R ffdfdply追加错误

我想将我的数据（data.frame）从长格式转换为宽格式，其中“ITEM”的值作为列和值（“ITEM2”）（见下文）：

因此，我使用包2中的dcast函数：

df <= dcast(df,SEQUENCEID + EVENTID ~ ITEM, value.var="ITEM2")

只计算一次拆分，记录少，不追加，效果很好

有人能帮忙吗

多谢各位

#Extract items 
item<-as.character(unique(lo_raw$ITEM))

#Transform to ffdf
ff_raw<-as.ffdf(lo_raw)
ff_raw$SEQUENCEID<-as.character.ff(ff_raw$SEQUENCEID)

#Function dcast
castff<-function(df,item){
   df=dcast(df,SEQUENCEID + EVENTID ~ ITEM, value.var="ITEM2")

   for(i in item){
      if (!(i %in% colnames(df))){
      df[,i]<-NA
   }
  }

  df<-df[,order(colnames(df))]
  df
}

#Apply dcast
ff_pivot<-ffdfdply(x=ff_raw,split=ff_raw$SEQUENCEID,FUN=function(df,item) castff(df,item),item=item,BATCHBYTES=1000000,trace=TRUE)

2016-12-08 09:25:35, calculating split sizes
2016-12-08 09:25:37, building up split locations
2016-12-08 09:25:51, working on split 1/139, extracting data in RAM of 106 split elements, totalling, 0.00093 GB, while max specified data specified using BATCHBYTES is 0.00093 GB
2016-12-08 09:25:52, ... applying FUN to selected data
2016-12-08 09:25:55, ... appending result to the output ffdf
2016-12-08 09:26:02, working on split 2/139, extracting data in RAM of 172 split elements, totalling, 0.00093 GB, while max specified data specified using BATCHBYTES is 0.00093 GB
2016-12-08 09:26:03, ... applying FUN to selected data
2016-12-08 09:26:05, ... appending result to the output ffdf
Error in ff(vmode = "integer", length = length(x), levels = as.character(levs)) : unable to open
In addition: Warning message:
In is.na(levs) : is.na() applied to non-(list or vector) of type 'NULL'