# Numpy Autocorrelation

In data science, variables of a dataset can be related to each other in some way or the other. The relationship could be directly proportional or indirectly proportional. A simple change in one variable might change some variable slightly or maybe, drastically. This phenomenon is known as correlation.

Autocorrelation refers to a correlation between a set of time signals with an outdated or old version of itself. The two sets of time signals have some time difference between them.

## Calculate Autocorrelation in NumPy

The robust data science library, NumPy, has an in-built function, `correlate()`, that can be used to find a correlation between two 1D sequences. It accepts two 1D arrays and a type of mode.

The mode type can be `valid`, `same`, and `full`, and this parameter is optional. The default value for this parameter is `valid`.

``````import numpy

myArray = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
myArray = numpy.array(myArray)
result = numpy.correlate(myArray, myArray, mode = 'full')
result = result[result.size // 2 :]
print(result)
``````

Output:

``````[385 330 276 224 175 130  90  56  29  10]
``````

In the above code, we first define a list of numbers and then convert it to a NumPy array using the NumPy’s `array()` method. Then we call our method of interest `correlate()` to compute our data’s autocorrelation. We are using the `full` mode for the calculations.

The results are stored in a `result` variable and then sliced. The slicing part is crucial since the `correlate()` method returns an array of size `2 * length of our array - 1`, and the values of our interest lie in the second half, that is, `[(result.size // 2), result.size)`.

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