Fix confusion matrix using only predictions as source for labels

src/dataset/mod.rs

@@ -323,6 +323,18 @@ pub trait Labels {
     fn labels(&self) -> Vec<Self::Elem> {
         self.label_set().into_iter().flatten().collect()
     }
+
+    fn combined_labels(&self, other: Vec<Self::Elem>) -> Vec<Self::Elem> {
+        let mut combined = self.labels();
+        combined.extend(other);
+
+        combined
+            .iter()
+            .collect::<HashSet<_>>()
+            .into_iter()
+            .cloned()
+            .collect()
+    }
 }
 
 #[cfg(test)]

src/metrics_classification.rs

@@ -290,7 +290,7 @@ where
             return Err(Error::MismatchedShapes(targets.len(), ground_truth.len()));
         }
 
-        let classes = self.labels();
+        let classes = self.combined_labels(ground_truth.labels());
 
         let indices = map_prediction_to_idx(
             targets.as_slice().unwrap(),
@@ -636,6 +636,17 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_division_by_zero_cm() {
+        let ground_truth = Array1::from(vec![1, 1, 0, 1, 0, 1]);
+        let predicted = Array1::from(vec![0, 0, 0, 0, 0, 0]);
+
+        let x = ground_truth.confusion_matrix(predicted).unwrap();
+        let f1 = x.f1_score();
+
+        assert_eq!(f1, 0.5);
+    }
+
     #[test]
     fn test_roc_curve() {
         let predicted = ArrayView1::from(&[0.1, 0.3, 0.5, 0.7, 0.8, 0.9]).mapv(Pr::new);