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# NumPy | roll method

schedule Aug 11, 2023
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Numpy's roll(~) method shifts an array along the specified axis.

# Parameters

1. a | array_like

✜ The array to perform the method on.

2. shiftlink | integer or tuple of integers

✜ The desired number of shifts.

3. axislink | number | optional

✜ The axis along which to shift the input array.

# Return value

A Numpy array with elements shifted by the specified amount.

# Examples

## Rolling a flat array

To shift a flat array by 1, use the following:

 np.roll([1,2,3,4], 1) array([4, 1, 2, 3]) 

Notice how the final value of the array (i.e. 4 in this case), got placed in the very front.

## Rolling a 2D array

### By value

To shift a 2D array by 1, use the following:

 np.roll([[1,2], [3,4]], 1) array([[4, 1], [2, 3]]) 

Essentially, what we are doing is as follows:

$$\begin{pmatrix} 1&2\\ 3&4\\ \end{pmatrix}\; \to \; \begin{pmatrix} 4&1\\ 2&3\\ \end{pmatrix}$$

### By row

To shift a 2D array by 1 row-wise, add the axis=0 parameter:

 np.roll([[1,2], [3,4], [5,6]], 1, axis=0) array([[5, 6], [1, 2], [3, 4]]) 

Essentially, we're doing the following:

$$\begin{pmatrix} 1&2\\ 3&4\\ 5&6\\ \end{pmatrix}\; \to \; \begin{pmatrix} 5&6\\ 1&2\\ 3&4\\ \end{pmatrix}$$

### By column

To shift a 2D array by 1 column-wise, add the axis=1 parameter:

 np.roll([[1,2,3], [4,5,6]], 1, axis=1) array([[3, 1, 2], [6, 4, 5]]) 

Essentially, we're doing the following:

$$\begin{pmatrix} 1&2&3\\ 4&5&6\\ \end{pmatrix}\; \to \; \begin{pmatrix} 3&1&2\\ 6&4&5\\ \end{pmatrix}$$
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