Loop over rows of dataframe applying function with if-statement

Question

I\'m new to R and I\'m trying to sum 2 columns of a given dataframe, if both the elements to be summed satisfy a given condition. To make things clear, what I want to do is:

Answer 1

ifelse is your friend here:

t.d$V4<-ifelse((t.d$V1>1)&(t.d$V3<9), t.d$V1+ t.d$V3, 0)