# NumPy Tutorial - NumPy Array Reshape and Resize

NumPy has two functions (and also methods) to change array shapes - `reshape`

and `resize`

. They have a significant difference that will our focus in this chapter.

`numpy.reshape()`

Let’s start with the function to change the shape of array - `reshape()`

.

```
import numpy as np
arrayA = np.arange(8)
# arrayA = array([0, 1, 2, 3, 4, 5, 6, 7])
np.reshape(arrayA, (2, 4))
#array([[0, 1, 2, 3],
# [4, 5, 6, 7]])
```

It converts a vector of 8 elements to the array of the shape of `(4, 2)`

. It could be executed successfully because the amount of elements before and after reshape is identical. It raises `ValueError`

if the amounts are different.

```
In [1]: np.reshape(arrayA, (3, 4))
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
ValueError: cannot reshape array of size 8 into shape (3,4)
```

Let’s take a closer look of the reshaped array. The first row is the first 4 data of `arrayA`

and the second row takes the last 4. It fills the data in the order of row in this reshape conversion.

You need to change the parameter `order`

if you want the order of filling data to be column.

```
In [1]: np.reshape(arrayA, (2, 4), order='F')
Out[1]: array([[0, 2, 4, 6],
[1, 3, 5, 7]])
```

The default of `order`

is `C`

that means to read or write data in C-like index order, or in simple words, in the order of row. `F`

means to read or write data in Fortan-like index order, or let’s say, in the order of `column`

. You could refer to official NumPy documentation to know more details of different indexing methods.

`ndarray.reshape()`

Besides the `reshape`

function, NumPy has also `reshape`

method in the `ndarray`

object. The method has the same parameters as the function but without given array as a parameter.

```
In [1]: arrayB = arrayA.reshape((2, 4))
In [2]: arrayB
Out[2]: array([[0, 1, 2, 3],
[4, 5, 6, 7]])
In [1]: arrayA
Out[2]: array([0, 1, 2, 3, 4, 5, 6, 7])
```

You could see, `reshape`

method is similar to `reshape`

function. And you should also be aware that `ndarray.reshape()`

method doesn’t change data and shape of the original array but returns a new `ndarray`

instance.

`Reshape()`

Function/Method Shared Memory

The converted array in `reshape`

function or method shares the same memory of the original array. You could think it as `shallow copy`

in Python, where if you change the data in one array, the corresponding data in the other array is also modified.

```
In [1]: arrayA = np.arange(8)
arrayB = arrayA.reshape((2, 4))
arrayB
Out[2]: array([[0, 1, 2, 3],
[4, 5, 6, 7]])
In [2]: arrayA[0] = 10
arrayA
Out[2]: array([10, 1, 2, 3, 4, 5, 6, 7])
In [3]: arrayB
Out[3]: array([[10, 1, 2, 3],
[4, 5, 6, 7]])
```

`numpy.resize()`

`numpy.resize()`

is a bit similar to `reshape`

in the sense of shape conversion. But it has some significant differences.

- It doesn’t have
`order`

parameter. The order of`resize`

is the same as`order='C'`

in`reshape`

. - If the number of elements of target array is not the same as original array, it will force to resize but not raise errors.

Let’s focus on the second difference.

```
In [1]: arrayA = np.arange(8)
arrayB = np.resize(arrayA, (2, 4))
Out[1]: array([[0, 1, 2, 3],
[4, 5, 6, 7]])
```

The result is the same as that in `reshpae`

if the element numbers are the same.

```
In [1]: arrayC = np.resize(arrayA, (3, 4))
arrayC
Out[1]: array([[0, 1, 2, 3],
[4, 5, 6, 7],
[0, 1, 2, 3]])
In [2]: arrayD = np.resize(arrayA, (4, 4))
arrayD
Out[2]: array([[0, 1, 2, 3],
[4, 5, 6, 7],
[0, 1, 2, 3],
[4, 5, 6, 7]])
```

If the new array has more rows, it will repeat the data in the original array but not raise the error.

```
In [1]: arrayE = np.resize(arrayA, (2, 2))
arrayE
Out[1]: array([[0, 1],
[2, 3]])
In [2]: np.resize(arrayA, (1,4))
Out[2]: array([[0, 1, 2, 3]])
```

If the number of elements in the new array is smaller, it fetches the number of elements it needs to fill in the new array in the order of row.

`resize`

Function/Method Memory

The new array doesn’t share the same memory with the original array in `resize`

function/method. The data change in one array is not mapped to the other.

```
In [1]: arrayA = np.arange(8)
arrayB = arrayA.reshape((2, 4))
arrayB
Out[2]: array([[0, 1, 2, 3],
[4, 5, 6, 7]])
In [2]: arrayA[0] = 10
arrayA
Out[2]: array([10, 1, 2, 3, 4, 5, 6, 7])
In [3]: arrayB
Out[3]: array([[0, 1, 2, 3],
[4, 5, 6, 7]])
```

**Jinku Hu**

Founder of DelftStack.com. Jinku has worked in the robotics and automotive industries for over 8 years. He sharpened his coding skills when he needed to do the automatic testing, data collection from remote servers and report creation from the endurance test. He is from an electrical/electronics engineering background but has expanded his interest to embedded electronics, embedded programming and front-/back-end programming.

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