Github user ProtD commented on the issue:
https://github.com/apache/spark/pull/18872
To reproduce the bug on v2.2 and v2.3:
```scala
import org.apache.spark.ml.linalg.Vectors
val rawData = Seq((1.0, Vectors.sparse(3, Seq((0, 2.0), (1, 3.0)))),
(4.0, Vectors.sparse(3, Seq((0, 5.0), (2, 6.0)))))
val dfTemp = spark.sparkContext.parallelize(rawData).toDF("label",
"features")
dfTemp.coalesce(1).write.format("libsvm").save("...filename...")
```
This causes `java.util.NoSuchElementException: key not found: numFeatures`.
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