how to add variables in a print method in R code example

Example 1: print in r

cat(c("hello", "world", "\n"))
#hello world

Example 2: print in r

> a=c(1,2,NA,4,NA,6,7)
> a
[1]  1  2 NA  4 NA  6  7
> print(a,na.print="-999")
[1]    1    2 -999    4 -999    6    7