在进行wilcox.test之前，我无法对变量（food.type和inter.distance）进行编码。完全新手，有人可以帮忙吗？
food.type inter.distance
leaves 25
fruit 15
none 25
none 25
fruit 10
none 15
none 10
none 10
bark 5
fruit 15
fruit 10
none 20
leaves 25
none 25
none 15
bark 25
fruit 15
leaves 15
fruit 25
none 20
fruit 25
fruit 15
none 5
none 5
none 5
none 5
none 10
none 10
none 25
none 20
leaves 5
none 10
none 10
fruit 20
bark 20
none 15
leaves 5
none 5
leaves 20
fruit 15
leaves 25
bark 10
none 20
none 10
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GROUP<-factor(GROUP,c(1,2,3,4),labels=c('leaves','fruit','bark','none'))
#Error in factor(GROUP, c(1, 2, 3, 4), labels = c("leaves", "fruit", "bark", :
# object 'GROUP' not found