numpy.reshape() function for reshaping array in Python

Numpy Reshape

You might frequently want to reshape an existing array into an array with a different number of dimensions while working with Numpy arrays. This might be very helpful if you transform data in stages.

The NumPy array reshape() function in Python is used to change the shape of an array without changing its data.We can add or remove dimensions from an array by reshaping it. Change the amount of elements in each dimension as well.

In this article, I'll show you how to use the numpy.reshape() function to reshape a Python NumPy array using examples.

But Before Diving into numpy reshape let us first discuss about array shape

Array Shape in Numpy

One dimensional array

As we all know in numpy arrays have a particular shape. Whenever we create an array using numpy.array() numpy automatically deduce the shape of the array. For example let us create an 1D array:

import numpy as np arr = np.array([1,2,3,4,5]) print(arr.shape) # Output: (5,)

Since it is an 1D array here the shape of the array will be (5).

Two dimensional array

Now let's us create two dimensional numpy array:

import numpy as np arr = np.array([[1,2,3,4], [5,6,7,9], [9,10,11,12]]) print(arr.shape) # Output: (3,4)

In 2D the shape of the array will be (Row,Column) or in this case (3,4). Suppose we want to access element "2" we would do something like arr[0,1]. Here the "0" indicates the outer index of the array and "1" indicated the inner index wrapped inside the brackets.

More dimensional arrays

We may apply the same approach to higher-dimensional arrays. The first index is the one with the most brackets, and the last index is the one with the fewest.

What is meant by reshaping numpy array or np.reshape()

You may want to start with a 1-dimensional array of numbers when working with NumPy arrays. After that, reshape it into an array of the 2-dimension.

This is especially useful when the dimensions of the new array are unknown at the start or are inferred during execution. It's also feasible that a given data processing step requires the input to be of a certain shape.

This is where reshaping comes in.NumPy's reshape function lets you change the shape of an array without altering its data.

Syntax of Numpy reshape()

Here is the syntax of numpy reshape function:

numpy.reshape(arr,newshape,order = 'C'|'F'|'A' )

numpy.reshape function take 3 arguments:

  • arr : Numpy array which you want to reshape.
  • newshape : the shape you want (1D,2D,3D etc) that will be return by reshape function.
  • order: refers to the order in which you’d like to read in the elements of the array to be reshaped.

Before learning how to use this NumPy method, let's first discuss a little bit more about the order parameter and how this order is carried out behind the scene.

Order C

import numpy as np np.reshape(arr,newshape,order = 'C')

Here the order = 'C' actually stands for programming language 'C'. This is the default order of np.reshape i.e if we don't specify order by default it will use order = C.

If the value is C, the reshape method will be instructed to read and write the elements using an index order similar to C, with the final axis index changing the fastest.

For 1-dimensional array we both unroll and roll back up, with the last index moving quickly and the first index changing gradually.

For 2-dimensional array first we convert array to 1-dimensional, we first unroll the array with the last index or row changing fastest which gives us 1D array. We than roll back up with last index moving fast(which in 1D array is just a single row).

Numpy reshape order C
Numpy reshape order C

Similarly for 3-dimensional array we unroll the array with the last(inner most) index changing fast, which will again gives us 1D array, then we roll back up with last index moving fast.

Once you have a grasp of how it operates, you can also bypass the unrolling and go straight from the input array to the output array. Additionally, we have the ability to transform any two-dimensional array into a three-dimensional array, as well as into any other combination of dimensions.

Order F

import numpy as np np.reshape(arr,newshape,order = 'F')

Here order = F stands for programming language "Fortan". With an index order similar to Fortran, the reshape function is instructed to read and write the items with the first index changing the fastest and the last index changing the slowest when given the value F.

The first index is changed first and most quickly, followed by the second index and then the final index.

The first index changes the most quickly in a two-dimensional array, and the second index changes the least quickly as shown below:

Numpy reshape order F
Numpy reshape order F

Similarly for 3-dimensional array, the first index changes the quickest, whereas the third index changes the slowest.

Order A

import numpy as np np.reshape(arr,newshape,order = 'A')

Last but not least, a value of "A" instructs the reshape function to read and write the elements in accordance with the following rules:

  • If the memory for the NumPy array is Fortran contiguous, read and write the elements in Fortran-like index order.

  • Otherwise, read and write the elements in C-like order.

Now that we have that out of the way, let's look at some np.reshape function examples

Example of np.reshape

Convert Numpy array from 1D to 2D

import numpy as np arr = np.arange(12) print(arr) # Output: array([ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]) print(np.reshape(arr,(4,3))) #Output: ''' [[ 0 1 2] [ 3 4 5] [ 6 7 8] [ 9 10 11]] '''

Convert Numpy array from 1D to 2D

import numpy as np arr = np.arange(12) print(arr) # Output: array([ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]) print(np.reshape(arr,(1,4,3))) #Output: ''' [[[ 0 1 2] [ 3 4 5] [ 6 7 8] [ 9 10 11]]] '''

Using numpy reshape() to flatten an array

Flattening an array means reducing a multidimensional array to a single dimension. To obtain 1D array from any multidimensional array we use np.reshape(-1):

-1 in np.reshape

In np.reshape, we can use -1 in a shape. -1 is a placeholder that takes the appropriate value so that the input and output shapes match. This is especially useful when writing a function and not knowing the exact dimensions of the input array, but knowing that the output should have two columns, for example. However, only one value can be replaced by -1 at a time.

import numpy as np arr = np.array([[1,2,3,4],[5,6,7,8]]) print(arr) #Output: ''' [[1 2 3 4] [5 6 7 8]] ''' print(np.reshape(-1)) #Output: [1 2 3 4 5 6 7 8]

Conclusion

As long as the elements needed for reshaping are identical in both shapes, we can transform any array into any shape. We are permitted to have one "unknown" dimension, which is interesting. That implies that for one of the dimensions in the reshape technique, you are not need to provide an example number. In this situation, supply -1 as the value and NumPy will generate the number for you.

References

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