- Converting Other Structures to NumPy Arrays
- Built-in Values
- Indexing/Slicing
- arange
- 2D NumPy Arrays
- Previous Section
- Next Section

NumPy will be the linear algebra library we will use. We will soon get into its functionalities - it has a lot of them - but here we are going to discuss a NumPy array. A NumPy array is used in a lot of plotting functionalities, and we'll be converting data from a Pandas Dataframe to a NumPy array when we reach more of the core ML parts.

__Converting Other Structures to NumPy Arrays__

First, let's consider a list.

`a = [1, 2, 3, 4, 5]`

We can make this a NumPy array through the following function call:

`numpy_arr = np.array(a)`

When we want to convert a dataframe column to a NumPy Array, we can use the `np.asanyarray`

* *function:

`numpyArray = np.asanyarray(df[["col"]])`

__Built-in Values__

If `a`

is a numpy array,

`a.size`

produces the number of elements in the numpy array and

`a.shape`

produces a tuple with the size of each dimension.

__Indexing/Slicing__

The NumPy array has a fixed size, meaning we cannot insert or delete elements from it. However, we can still change the already existing elements of a numpy array. For instance, if we have an array called `numpyArray`

and `len(numpyArray)`

`numpyArray[n] = m`

sets the th index to in the array*.*

Additionally, if you query a list of values in place of the number n, you can update the NumPy array to contains the values at those indices (plural for index).

`numpyArray[1, 2, 3] = [a1, a2, a3]`

After the above code is run, , , and will be the elements of `numpyArray`

* *at indices 1, 2 and 3 respectively.

__arange__

`np.arange(a, b)`

produces a list of all integers from and , excluding .

```
# the following code returns [1, 2, 3, 4]
np.arange(1, 5)
```

__2D NumPy Arrays__

2D NumPy Arrays are a lot like 1D arrays. To create an array, we can initialize an array (of size of arrays (each of size ). For instance,

can be created with the following line:

`numpyArray = np.array([[1, 2, 3], [4, 5, 6]])`

In essence, we create the corresponding 2D list, and then convert them to NumPy arrays.

In the next lesson, we'll some benefits of using NumPy arrays over python lists.

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