Multiclass Confusion Matrix & Macro-F1

For multiclass problems, accuracy hides which classes the model confuses. A confusion matrix shows it: row $i$, column $j$ counts examples whose true label is labels[i] and predicted label is labels[j].

Implement:

• confusion_matrix(y_true, y_pred, labels), returning a NumPy integer array of shape (len(labels), len(labels)). Ignore any example whose true or predicted label isn't in labels.
• macro_f1(cm): compute F1 for each class from the matrix (precision = diagonal / column sum, recall = diagonal / row sum), then average over classes. A class with no predictions or no true examples has F1 of 0 for that class.

Example

confusion_matrix(["cat", "dog", "cat", "bird"], ["cat", "cat", "cat", "bird"], ["bird", "cat", "dog"])
# [[1, 0, 0],
#  [0, 2, 0],
#  [0, 1, 0]]