R matrix replace NA with null code example

Example 1: r replace na with 0

d[is.na(d)] <- 0

Example 2: replace any NA in a data frame in r

a = data.frame(a=c(NA,1,2,NA), b=c(1,1,2,NA),c=c(NA,NA,2,NA))
for (i in 1:3){
  a[which(is.na(a[,i])),i] = 0
}