# Numpy Tutorial - NumPy Multidimensional Array-Ndarray

NumPy is a library that uses multidimensional arrays as the basic data structure. The only data structure in NumPy is `ndarray`

but not Python primitive `list`

data type, because `list`

runs relatively slowly.

After you have learned `ndarray`

that is cornerstone of NumPy, you will understand why NumPy can achieve high-speed computing.

## Ndarray Definition

`ndarray`

is the abbreviation of n-dimension array, or in other words - multidimensional arrays. `ndarray`

is an array object representing a multidimensional, homogeneous array of fixed-size items.

The dimensions and the number of elements are defined by the shape, that is a tuple of N integers that represents the number of elements in each dimension. The element type in the array is defined by `dtype`

- `data-type object`

.

Let’s explain the sentences above in layman language. All the elements stored in the `ndarray`

object must have the same data type and size.

The characteristics of `ndarray`

data type are summarized as follows.

- Can only store elements of the same type
- The amount of data in each dimension must be the same, for example 2D
`ndarray`

must have the same amount of elements in every column, and of course also in each row. - It is written in C language and it could execute matrix operation optimally

## Ndarray Attributes

Let’s list the attributes of `ndarray`

.

Attributes | Description |
---|---|

`T` |
Transpose matrix. When the array is 1 D, the original array is returned. |

`data` |
A Python buffer object that points to the starting position of the data in the array. |

`dtype` |
The data type of the element contained in the ndarray. |

`flags` |
Information about how to store ndarray data in memory (memory layout). |

`flat` |
An iterator that converts ndarray to a one-dimensional array. |

`imag` |
The imaginary part of ndarray data |

`real` |
Real part of ndarray data |

`size` |
The number of elements contained in the ndarray. |

`itemsize` |
The size of each element in bytes. |

`nbytes` |
The total memory (in bytes) occupied by the ndarray. |

`ndim` |
The number of dimensions contained in the ndarray. |

`shape` |
The shape of the ndarray (results are tuples). |

`strides` |
The number of bytes required to move to the next adjacent element in each dimension direction is represented by a tuple. |

`ctypes` |
An iterator that is processed in the ctypes module. |

`base` |
The object on which ndarray is based (which memory is being referenced). |

When you access the attributes of `ndarray`

, the data of `ndarray`

instance is not modified, even if when you use `.T`

to get the transpose of the object. You get a new `ndarray`

object but not modified original data.

Let’s take a look at the specific meaning of each attribute through example codes.

```
>>> import numpy as np
>>> a = np.array([1, 2, 3])
```

We need to import `numpy`

library and create a new 1-D array. You could check its data type and the data type of its element.

```
>>> type(a)
numpy.ndarray
>>> a.dtype
dtype('int32')
```

Let’s create a new 2-D array and then check its attributes.

```
>>> b = np.array([[4, 5, 6], [7, 8, 9]])
>>> b
array([[4, 5, 6],
[7, 8, 9]])
>>> b.T # get the transpose of b
array([[4, 7],
[5, 8],
[6, 9]])
>>> b # b keeps unmodified
array([[4, 5, 6],
[7, 8, 9]])
>>> a.size # a has 3 elements
3
>>> b.size # b has 6 elements
6
>>> a.itemsize # The size of element in a. The data type here is int64 - 8 bytes
8
>>> b.nbytes # check how many bytes in b. It is 48, where 6x8 = 48
48
>>> b.shape # The shape of b
(2, 3)
>>> b.dnim # The dimensions of b
2
```