[R] Why the result is coming as NULL?

[Sri krishna Devarayalu Balanagu]

Why the result is coming as NULL. Can anyone help. I want to find the outliers for a reference


setwd("D:/AZ")

library("RODBC")
cdb_cnct <- odbcConnectExcel("Book1.xls")
cdb_frame <- sqlFetch(cdb_cnct, "Sheet1")
odbcClose(cdb_cnct)
rm(cdb_cnct)

x<- cdb_frame$Publication >=1990 & cdb_frame$Publication <=2012
invalid <- cdb_frame[!x, c("Reference")]
invalid

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