# 4. Unsupervised Metric Learning¶

Unsupervised metric learning algorithms only take as input an (unlabeled)
dataset `X`

. For now, in metric-learn, there only is Covariance, which is a
simple baseline algorithm (see below).

## 4.1. Algorithms¶

### 4.1.1. Covariance¶

Covariance does not “learn” anything, rather it calculates the covariance matrix of the input data. This is a simple baseline method. It can be used for ZCA whitening of the data (see the Wikipedia page of whitening transformation).

```
from metric_learn import Covariance
from sklearn.datasets import load_iris
iris = load_iris()['data']
cov = Covariance().fit(iris)
x = cov.transform(iris)
```

References:

[1] | On the Generalized Distance in Statistics, P.C.Mahalanobis, 1936 |