Github user felixcheung commented on a diff in the pull request:
https://github.com/apache/spark/pull/20464#discussion_r172033037
--- Diff: docs/sparkr.md ---
@@ -663,3 +663,7 @@ You can inspect the search path in R with
[`search()`](https://stat.ethz.ch/R-ma
- The `stringsAsFactors` parameter was previously ignored with `collect`,
for example, in `collect(createDataFrame(iris), stringsAsFactors = TRUE))`. It
has been corrected.
- For `summary`, option for statistics to compute has been added. Its
output is changed from that from `describe`.
- A warning can be raised if versions of SparkR package and the Spark JVM
do not match.
+
+## Upgrading to Spark 2.4.0
+
+ - The `start` parameter of `substr` method was wrongly subtracted by one,
previously. This can lead to inconsistent substring results and also does not
match with the behaviour with `substr` in R. It has been corrected.
--- End diff --
2. in the migration guide we should give a concrete example with non-0
start index, eg.
`substr(df$a, 1, 6)` should be changed to `substr(df$a, 0, 5)`
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