Deal with missing data: I want to show NA
I want to extract variables with missing value. But in my original dataset, missing case has already been assigned value like -1, -9, -8, etc. I want to recode them to make missing case i.e. na appear so I could follow analyze the trend of missing value. How can I revive those missing value?

I tried 

> mydata[mydata==-3 -4 -8 -9] <- NA

It seems work(no error message), but when I want to test whether type:

> is.na(mydata$BYSEX)

all showed FALSE...

Thanks !
Deal with missing data: I want to show NA
===

I want to extract variables with missing value. But in my original dataset, missing case has already been assigned value like -1, -9, -8, etc. I want to recode them to make missing case i.e. na appear so I could follow analyze the trend of missing value. How can I revive those missing value?

I tried 

> mydata[mydata==-3 -4 -8 -9] <- NA

It seems work(no error message), but when I want to test whether type:

> is.na(mydata$BYSEX)

all showed FALSE...

Thanks !