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Possible Duplicate:
Filtering a data frame by factors in R

I wrote a simple function to map a variable against a table and return the value in a corresponding column. The idea is very the same as vlookup in Excel. The function I wrote is the following:

ApplyMap <- function(mappingtable,variable){
  if(ncol(mappingtable)!=2){
    "Mapping table needs to have two columns"
  }else{
    names(mappingtable) <- c("col1","col2")
    output <- mappingtable[col1==variable]$col2[1]
    output <- as.character(if(is.na(output)){variable}else{output})
    return(output)
  }
}

My map:

require(data.table)

mapping <- structure(list(Field1 = structure(c(4L, 1L, 2L, 3L), .Label = c("Amsterdam", 
"Arnhem", "Groningen", "Rotterdam"), class = "factor"), Field2 = structure(c(4L, 
3L, 1L, 2L), .Label = c("Gelderland", "Groningen", "Noord-Holland", 
"Zuid-Holland"), class = "factor")), .Names = c("Field1", "Field2"
), row.names = c(NA, -4L), class = c("data.table", "data.frame"
), .internal.selfref = <pointer: 0x7f93f1018578>)

When I apply this to a simple variable such as:

ApplyMap(mapping,"Arnhem")
[1] "Gelderland"

or 

ApplyMap(mapping,"New York")
[1] "New York"

it seems to be working fine. However, now I would like to apply it to the following data.table and add another column with the second column in the map where appropriate. The list is the following:

list <- structure(list(List = structure(c(6L, 1L, 2L, 3L, 4L, 5L, 7L), .Label = c("Amsterdam", 
"Arnhem", "Groningen", "Haarlem", "Maastricht", "Rotterdam", 
"Utrecht"), class = "factor")), .Names = "List", class = "data.frame", row.names = c(NA, 
-7L))

I try to do this with the following code:

list$Province <- ApplyMap(mapping,list$List)

Unfortunately this does not seem to work appropriately. Is there anybody who can help me to understand why it is not working correctly? I get the following error message when I execute the line above:

Error in Ops.factor(col1, variable) : level sets of factors are different

Obvious, I would like to get second column to the table with the values from the second column of the mapping-table.

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