Logical Operators
== Equal != Different
> Greater than < Lower than
>= Greater or equal <= Lower or equal
& and | or ! not
T true F false
dat[dat$v1 == 'yes',]
dat[dat$v1 == 'yes' & dat$v2 !='Q1',]
if(!error) dat$dv=log(dat$dv) else dat$dv=NA
dat[scale(dat$dv) >= 2 | scale(dat$dv <= -2),'dv']=NA
dat[abs(scale(dat$dv)) >= 2, 'dv']=NA