Data Manipulation

I still struggling with this:

 error massage:

 by(AlexETF,AlexETF$Industry,function(a) {filename = paste("C:/ab/",gsub("
","",a$Industry[1]),".txt",sep="")

+         print(filename)
+ write.table(a[,3,drop=FALSE],quote=FALSE,col.names=FALSE,row.names=FALSE)    
+     }
+  )  

[1] "C:/ab/Accident&HealthInsurance.txt"
Error in `[.data.frame`(a, , 3, drop = FALSE) :
  undefined columns selected

Best,
Peter

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