Change NA value into 0

Hi R-users,
I have problem regarding my function. My function as below:
gen.m <- function(n,itr){

     set.seed(1234)

     m <- matrix(nrow = n, ncol=4)
        a <- matrix(nrow = n, ncol = 1)
        b <- matrix(nrow = n, ncol = 1)
        C <- matrix(nrow = n, ncol = 1)
        d <- matrix(nrow = n, ncol = 1)
   lambda <- 0.342
     day0 <- 0.1134

       x = matrix(runif(n*itr, 0, 1),nrow = n, ncol = itr)

   y = 0
   for(i in 1:n){
     a[i] = x[i,1]
     b[i] = y - (log (a[i])) / lambda
        y =  b[i]
     }

   if (a[1] < day0){
     C[1] = 0
     }else{
     C[1] = 1}

     for (i in 1:n){
        w = as.integer(b[i])
        if (w <= n){
          C[w] = 1
          }else{
          C[w] = 0}
      }


     for (i in 2:n)
        rain2 <- cbind(a,b,C,d)
        y <- cbind(a,b,C)
        return(y)
    }


   gen.m(31,10)

When i run my output, it gave the result NA. I want the NA values is zero. Hope anyone can help me. Thanks so much.

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