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# Ignoring negative and extreme values

Masked arrays are useful when we want to ignore negative values, for instance, when taking the logarithm of array values. Another use case for masked arrays is excluding extreme values. This works based on upper and lower bounds for extreme values.

We will apply these techniques to stock price data. We will skip the steps for downloading data, as they were already covered in the previous chapters.

## How to do it...

We will take the logarithm of an array that contains negative numbers:

1. Create an array containing numbers divisible by three:
```triples = np.arange(0, len(close), 3)
print("Triples", triples[:10], "...")```

Next, create an array with the ones that have the same size as the price data array:

`signs = np.ones(len(close)) ...`

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