Detect and replace omitted data

Hi Jonny,

On Tue, Oct 18, 2011 at 1:02 PM, Jonny Armstrong
<jonny5armstrong at gmail.com> wrote:

I am analyzing the spatial distribution of fish in a stream. The  
stream is
divided into equally sized units, and the number of fish in each  
unit is
counted. My problem is that my dataset is missing rows where the  
count in a
unit equals zero. I need to create zero data for the missing units.

For example:
day<-(c(rep(4,8),rep(6,8)))
unit<-c(seq(1,8,1),seq(2,16,2))
value<-floor(rnorm(16,25,10))
x<-cbind(day,unit,value)

Thanks for the actual reproducible example.

x
     day unit value
 [1,]   4    1    19
 [2,]   4    2    15
 [3,]   4    3    16
 [4,]   4    4    20
 [5,]   4    5    17
 [6,]   4    6    15
 [7,]   4    7    14
 [8,]   4    8    29
 [9,]   6    2    18
[10,]   6    4    22
[11,]   6    6    27
[12,]   6    8    16
[13,]   6   10    45
[14,]   6   12    36
[15,]   6   14    34
[16,]   6   16    13

Lets say the stream has 16 units. For each day, I want to fill in  
rows for
any missing units (e.g., units 9-16 for day 4, the odd numbered  
units on day
6) with values of zero.