Array of ones (
`numpy.ones(shape)`

) - Array of zeros (
`numpy.zeros(shape)`

) - Array of any value (
`numpy.full(value)`

) - Copy an array (
`numpy.copy(array)`

) - Sequential or evenly spaced values (
`numpy.arange`

,`numpy.linspace`

,`numpy.geomspace`

) - Array of random values (
`numpy.random`

) - Array of repeaded values (
`numpy.repeat`

)

At first, `numpy`

will have a steep learning curve, but stick with it. Learning `numpy`

is a skill that will greatly improve your Python programming.

## How to Create Numpy Arrays in 10 Different Ways

The very first thing we must do is import `numpy`

.

```
import numpy as np
```

## Array from Values¶

The most basic way to create a `numpy`

array is to specify the exact values you would like to include in the array. This is done with the `numpy.array()`

function. The desired values are passed to `numpy.array()`

as lists defined by square brackets (`[]`

).

Below, I’ve specified values to intialize a simple 1-dimension `numpy`

array.

```
np.array([8, 3, 13, 1])
```

array([ 8, 3, 13, 1])

More complicated, multidimensional arrays can also be created. Below, I’ve created a 2D array.

```
np.array([[4, 23, 98, 7],
[38, 54, 6, 18],
[5, 48, 4, 56]])
```

array([[ 4, 23, 98, 7], [38, 54, 6, 18], [ 5, 48, 4, 56]])

Just make sure you have the same number of values in each row. If you don’t you’ll get an error like this.

```
np.array([[1, 2, 3, 4], # first row as 4 values
[1, 2, 3]]) # second row only has 3 values
```

<ipython-input-4-3bc2f09689e3>:1: VisibleDeprecationWarning: Creating an ndarray from ragged nested sequences (which is a list-or-tuple of lists-or-tuples-or ndarrays with different lengths or shapes) is deprecated. If you meant to do this, you must specify 'dtype=object' when creating the ndarray. np.array([[1, 2, 3, 4], # first row as 4 values

array([list([1, 2, 3, 4]), list([1, 2, 3])], dtype=object)

You will often find it useful to save the array as a variable. Here, a 2D array is saved to the variable `a`

.

```
a = np.array([[4, 23, 98, 7],
[38, 54, 6, 18],
[5, 48, 4, 56]])
a
```

array([[ 4, 23, 98, 7], [38, 54, 6, 18], [ 5, 48, 4, 56]])

`numpy`

Array from a Python List (or tuple)¶

Converting values stored in another Python data structure, like a list or tupe, to a `numpy`

array can be done with `numpy.asarray()`

. Pass the list (or other data structure) to `numpy.asarray()`

and a `numpy`

array is returned.

These two examples demonstrate how Python lists can be converted to 1D and 2D `numpy`

arrays.

First, a 1D array.

```
my_list = [2, 45, 3, 2, 56]
my_array = np.asarray(my_list)
print('The intial object type of', type(my_list), 'was converted to ', type(my_array))
my_array
```

The intial object type of <class 'list'> was converted to <class 'numpy.ndarray'>

array([ 2, 45, 3, 2, 56])

As you can see from the output above, the intial Python list was converted into a `numpy`

array with the same values as the original.

Now, let’s take a look at a 2D array.

```
my_2d_list = [[445, 2, 442, 56, 23], [64, 556, 29, 34, 23]]
my_2d_array = np.asarray(my_2d_list)
my_array
```

array([ 2, 45, 3, 2, 56])

## Create an Empty Array¶

If you don’t have data to create a `numpy`

array then you can create an empty array. In truth, and empty array isn’t actually empty, it just contains very small, meaningless values.

To create and empty `numpy`

array, call the `numpy.empty()`

function and pass it a shape tuple. The code below demonstrates how this is done. Notice that the output array does contain values.

```
a_shape = (3, 4) # 3 rows and 4 columns
a = np.empty(a_shape)
a
```

array([[9.30800404e-312, 2.47032823e-322, 0.00000000e+000, 0.00000000e+000], [4.00766127e+175, 4.47032019e-038, 6.87008009e-091, 9.14796435e-071], [1.00554692e-047, 4.47042917e-033, 3.99910963e+252, 1.93300839e+184]])

More information and examples for `numpy.empty`

can be found in the numpy documentation.

## Array of Ones¶

Use the `numpy.ones()`

function to create a `numpy`

array of a specified shape that is is filled with the value one (1). The `numpy.ones`

function is very similar to `numpy.empty`

. You just need to pass it the shape of the output array.

The code below creates an array with 3 rows and 4 columns. Each element in the array contains the value 1.

You can change the value type (integer, double, byte, etc) by passing the `dtype`

argument to the function.

```
a_shape = (3, 4) # 3 rows and 4 columns
a = np.ones(a_shape)
a
```

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

More information and examples for `numpy.ones`

can be found in the numpy documentation.

## Array of Zeros¶

Use the `numpy.zeros()`

function to create a `numpy`

array of a specified shape that is filled with the value zero (0). The `numpy.zeros`

function is nearly the same as `numpy.ones`

and `numpy.empty`

except that the resulting array is filled the value of zero. Once again, you just need to pass a shape tuple to the `numpy.zeros`

function.

```
a_shape = (3, 4) # 3 rows and 4 columns
a = np.zeros(a.shape)
a
```

array([[0., 0., 0., 0.], [0., 0., 0., 0.], [0., 0., 0., 0.]])

More information and examples for `numpy.zeros`

can be found in the numpy documentation

## Array of Any Value¶

With `numpy.full()`

you can create an array where each element contains the same value. The `numpy.full`

function is very similar to the previous three functions (`numpy.empty`

, `numpy.zeros`

, and `numpy.ones`

) but it requires two arguments, the shape of the resulting array and the fill value.

The code below creates and array with 3 rows and 4 columns where each element contains the value 38.7.

```
a_shape = (3, 4) # 3 rows and 4 columns
fill_value = 38.7
a = np.full(a_shape, fill_value)
a
```

array([[38.7, 38.7, 38.7, 38.7], [38.7, 38.7, 38.7, 38.7], [38.7, 38.7, 38.7, 38.7]])

More information and examples for `numpy.full`

can be found in the numpy documentation.

## Copy an Existing `numpy`

Array¶

```
a = np.array([8, 3, 13, 1])
a_copy = np.copy(a)
a_copy
```

array([ 8, 3, 13, 1])

## Create an Array of Sequential (or evenly spaced) Values¶

There are several ways to create arrays of sequential or evenly spaced values with `numpy`

. In this exaple I’ll go over how to do this `numpy.arange`

for evenly spaced values over a range, `numpy.linspace`

for a certain number of evenly spaced values between two endpoints, and `numpy.geomspace`

for evenly spaces values on a log scale.

### numpy.arange¶

Let’s start with `numpy.arange`

. `numpy.arange`

returns an array of values over a specified interval. There are a few ways to use it. In this first example, we’ll create a 1D array with 9 elements by passing only the number 9 to `arange`

```
np.arange(9)
```

array([0, 1, 2, 3, 4, 5, 6, 7, 8])

As you can see, an array with 9 values (0-8) was returned. This array can be reshaped in place using `numpy.reshape`

after the call to `arange`

. Like this.

```
np.arange(9).reshape((3, 3))
```

array([[0, 1, 2], [3, 4, 5], [6, 7, 8]])

Now we have a 2D array with the same number of elements and same values but in a different shape. When you do this be sure the product of the shape tuple equals the number of elements in the array, or you will get an error.

With `numpy.arange`

we can also specify the interval for the values to be generated from. For example, if I want to generate an array with values 12-19 I would use the following code, which specifies a start value of 12 and an end value (non-inclusive) of 20.

```
np.arange(12, 20)
```

array([12, 13, 14, 15, 16, 17, 18, 19])

Finally, a step value can be passed to `arange`

so that not all values in the range are included. For example, the following code will return an array with all even values in the range 20-40.

```
np.arange(20, 41, 2)
```

array([20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40])

More information and examples for `numpy.arange`

can be found in the numpy documentation

`numpy.linspace`

¶

Similar to `numpy.arange`

, `numpy.linspace`

generates an interval of values. The difference is that `numpy.linspace`

will generate a specified number of values over an interval. I’ll demonstrate this with an example. To generate 11 evenly spaced values on the interval 0-10 we would use the following code.

```
np.linspace(0, 10, 11)
```

array([ 0., 1., 2., 3., 4., 5., 6., 7., 8., 9., 10.])

Let’s consider one more example. This time we’ll generate 10 values on the interval 22-27.

```
np.linspace(22, 27, 10)
```

array([22. , 22.55555556, 23.11111111, 23.66666667, 24.22222222, 24.77777778, 25.33333333, 25.88888889, 26.44444444, 27. ])

We can also exclude the endpoint from the interval. This will give us values that are multiples of 0.5.

```
np.linspace(22, 27, 10, endpoint=False)
```

array([22. , 22.5, 23. , 23.5, 24. , 24.5, 25. , 25.5, 26. , 26.5])

More information and examples for `numpy.linspace`

can be found in the numpy documentation.

### numpy.geomspace¶

`numpy.geomspace`

uses the same arguments as `numpy.linspace`

, but produces values evenly spaced on a log scale. Let’s take a look at a couple of examples.

```
np.geomspace(1, 1000, 4)
```

array([ 1., 10., 100., 1000.])

```
np.geomspace(1, 256, 9)
```

array([ 1., 2., 4., 8., 16., 32., 64., 128., 256.])

Notice that `numpy.geomspace`

automatically infers the the shortest log base. More information and examples for `numpy.geomspace`

can be found in the numpy documentation

## Create an Array of Random Values¶

First lets create a random number generator, `rng`

.

```
rng = np.random.default_rng()
```

We can now use `rng`

to generate random values. The following code will generate a random float between 0.0 and 1.0. First we’ll generate a single random value. Notice that this line of code will generate a new number each time it is run.

```
rng.random()
```

0.5849737472063717

An array of random values can be generated by passing an integer to `random`

, like this.

```
rng.random(10)
```

array([0.49100274, 0.79170392, 0.12288195, 0.4785899 , 0.3978777 , 0.0933146 , 0.19365217, 0.73930889, 0.68300462, 0.18180961])

And the array can be reshaped like this.

```
rng.random(9).reshape((3,3))
```

array([[0.58577718, 0.62177914, 0.25939382], [0.30153055, 0.5661081 , 0.03098927], [0.34069052, 0.27714148, 0.68044703]])

To generate random integers we’ll take a slightly different approach. Random integers can be generated by passing an interval start, interval end, and number of values to the `integers`

method. The following code will generate 5 random integers between 0 and 100.

```
rng.integers(0, 100, 5)
```

array([ 5, 32, 72, 5, 77], dtype=int64)

These are some basic example of random number generation with `numpy`

. More information and examples for random number generation with numpy can be found in the numpy documentation.

## Create an Array of Repeated Values¶

Finally, we’ll create an array of a repeated value or pattern with `numpy.repeat`

. There are many ways to apply `numpy.repeat`

, let’s walk through a few examples.

First, is a simple example where we’ll create an array be repeating the number 13 four times, like this.

```
np.repeat(13, 4)
```

array([13, 13, 13, 13])

We can also repeat each number in an array-like object a specified number of times.

```
np.repeat([7, 8, 9], 3)
```

array([7, 7, 7, 8, 8, 8, 9, 9, 9])

Alternatively, each element in the array can be repeated a different number of times.

```
np.repeat([7, 8, 9], [2, 3, 4])
```

array([7, 7, 8, 8, 8, 9, 9, 9, 9])

More information and examples for `numpy.repeat`

can be found in the numpy documentation.

## Final Thoughts

There are many different ways to create `numpy`

arrays. This article has demonstrated what I think are the most common and useful ways to initialize `numpy`

arrays. If one of these methods doesn’t meet your needs, consult the `numpy`

documentation for more array creation options.

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