任何人都可以告诉我这怎么可能？
> levels(x$Species)
[1] "alisterus amboinensis" "amazona viridigenalis" "aprosmictus erythropterus"
[4] "ara macao" "aratinga pertinax" "bolbopsittacus lunulatus"
[7] "brotogeris jugularis" "cacatua sulphurea" "charmosyna papou"
[10] "coracopsis vasa" "cyanoliseus patagonus" "cyanoramphus auriceps"
[13] "cyclopsitta diophthalma" "deroptyus accipitrinus" "diopsittaca nobilis"
[16] "eclectus roratus" "enicognathus leptorhynchus" "eos reticulata"
[19] "forpus passerinus" "geoffroyus heteroclitus" "guaruba guarouba"
[22] "hapalopsittaca amazonina" "leptosittaca branickii" "loriculus galgulus"
[25] "lorius albidinuchus" "melopsittacus undulatus" "micropsitta finschii"
[28] "myiopsitta monachus" "nandayus nenday" "nannopsittaca panychlora"
[31] "neopsephotus bourkii" "neopsittacus musschenbroekii" "nestor notabilis"
[34] "northiella haematogaster" "orthopsittaca manilata" "phigys solitarius"
[37] "pionites melanocephala" "pionopsitta caica" "pionus menstruus"
[40] "platycercus adscitus" "poicephalus robustus" "polytelis alexandrae"
[43] "prioniturus luconensis" "prosopeia tabuensis" "pseudeos fuscata"
[46] "psittacula columboides" "psittaculirostris edwardsii" "psittacus erithacus"
[49] "psittrichas fulgidus" "purpureicephalus spurius" "pyrrhura picta"
[52] "rhynchopsitta pachyrhyncha" "tanygnathus lucionensis" "touit batavica"
[55] "vini australis"
> x[x$species=="vini australis",]
[1] Species Longitude Latitude
<0 rows> (or 0-length row.names)
> x$Species=factor(x$Species)
> levels(x$Species)
[1] "alisterus amboinensis" "amazona viridigenalis" "aprosmictus erythropterus"
[4] "ara macao" "aratinga pertinax" "bolbopsittacus lunulatus"
[7] "brotogeris jugularis" "cacatua sulphurea" "charmosyna papou"
[10] "coracopsis vasa" "cyanoliseus patagonus" "cyanoramphus auriceps"
[13] "cyclopsitta diophthalma" "deroptyus accipitrinus" "diopsittaca nobilis"
[16] "eclectus roratus" "enicognathus leptorhynchus" "eos reticulata"
[19] "forpus passerinus" "geoffroyus heteroclitus" "guaruba guarouba"
[22] "hapalopsittaca amazonina" "leptosittaca branickii" "loriculus galgulus"
[25] "lorius albidinuchus" "melopsittacus undulatus" "micropsitta finschii"
[28] "myiopsitta monachus" "nandayus nenday" "nannopsittaca panychlora"
[31] "neopsephotus bourkii" "neopsittacus musschenbroekii" "nestor notabilis"
[34] "northiella haematogaster" "orthopsittaca manilata" "phigys solitarius"
[37] "pionites melanocephala" "pionopsitta caica" "pionus menstruus"
[40] "platycercus adscitus" "poicephalus robustus" "polytelis alexandrae"
[43] "prioniturus luconensis" "prosopeia tabuensis" "pseudeos fuscata"
[46] "psittacula columboides" "psittaculirostris edwardsii" "psittacus erithacus"
[49] "psittrichas fulgidus" "purpureicephalus spurius" "pyrrhura picta"
[52] "rhynchopsitta pachyrhyncha" "tanygnathus lucionensis" "touit batavica"
[55] "vini australis"
> x[x$species=="vini australis",]
[1] Species Longitude Latitude
<0 rows> (or 0-length row.names)
答案 0 :(得分：3)
您正在寻找功能droplevels。它从一个因子中删除未使用的级别。
如您所见，默认情况下不会删除级别。您可能想知道有些值没有观察值。
以下是一个例子：
(x <- factor(LETTERS))
## [1] A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
## Levels: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
现在选择一个子集：
(y <- x[10:15])
## [1] J K L M N O
## Levels: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
降低等级会导致：
(z <- droplevels(y))
## [1] J K L M N O
## Levels: J K L M N O
不再使用的级别不再存在。如果您使用因子的数值，请小心，因为droplevels更改了表示形式：
as.numeric(y)
## [1] 10 11 12 13 14 15
as.numeric(z)
## [1] 1 2 3 4 5 6