Fill a missing value of a variable with the value of the previous column (same row)

[eugenio.alladio]

I'd like to fill the missing values of the variable xxx.2 following dataset x:

  sample.code day.code    var xxx.1 xxx.2
1           a      a_1    rff    10   11
2           a      a_1   drav    10   NA
3           a      a_1 proint    15   15
4           a      a_1   jock    NA   NA
5           a      a_1    saf     8    8
6           a      a_1   prut     9   NA

x <- data.frame(
      sample.code = as.factor(c("a", "a", "a", "a", "a", "a")),
      day.code = as.factor(c("a_1", "a_1", "a_1", "a_1", "a_1", "a_1")),
           var = as.factor(c("rff", "drav", "proint", "jock", "saf", "prut")),
      xxx.1 = c(10L, 10L, 15L, NA, 8L, 9L),
      xxx.2 = c(11L, NA, 15L, NA, 8L, NA)
)

in order to obtain the following matrix:

  sample.code day.code    var xxx.1 xxx.2
1           a      a_1    rff    10   11
2           a      a_1   drav    10   10
3           a      a_1 proint    15   15
4           a      a_1   jock    NA   NA
5           a      a_1    saf     8    8
6           a      a_1   prut     9    9

so that the values of the column xxx.2 have to be filled by the value of the same row, reported in the variable xxx.1.
If both xxx.1 and xxx.2 are missing, I'd like to have NA for those samples.

I tried to use the following code:

while(length(ind <- which(data$Allele.2 == "")) > 0){
  data$Allele.2[ind] <- data$Allele.1[ind]
}

but I didn't succed.
I'd be very grateful if anybody could be attend to this matter.

[martin.R]

This is one way:

x$xxx.2 <- ifelse(is.na(x$xxx.1), x$xxx.2, x$xxx.1)

[eugenio.alladio]

Thank you very much for your help, you actually solved my issue!