R in U-SQL in azure data lake analytics - r

Question

I have been experimenting for a while now with R in Azure Data Lake Analytics and I keep getting stuck on installaing extra R-packages like dlplyr and tidyr. I asked the following question, but that did not solve the issue: Installing R-packages in Azure Data Lake Analytics
Now I am trying to follow this tutorial https://blog.revolutionanalytics.com/2017/10/adla-with-r.html but get an error when executing the third script usqlscriptEx3a.usql (https://github.com/Azure/ADLAwithR-GettingStarted/tree/master/Tutorial/Exercise3). I am using te script as is and I have been debugging the R-code locally, and that seems to do the right thing, so now I do not know where to look. The error is:
An unhandled exception from user code has been reported when invoking the method 'Reduce' on the user type 'Extension.R.Reducer'
Unhandled exception from user code: "Output column 'Par' is missing from the data frame"
However Par seems to be there as it should be.
Here follows some of the code from usqlscriptEx3a.usql:
REFERENCE ASSEMBLY [ExtR];
//declare the R script as a string variable and pass it as a parameter to
the Reducer:
DECLARE #myRScript = #"
temp = inputFromUSQL
t1 = loadedNamespaces()
temp = data.frame(loadedNamespaces = t1)
t2 = data.frame(apply(temp[1],1,FUN=function(x)
{paste(unlist(packageVersion(as.character(x))),collapse='.')}) )
names(t2) = 'packageVersion'
temp$packageVersion = t2$packageVersion
t3 = sessionInfo()[1]
t3 = t3$R.version$version.string
t3 = as.character(t3)
temp$Rversion = t3
temp$Rversion[2:nrow(temp)]=''
outputToUSQL = temp
";
DECLARE #myOutputFile string = #"/TutorialMaterial/outex3a.txt";
#somedata =
SELECT * FROM
(VALUES
("Contoso", 1500.0),
("Woodgrove", 2700.0)
) AS
D( customer, amount );
#ExtendedData = SELECT
0 AS Par,
*
FROM #somedata;
#RScriptOutput = REDUCE #ExtendedData ON Par PRODUCE
Par,
loadedNamespaces string,
packageVersion string,
Rversion string
USING new Extension.R.Reducer(command:#myRScript, rReturnType:"dataframe");

Answer 1

It turns out, that what was missing, at least in my case, is the following line:
READONLY Par
In order to make the script usqlscriptEx3a.usql work I had to insert READONLY Par in the following lines:
#RScriptOutput = REDUCE #ExtendedData ON Par
PRODUCE Par,
loadedNamespaces string,
packageVersion string,
Rversion string
READONLY Par
USING new Extension.R.Reducer(command:#myRScript, rReturnType:"dataframe");