[R] multiple selection syntax

[anne]

Hello

This is a very newbie question on R syntax, but I do not find the answer....

I want to make a selection on an interval say choose Xint in the 
interval of temperatures 390-399

I tried this syntax
Xint<- X[t>=390 && t< 400] 
typing >XintI get the answer numeric(0)

it did not select any object! 'though I verified that there indeed are 
occrencies of X in this interval


Any idea?

thanks

Anne