Using if else statement for multiple conditions

This can be done using either ifelse

with(dat, ifelse(x < 0.15 & dif <0, 3, ifelse(x > 0.15 & dif < 0, 2, 1)))

Or

with(dat, as.numeric(factor(1+4*(x < 0.15 & dif < 0) + 2*(x>=0.15 & dif < 0))))

The problem with your statement is that if only checks the first element of the expression tested -- you should have received a warning. ifelse is vectorized.

In addition, you can perform the tests in the reverse order for a simpler, equivalent expression:

with(dat,
     ifelse(dif >= 0 , 1 , ifelse(x < 0.15, 3, 2))
)