Replace NA with mean value for specific day and grid

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R

I am using a large dataset and I am not used to using one this big (286,212 rows, 19 columns) and I am not sure how to go about my problem. the data is made up of values for each day of the year for 782 grid references and I have this for 15 years. It looks as follows

**Month  Day  Grid   x2004    x2005    x2006     x2007**
 1       1    A10    0.091   0.134     NA       0.066
 1       2    A10    0.12    0.10      0.23     0.054
 1       3    A10    0.55    NA        NA       0.08
 1       1    B10    NA      0.134     NA       0.17
 1       2    B10    0.14    0.151     NA       0.21
 1       3    B10    0.43    0.162     0.24      NA

However some of the days are missing and I want to insert the mean of that day for that specific grid using values from the other years. So if the Grid A10 for day 1 in 2006 is missing. I want to insert the mean for day 1 grid A10 from 2004, 2005, 2007, in this case 0.097.

What I have tried:

I am trying the following code

x<-for(i in 1:ncol(data)){
  data[is.na(data[,i]) ,i] <- mean(data[,i], na.rm = TRUE)
}

but it seems to be finding the column mean i think and adding it in. I have also tried to change it to

x<-for(i in 1:nrow(data)){
data[is.na(data[i,]) ,i] <- mean(data[i,], na.rm = TRUE)
}

and that didn't work either. I have already asked on stackoverflow but have not got a solution yet. I am not a computer programmer, and this is the last bit of coding I need to do the stats analysis for my PhD so I am quite desperate to figure it out, but I am just not sure how to go about it. Iknow this forum is for other progamming languages but Please help if you can.