set.seed(1)
example <- data.frame(col1 = rnorm(10, 0, 1), col2 = rnorm(10, 2, 3))
namevector <- c("col3", "col4")
example[ , namevector] <- NA
example
# col1 col2 col3 col4
# 1 -0.6264538 6.5353435 NA NA
# 2 0.1836433 3.1695297 NA NA
# 3 -0.8356286 0.1362783 NA NA
# 4 1.5952808 -4.6440997 NA NA
# 5 0.3295078 5.3747928 NA NA
# 6 -0.8204684 1.8651992 NA NA
# 7 0.4874291 1.9514292 NA NA
# 8 0.7383247 4.8315086 NA NA
# 9 0.5757814 4.4636636 NA NA
# 10 -0.3053884 3.7817040 NA NA